This is ../../info/eintr, produced by makeinfo version 4.11 from emacs-lisp-intro.texi. INFO-DIR-SECTION Emacs START-INFO-DIR-ENTRY * Emacs Lisp Intro: (eintr). A simple introduction to Emacs Lisp programming. END-INFO-DIR-ENTRY This is an `Introduction to Programming in Emacs Lisp', for people who are not programmers. Edition 3.10, 28 October 2009 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc. Published by the: GNU Press, Website: http://www.gnupress.org a division of the General: press@gnu.org Free Software Foundation, Inc. Orders: sales@gnu.org 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 ISBN 1-882114-43-4 Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; there being no Invariant Section, with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom."  File: eintr, Node: Digression into C, Next: defvar, Prev: copy-region-as-kill, Up: Cutting & Storing Text 8.4 Digression into C ===================== The `copy-region-as-kill' function (*note `copy-region-as-kill': copy-region-as-kill.) uses the `filter-buffer-substring' function, which in turn uses the `delete-and-extract-region' function. It removes the contents of a region and you cannot get them back. Unlike the other code discussed here, the `delete-and-extract-region' function is not written in Emacs Lisp; it is written in C and is one of the primitives of the GNU Emacs system. Since it is very simple, I will digress briefly from Lisp and describe it here. Like many of the other Emacs primitives, `delete-and-extract-region' is written as an instance of a C macro, a macro being a template for code. The complete macro looks like this: DEFUN ("buffer-substring-no-properties", Fbuffer_substring_no_properties, Sbuffer_substring_no_properties, 2, 2, 0, doc: /* Return the characters of part of the buffer, without the text properties. The two arguments START and END are character positions; they can be in either order. */) (start, end) Lisp_Object start, end; { register int b, e; validate_region (&start, &end); b = XINT (start); e = XINT (end); return make_buffer_string (b, e, 0); } Without going into the details of the macro writing process, let me point out that this macro starts with the word `DEFUN'. The word `DEFUN' was chosen since the code serves the same purpose as `defun' does in Lisp. (The `DEFUN' C macro is defined in `emacs/src/lisp.h'.) The word `DEFUN' is followed by seven parts inside of parentheses: * The first part is the name given to the function in Lisp, `delete-and-extract-region'. * The second part is the name of the function in C, `Fdelete_and_extract_region'. By convention, it starts with `F'. Since C does not use hyphens in names, underscores are used instead. * The third part is the name for the C constant structure that records information on this function for internal use. It is the name of the function in C but begins with an `S' instead of an `F'. * The fourth and fifth parts specify the minimum and maximum number of arguments the function can have. This function demands exactly 2 arguments. * The sixth part is nearly like the argument that follows the `interactive' declaration in a function written in Lisp: a letter followed, perhaps, by a prompt. The only difference from the Lisp is when the macro is called with no arguments. Then you write a `0' (which is a `null string'), as in this macro. If you were to specify arguments, you would place them between quotation marks. The C macro for `goto-char' includes `"NGoto char: "' in this position to indicate that the function expects a raw prefix, in this case, a numerical location in a buffer, and provides a prompt. * The seventh part is a documentation string, just like the one for a function written in Emacs Lisp, except that every newline must be written explicitly as `\n' followed by a backslash and carriage return. Thus, the first two lines of documentation for `goto-char' are written like this: "Set point to POSITION, a number or marker.\n\ Beginning of buffer is position (point-min), end is (point-max)." In a C macro, the formal parameters come next, with a statement of what kind of object they are, followed by what might be called the `body' of the macro. For `delete-and-extract-region' the `body' consists of the following four lines: validate_region (&start, &end); if (XINT (start) == XINT (end)) return build_string (""); return del_range_1 (XINT (start), XINT (end), 1, 1); The `validate_region' function checks whether the values passed as the beginning and end of the region are the proper type and are within range. If the beginning and end positions are the same, then return and empty string. The `del_range_1' function actually deletes the text. It is a complex function we will not look into. It updates the buffer and does other things. However, it is worth looking at the two arguments passed to `del_range'. These are `XINT (start)' and `XINT (end)'. As far as the C language is concerned, `start' and `end' are two integers that mark the beginning and end of the region to be deleted(1). In early versions of Emacs, these two numbers were thirty-two bits long, but the code is slowly being generalized to handle other lengths. Three of the available bits are used to specify the type of information; the remaining bits are used as `content'. `XINT' is a C macro that extracts the relevant number from the longer collection of bits; the three other bits are discarded. The command in `delete-and-extract-region' looks like this: del_range_1 (XINT (start), XINT (end), 1, 1); It deletes the region between the beginning position, `start', and the ending position, `end'. From the point of view of the person writing Lisp, Emacs is all very simple; but hidden underneath is a great deal of complexity to make it all work. ---------- Footnotes ---------- (1) More precisely, and requiring more expert knowledge to understand, the two integers are of type `Lisp_Object', which can also be a C union instead of an integer type.  File: eintr, Node: defvar, Next: cons & search-fwd Review, Prev: Digression into C, Up: Cutting & Storing Text 8.5 Initializing a Variable with `defvar' ========================================= The `copy-region-as-kill' function is written in Emacs Lisp. Two functions within it, `kill-append' and `kill-new', copy a region in a buffer and save it in a variable called the `kill-ring'. This section describes how the `kill-ring' variable is created and initialized using the `defvar' special form. (Again we note that the term `kill-ring' is a misnomer. The text that is clipped out of the buffer can be brought back; it is not a ring of corpses, but a ring of resurrectable text.) In Emacs Lisp, a variable such as the `kill-ring' is created and given an initial value by using the `defvar' special form. The name comes from "define variable". The `defvar' special form is similar to `setq' in that it sets the value of a variable. It is unlike `setq' in two ways: first, it only sets the value of the variable if the variable does not already have a value. If the variable already has a value, `defvar' does not override the existing value. Second, `defvar' has a documentation string. (Another special form, `defcustom', is designed for variables that people customize. It has more features than `defvar'. (*Note Setting Variables with `defcustom': defcustom.) * Menu: * See variable current value:: * defvar and asterisk::  File: eintr, Node: See variable current value, Next: defvar and asterisk, Prev: defvar, Up: defvar Seeing the Current Value of a Variable -------------------------------------- You can see the current value of a variable, any variable, by using the `describe-variable' function, which is usually invoked by typing `C-h v'. If you type `C-h v' and then `kill-ring' (followed by ) when prompted, you will see what is in your current kill ring--this may be quite a lot! Conversely, if you have been doing nothing this Emacs session except read this document, you may have nothing in it. Also, you will see the documentation for `kill-ring': Documentation: List of killed text sequences. Since the kill ring is supposed to interact nicely with cut-and-paste facilities offered by window systems, use of this variable should interact nicely with `interprogram-cut-function' and `interprogram-paste-function'. The functions `kill-new', `kill-append', and `current-kill' are supposed to implement this interaction; you may want to use them instead of manipulating the kill ring directly. The kill ring is defined by a `defvar' in the following way: (defvar kill-ring nil "List of killed text sequences. ...") In this variable definition, the variable is given an initial value of `nil', which makes sense, since if you have saved nothing, you want nothing back if you give a `yank' command. The documentation string is written just like the documentation string of a `defun'. As with the documentation string of the `defun', the first line of the documentation should be a complete sentence, since some commands, like `apropos', print only the first line of documentation. Succeeding lines should not be indented; otherwise they look odd when you use `C-h v' (`describe-variable').  File: eintr, Node: defvar and asterisk, Prev: See variable current value, Up: defvar 8.5.1 `defvar' and an asterisk ------------------------------ In the past, Emacs used the `defvar' special form both for internal variables that you would not expect a user to change and for variables that you do expect a user to change. Although you can still use `defvar' for user customizable variables, please use `defcustom' instead, since that special form provides a path into the Customization commands. (*Note Specifying Variables using `defcustom': defcustom.) When you specified a variable using the `defvar' special form, you could distinguish a variable that a user might want to change from others by typing an asterisk, `*', in the first column of its documentation string. For example: (defvar shell-command-default-error-buffer nil "*Buffer name for `shell-command' ... error output. ... ") You could (and still can) use the `set-variable' command to change the value of `shell-command-default-error-buffer' temporarily. However, options set using `set-variable' are set only for the duration of your editing session. The new values are not saved between sessions. Each time Emacs starts, it reads the original value, unless you change the value within your `.emacs' file, either by setting it manually or by using `customize'. *Note Your `.emacs' File: Emacs Initialization. For me, the major use of the `set-variable' command is to suggest variables that I might want to set in my `.emacs' file. There are now more than 700 such variables -- far too many to remember readily. Fortunately, you can press after calling the `M-x set-variable' command to see the list of variables. (*Note Examining and Setting Variables: (emacs)Examining.)  File: eintr, Node: cons & search-fwd Review, Next: search Exercises, Prev: defvar, Up: Cutting & Storing Text 8.6 Review ========== Here is a brief summary of some recently introduced functions. `car' `cdr' `car' returns the first element of a list; `cdr' returns the second and subsequent elements of a list. For example: (car '(1 2 3 4 5 6 7)) => 1 (cdr '(1 2 3 4 5 6 7)) => (2 3 4 5 6 7) `cons' `cons' constructs a list by prepending its first argument to its second argument. For example: (cons 1 '(2 3 4)) => (1 2 3 4) `funcall' `funcall' evaluates its first argument as a function. It passes its remaining arguments to its first argument. `nthcdr' Return the result of taking CDR `n' times on a list. The `rest of the rest', as it were. For example: (nthcdr 3 '(1 2 3 4 5 6 7)) => (4 5 6 7) `setcar' `setcdr' `setcar' changes the first element of a list; `setcdr' changes the second and subsequent elements of a list. For example: (setq triple '(1 2 3)) (setcar triple '37) triple => (37 2 3) (setcdr triple '("foo" "bar")) triple => (37 "foo" "bar") `progn' Evaluate each argument in sequence and then return the value of the last. For example: (progn 1 2 3 4) => 4 `save-restriction' Record whatever narrowing is in effect in the current buffer, if any, and restore that narrowing after evaluating the arguments. `search-forward' Search for a string, and if the string is found, move point. With a regular expression, use the similar `re-search-forward'. (*Note Regular Expression Searches: Regexp Search, for an explanation of regular expression patterns and searches.) `search-forward' and `re-search-forward' take four arguments: 1. The string or regular expression to search for. 2. Optionally, the limit of the search. 3. Optionally, what to do if the search fails, return `nil' or an error message. 4. Optionally, how many times to repeat the search; if negative, the search goes backwards. `kill-region' `delete-and-extract-region' `copy-region-as-kill' `kill-region' cuts the text between point and mark from the buffer and stores that text in the kill ring, so you can get it back by yanking. `copy-region-as-kill' copies the text between point and mark into the kill ring, from which you can get it by yanking. The function does not cut or remove the text from the buffer. `delete-and-extract-region' removes the text between point and mark from the buffer and throws it away. You cannot get it back. (This is not an interactive command.)  File: eintr, Node: search Exercises, Prev: cons & search-fwd Review, Up: Cutting & Storing Text 8.7 Searching Exercises ======================= * Write an interactive function that searches for a string. If the search finds the string, leave point after it and display a message that says "Found!". (Do not use `search-forward' for the name of this function; if you do, you will overwrite the existing version of `search-forward' that comes with Emacs. Use a name such as `test-search' instead.) * Write a function that prints the third element of the kill ring in the echo area, if any; if the kill ring does not contain a third element, print an appropriate message.  File: eintr, Node: List Implementation, Next: Yanking, Prev: Cutting & Storing Text, Up: Top 9 How Lists are Implemented *************************** In Lisp, atoms are recorded in a straightforward fashion; if the implementation is not straightforward in practice, it is, nonetheless, straightforward in theory. The atom `rose', for example, is recorded as the four contiguous letters `r', `o', `s', `e'. A list, on the other hand, is kept differently. The mechanism is equally simple, but it takes a moment to get used to the idea. A list is kept using a series of pairs of pointers. In the series, the first pointer in each pair points to an atom or to another list, and the second pointer in each pair points to the next pair, or to the symbol `nil', which marks the end of the list. A pointer itself is quite simply the electronic address of what is pointed to. Hence, a list is kept as a series of electronic addresses. * Menu: * Lists diagrammed:: * Symbols as Chest:: Exploring a powerful metaphor. * List Exercise::  File: eintr, Node: Lists diagrammed, Next: Symbols as Chest, Prev: List Implementation, Up: List Implementation Lists diagrammed ================ For example, the list `(rose violet buttercup)' has three elements, `rose', `violet', and `buttercup'. In the computer, the electronic address of `rose' is recorded in a segment of computer memory along with the address that gives the electronic address of where the atom `violet' is located; and that address (the one that tells where `violet' is located) is kept along with an address that tells where the address for the atom `buttercup' is located. This sounds more complicated than it is and is easier seen in a diagram: ___ ___ ___ ___ ___ ___ |___|___|--> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup In the diagram, each box represents a word of computer memory that holds a Lisp object, usually in the form of a memory address. The boxes, i.e. the addresses, are in pairs. Each arrow points to what the address is the address of, either an atom or another pair of addresses. The first box is the electronic address of `rose' and the arrow points to `rose'; the second box is the address of the next pair of boxes, the first part of which is the address of `violet' and the second part of which is the address of the next pair. The very last box points to the symbol `nil', which marks the end of the list. When a variable is set to a list with a function such as `setq', it stores the address of the first box in the variable. Thus, evaluation of the expression (setq bouquet '(rose violet buttercup)) creates a situation like this: bouquet | | ___ ___ ___ ___ ___ ___ --> |___|___|--> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup In this example, the symbol `bouquet' holds the address of the first pair of boxes. This same list can be illustrated in a different sort of box notation like this: bouquet | | -------------- --------------- ---------------- | | car | cdr | | car | cdr | | car | cdr | -->| rose | o------->| violet | o------->| butter- | nil | | | | | | | | cup | | -------------- --------------- ---------------- (Symbols consist of more than pairs of addresses, but the structure of a symbol is made up of addresses. Indeed, the symbol `bouquet' consists of a group of address-boxes, one of which is the address of the printed word `bouquet', a second of which is the address of a function definition attached to the symbol, if any, a third of which is the address of the first pair of address-boxes for the list `(rose violet buttercup)', and so on. Here we are showing that the symbol's third address-box points to the first pair of address-boxes for the list.) If a symbol is set to the CDR of a list, the list itself is not changed; the symbol simply has an address further down the list. (In the jargon, CAR and CDR are `non-destructive'.) Thus, evaluation of the following expression (setq flowers (cdr bouquet)) produces this: bouquet flowers | | | ___ ___ | ___ ___ ___ ___ --> | | | --> | | | | | | |___|___|----> |___|___|--> |___|___|--> nil | | | | | | --> rose --> violet --> buttercup The value of `flowers' is `(violet buttercup)', which is to say, the symbol `flowers' holds the address of the pair of address-boxes, the first of which holds the address of `violet', and the second of which holds the address of `buttercup'. A pair of address-boxes is called a "cons cell" or "dotted pair". *Note Cons Cell and List Types: (elisp)Cons Cell Type, and *note Dotted Pair Notation: (elisp)Dotted Pair Notation, for more information about cons cells and dotted pairs. The function `cons' adds a new pair of addresses to the front of a series of addresses like that shown above. For example, evaluating the expression (setq bouquet (cons 'lily bouquet)) produces: bouquet flowers | | | ___ ___ ___ ___ | ___ ___ ___ ___ --> | | | | | | --> | | | | | | |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil | | | | | | | | --> lily --> rose --> violet --> buttercup However, this does not change the value of the symbol `flowers', as you can see by evaluating the following, (eq (cdr (cdr bouquet)) flowers) which returns `t' for true. Until it is reset, `flowers' still has the value `(violet buttercup)'; that is, it has the address of the cons cell whose first address is of `violet'. Also, this does not alter any of the pre-existing cons cells; they are all still there. Thus, in Lisp, to get the CDR of a list, you just get the address of the next cons cell in the series; to get the CAR of a list, you get the address of the first element of the list; to `cons' a new element on a list, you add a new cons cell to the front of the list. That is all there is to it! The underlying structure of Lisp is brilliantly simple! And what does the last address in a series of cons cells refer to? It is the address of the empty list, of `nil'. In summary, when a Lisp variable is set to a value, it is provided with the address of the list to which the variable refers.  File: eintr, Node: Symbols as Chest, Next: List Exercise, Prev: Lists diagrammed, Up: List Implementation 9.1 Symbols as a Chest of Drawers ================================= In an earlier section, I suggested that you might imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice-verse. Actually, what is put in each drawer is the address of the value or function definition. It is as if you found an old chest in the attic, and in one of its drawers you found a map giving you directions to where the buried treasure lies. (In addition to its name, symbol definition, and variable value, a symbol has a `drawer' for a "property list" which can be used to record other information. Property lists are not discussed here; see *note Property Lists: (elisp)Property Lists.) Here is a fanciful representation: Chest of Drawers Contents of Drawers __ o0O0o __ / \ --------------------- | directions to | [map to] | symbol name | bouquet | | +---------------------+ | directions to | | symbol definition | [none] | | +---------------------+ | directions to | [map to] | variable value | (rose violet buttercup) | | +---------------------+ | directions to | | property list | [not described here] | | +---------------------+ |/ \|  File: eintr, Node: List Exercise, Prev: Symbols as Chest, Up: List Implementation 9.2 Exercise ============ Set `flowers' to `violet' and `buttercup'. Cons two more flowers on to this list and set this new list to `more-flowers'. Set the CAR of `flowers' to a fish. What does the `more-flowers' list now contain?  File: eintr, Node: Yanking, Next: Loops & Recursion, Prev: List Implementation, Up: Top 10 Yanking Text Back ******************** Whenever you cut text out of a buffer with a `kill' command in GNU Emacs, you can bring it back with a `yank' command. The text that is cut out of the buffer is put in the kill ring and the yank commands insert the appropriate contents of the kill ring back into a buffer (not necessarily the original buffer). A simple `C-y' (`yank') command inserts the first item from the kill ring into the current buffer. If the `C-y' command is followed immediately by `M-y', the first element is replaced by the second element. Successive `M-y' commands replace the second element with the third, fourth, or fifth element, and so on. When the last element in the kill ring is reached, it is replaced by the first element and the cycle is repeated. (Thus the kill ring is called a `ring' rather than just a `list'. However, the actual data structure that holds the text is a list. *Note Handling the Kill Ring: Kill Ring, for the details of how the list is handled as a ring.) * Menu: * Kill Ring Overview:: * kill-ring-yank-pointer:: The kill ring is a list. * yank nthcdr Exercises:: The `kill-ring-yank-pointer' variable.  File: eintr, Node: Kill Ring Overview, Next: kill-ring-yank-pointer, Prev: Yanking, Up: Yanking 10.1 Kill Ring Overview ======================= The kill ring is a list of textual strings. This is what it looks like: ("some text" "a different piece of text" "yet more text") If this were the contents of my kill ring and I pressed `C-y', the string of characters saying `some text' would be inserted in this buffer where my cursor is located. The `yank' command is also used for duplicating text by copying it. The copied text is not cut from the buffer, but a copy of it is put on the kill ring and is inserted by yanking it back. Three functions are used for bringing text back from the kill ring: `yank', which is usually bound to `C-y'; `yank-pop', which is usually bound to `M-y'; and `rotate-yank-pointer', which is used by the two other functions. These functions refer to the kill ring through a variable called the `kill-ring-yank-pointer'. Indeed, the insertion code for both the `yank' and `yank-pop' functions is: (insert (car kill-ring-yank-pointer)) (Well, no more. In GNU Emacs 22, the function has been replaced by `insert-for-yank' which calls `insert-for-yank-1' repetitively for each `yank-handler' segment. In turn, `insert-for-yank-1' strips text properties from the inserted text according to `yank-excluded-properties'. Otherwise, it is just like `insert'. We will stick with plain `insert' since it is easier to understand.) To begin to understand how `yank' and `yank-pop' work, it is first necessary to look at the `kill-ring-yank-pointer' variable.  File: eintr, Node: kill-ring-yank-pointer, Next: yank nthcdr Exercises, Prev: Kill Ring Overview, Up: Yanking 10.2 The `kill-ring-yank-pointer' Variable ========================================== `kill-ring-yank-pointer' is a variable, just as `kill-ring' is a variable. It points to something by being bound to the value of what it points to, like any other Lisp variable. Thus, if the value of the kill ring is: ("some text" "a different piece of text" "yet more text") and the `kill-ring-yank-pointer' points to the second clause, the value of `kill-ring-yank-pointer' is: ("a different piece of text" "yet more text") As explained in the previous chapter (*note List Implementation::), the computer does not keep two different copies of the text being pointed to by both the `kill-ring' and the `kill-ring-yank-pointer'. The words "a different piece of text" and "yet more text" are not duplicated. Instead, the two Lisp variables point to the same pieces of text. Here is a diagram: kill-ring kill-ring-yank-pointer | | | ___ ___ | ___ ___ ___ ___ ---> | | | --> | | | | | | |___|___|----> |___|___|--> |___|___|--> nil | | | | | | | | --> "yet more text" | | | --> "a different piece of text" | --> "some text" Both the variable `kill-ring' and the variable `kill-ring-yank-pointer' are pointers. But the kill ring itself is usually described as if it were actually what it is composed of. The `kill-ring' is spoken of as if it were the list rather than that it points to the list. Conversely, the `kill-ring-yank-pointer' is spoken of as pointing to a list. These two ways of talking about the same thing sound confusing at first but make sense on reflection. The kill ring is generally thought of as the complete structure of data that holds the information of what has recently been cut out of the Emacs buffers. The `kill-ring-yank-pointer' on the other hand, serves to indicate--that is, to `point to'--that part of the kill ring of which the first element (the CAR) will be inserted.  File: eintr, Node: yank nthcdr Exercises, Prev: kill-ring-yank-pointer, Up: Yanking 10.3 Exercises with `yank' and `nthcdr' ======================================= * Using `C-h v' (`describe-variable'), look at the value of your kill ring. Add several items to your kill ring; look at its value again. Using `M-y' (`yank-pop)', move all the way around the kill ring. How many items were in your kill ring? Find the value of `kill-ring-max'. Was your kill ring full, or could you have kept more blocks of text within it? * Using `nthcdr' and `car', construct a series of expressions to return the first, second, third, and fourth elements of a list.  File: eintr, Node: Loops & Recursion, Next: Regexp Search, Prev: Yanking, Up: Top 11 Loops and Recursion ********************** Emacs Lisp has two primary ways to cause an expression, or a series of expressions, to be evaluated repeatedly: one uses a `while' loop, and the other uses "recursion". Repetition can be very valuable. For example, to move forward four sentences, you need only write a program that will move forward one sentence and then repeat the process four times. Since a computer does not get bored or tired, such repetitive action does not have the deleterious effects that excessive or the wrong kinds of repetition can have on humans. People mostly write Emacs Lisp functions using `while' loops and their kin; but you can use recursion, which provides a very powerful way to think about and then to solve problems(1). * Menu: * while:: Causing a stretch of code to repeat. * dolist dotimes:: * Recursion:: Causing a function to call itself. * Looping exercise:: ---------- Footnotes ---------- (1) You can write recursive functions to be frugal or wasteful of mental or computer resources; as it happens, methods that people find easy--that are frugal of `mental resources'--sometimes use considerable computer resources. Emacs was designed to run on machines that we now consider limited and its default settings are conservative. You may want to increase the values of `max-specpdl-size' and `max-lisp-eval-depth'. In my `.emacs' file, I set them to 15 and 30 times their default value.  File: eintr, Node: while, Next: dolist dotimes, Prev: Loops & Recursion, Up: Loops & Recursion 11.1 `while' ============ The `while' special form tests whether the value returned by evaluating its first argument is true or false. This is similar to what the Lisp interpreter does with an `if'; what the interpreter does next, however, is different. In a `while' expression, if the value returned by evaluating the first argument is false, the Lisp interpreter skips the rest of the expression (the "body" of the expression) and does not evaluate it. However, if the value is true, the Lisp interpreter evaluates the body of the expression and then again tests whether the first argument to `while' is true or false. If the value returned by evaluating the first argument is again true, the Lisp interpreter again evaluates the body of the expression. The template for a `while' expression looks like this: (while TRUE-OR-FALSE-TEST BODY...) * Menu: * Looping with while:: Repeat so long as test returns true. * Loop Example:: A `while' loop that uses a list. * print-elements-of-list:: Uses `while', `car', `cdr'. * Incrementing Loop:: A loop with an incrementing counter. * Incrementing Loop Details:: * Decrementing Loop:: A loop with a decrementing counter.  File: eintr, Node: Looping with while, Next: Loop Example, Prev: while, Up: while Looping with `while' -------------------- So long as the true-or-false-test of the `while' expression returns a true value when it is evaluated, the body is repeatedly evaluated. This process is called a loop since the Lisp interpreter repeats the same thing again and again, like an airplane doing a loop. When the result of evaluating the true-or-false-test is false, the Lisp interpreter does not evaluate the rest of the `while' expression and `exits the loop'. Clearly, if the value returned by evaluating the first argument to `while' is always true, the body following will be evaluated again and again ... and again ... forever. Conversely, if the value returned is never true, the expressions in the body will never be evaluated. The craft of writing a `while' loop consists of choosing a mechanism such that the true-or-false-test returns true just the number of times that you want the subsequent expressions to be evaluated, and then have the test return false. The value returned by evaluating a `while' is the value of the true-or-false-test. An interesting consequence of this is that a `while' loop that evaluates without error will return `nil' or false regardless of whether it has looped 1 or 100 times or none at all. A `while' expression that evaluates successfully never returns a true value! What this means is that `while' is always evaluated for its side effects, which is to say, the consequences of evaluating the expressions within the body of the `while' loop. This makes sense. It is not the mere act of looping that is desired, but the consequences of what happens when the expressions in the loop are repeatedly evaluated.  File: eintr, Node: Loop Example, Next: print-elements-of-list, Prev: Looping with while, Up: while 11.1.1 A `while' Loop and a List -------------------------------- A common way to control a `while' loop is to test whether a list has any elements. If it does, the loop is repeated; but if it does not, the repetition is ended. Since this is an important technique, we will create a short example to illustrate it. A simple way to test whether a list has elements is to evaluate the list: if it has no elements, it is an empty list and will return the empty list, `()', which is a synonym for `nil' or false. On the other hand, a list with elements will return those elements when it is evaluated. Since Emacs Lisp considers as true any value that is not `nil', a list that returns elements will test true in a `while' loop. For example, you can set the variable `empty-list' to `nil' by evaluating the following `setq' expression: (setq empty-list ()) After evaluating the `setq' expression, you can evaluate the variable `empty-list' in the usual way, by placing the cursor after the symbol and typing `C-x C-e'; `nil' will appear in your echo area: empty-list On the other hand, if you set a variable to be a list with elements, the list will appear when you evaluate the variable, as you can see by evaluating the following two expressions: (setq animals '(gazelle giraffe lion tiger)) animals Thus, to create a `while' loop that tests whether there are any items in the list `animals', the first part of the loop will be written like this: (while animals ... When the `while' tests its first argument, the variable `animals' is evaluated. It returns a list. So long as the list has elements, the `while' considers the results of the test to be true; but when the list is empty, it considers the results of the test to be false. To prevent the `while' loop from running forever, some mechanism needs to be provided to empty the list eventually. An oft-used technique is to have one of the subsequent forms in the `while' expression set the value of the list to be the CDR of the list. Each time the `cdr' function is evaluated, the list will be made shorter, until eventually only the empty list will be left. At this point, the test of the `while' loop will return false, and the arguments to the `while' will no longer be evaluated. For example, the list of animals bound to the variable `animals' can be set to be the CDR of the original list with the following expression: (setq animals (cdr animals)) If you have evaluated the previous expressions and then evaluate this expression, you will see `(giraffe lion tiger)' appear in the echo area. If you evaluate the expression again, `(lion tiger)' will appear in the echo area. If you evaluate it again and yet again, `(tiger)' appears and then the empty list, shown by `nil'. A template for a `while' loop that uses the `cdr' function repeatedly to cause the true-or-false-test eventually to test false looks like this: (while TEST-WHETHER-LIST-IS-EMPTY BODY... SET-LIST-TO-CDR-OF-LIST) This test and use of `cdr' can be put together in a function that goes through a list and prints each element of the list on a line of its own.  File: eintr, Node: print-elements-of-list, Next: Incrementing Loop, Prev: Loop Example, Up: while 11.1.2 An Example: `print-elements-of-list' ------------------------------------------- The `print-elements-of-list' function illustrates a `while' loop with a list. The function requires several lines for its output. If you are reading this in a recent instance of GNU Emacs, you can evaluate the following expression inside of Info, as usual. If you are using an earlier version of Emacs, you need to copy the necessary expressions to your `*scratch*' buffer and evaluate them there. This is because the echo area had only one line in the earlier versions. You can copy the expressions by marking the beginning of the region with `C-' (`set-mark-command'), moving the cursor to the end of the region and then copying the region using `M-w' (`kill-ring-save', which calls `copy-region-as-kill' and then provides visual feedback). In the `*scratch*' buffer, you can yank the expressions back by typing `C-y' (`yank'). After you have copied the expressions to the `*scratch*' buffer, evaluate each expression in turn. Be sure to evaluate the last expression, `(print-elements-of-list animals)', by typing `C-u C-x C-e', that is, by giving an argument to `eval-last-sexp'. This will cause the result of the evaluation to be printed in the `*scratch*' buffer instead of being printed in the echo area. (Otherwise you will see something like this in your echo area: `^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil', in which each `^J' stands for a `newline'.) In a recent instance of GNU Emacs, you can evaluate these expressions directly in the Info buffer, and the echo area will grow to show the results. (setq animals '(gazelle giraffe lion tiger)) (defun print-elements-of-list (list) "Print each element of LIST on a line of its own." (while list (print (car list)) (setq list (cdr list)))) (print-elements-of-list animals) When you evaluate the three expressions in sequence, you will see this: gazelle giraffe lion tiger nil Each element of the list is printed on a line of its own (that is what the function `print' does) and then the value returned by the function is printed. Since the last expression in the function is the `while' loop, and since `while' loops always return `nil', a `nil' is printed after the last element of the list.  File: eintr, Node: Incrementing Loop, Next: Incrementing Loop Details, Prev: print-elements-of-list, Up: while 11.1.3 A Loop with an Incrementing Counter ------------------------------------------ A loop is not useful unless it stops when it ought. Besides controlling a loop with a list, a common way of stopping a loop is to write the first argument as a test that returns false when the correct number of repetitions are complete. This means that the loop must have a counter--an expression that counts how many times the loop repeats itself.  File: eintr, Node: Incrementing Loop Details, Next: Decrementing Loop, Prev: Incrementing Loop, Up: while Details of an Incrementing Loop ------------------------------- The test for a loop with an incrementing counter can be an expression such as `(< count desired-number)' which returns `t' for true if the value of `count' is less than the `desired-number' of repetitions and `nil' for false if the value of `count' is equal to or is greater than the `desired-number'. The expression that increments the count can be a simple `setq' such as `(setq count (1+ count))', where `1+' is a built-in function in Emacs Lisp that adds 1 to its argument. (The expression `(1+ count)' has the same result as `(+ count 1)', but is easier for a human to read.) The template for a `while' loop controlled by an incrementing counter looks like this: SET-COUNT-TO-INITIAL-VALUE (while (< count desired-number) ; true-or-false-test BODY... (setq count (1+ count))) ; incrementer Note that you need to set the initial value of `count'; usually it is set to 1. * Menu: * Incrementing Example:: Counting pebbles in a triangle. * Inc Example parts:: The parts of the function definition. * Inc Example altogether:: Putting the function definition together.  File: eintr, Node: Incrementing Example, Next: Inc Example parts, Prev: Incrementing Loop Details, Up: Incrementing Loop Details Example with incrementing counter ................................. Suppose you are playing on the beach and decide to make a triangle of pebbles, putting one pebble in the first row, two in the second row, three in the third row and so on, like this: * * * * * * * * * * (About 2500 years ago, Pythagoras and others developed the beginnings of number theory by considering questions such as this.) Suppose you want to know how many pebbles you will need to make a triangle with 7 rows? Clearly, what you need to do is add up the numbers from 1 to 7. There are two ways to do this; start with the smallest number, one, and add up the list in sequence, 1, 2, 3, 4 and so on; or start with the largest number and add the list going down: 7, 6, 5, 4 and so on. Because both mechanisms illustrate common ways of writing `while' loops, we will create two examples, one counting up and the other counting down. In this first example, we will start with 1 and add 2, 3, 4 and so on. If you are just adding up a short list of numbers, the easiest way to do it is to add up all the numbers at once. However, if you do not know ahead of time how many numbers your list will have, or if you want to be prepared for a very long list, then you need to design your addition so that what you do is repeat a simple process many times instead of doing a more complex process once. For example, instead of adding up all the pebbles all at once, what you can do is add the number of pebbles in the first row, 1, to the number in the second row, 2, and then add the total of those two rows to the third row, 3. Then you can add the number in the fourth row, 4, to the total of the first three rows; and so on. The critical characteristic of the process is that each repetitive action is simple. In this case, at each step we add only two numbers, the number of pebbles in the row and the total already found. This process of adding two numbers is repeated again and again until the last row has been added to the total of all the preceding rows. In a more complex loop the repetitive action might not be so simple, but it will be simpler than doing everything all at once.  File: eintr, Node: Inc Example parts, Next: Inc Example altogether, Prev: Incrementing Example, Up: Incrementing Loop Details The parts of the function definition .................................... The preceding analysis gives us the bones of our function definition: first, we will need a variable that we can call `total' that will be the total number of pebbles. This will be the value returned by the function. Second, we know that the function will require an argument: this argument will be the total number of rows in the triangle. It can be called `number-of-rows'. Finally, we need a variable to use as a counter. We could call this variable `counter', but a better name is `row-number'. That is because what the counter does in this function is count rows, and a program should be written to be as understandable as possible. When the Lisp interpreter first starts evaluating the expressions in the function, the value of `total' should be set to zero, since we have not added anything to it. Then the function should add the number of pebbles in the first row to the total, and then add the number of pebbles in the second to the total, and then add the number of pebbles in the third row to the total, and so on, until there are no more rows left to add. Both `total' and `row-number' are used only inside the function, so they can be declared as local variables with `let' and given initial values. Clearly, the initial value for `total' should be 0. The initial value of `row-number' should be 1, since we start with the first row. This means that the `let' statement will look like this: (let ((total 0) (row-number 1)) BODY...) After the internal variables are declared and bound to their initial values, we can begin the `while' loop. The expression that serves as the test should return a value of `t' for true so long as the `row-number' is less than or equal to the `number-of-rows'. (If the expression tests true only so long as the row number is less than the number of rows in the triangle, the last row will never be added to the total; hence the row number has to be either less than or equal to the number of rows.) Lisp provides the `<=' function that returns true if the value of its first argument is less than or equal to the value of its second argument and false otherwise. So the expression that the `while' will evaluate as its test should look like this: (<= row-number number-of-rows) The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found. Since the number of pebbles in the row is equal to the row number, the total can be found by adding the row number to the total. (Clearly, in a more complex situation, the number of pebbles in the row might be related to the row number in a more complicated way; if this were the case, the row number would be replaced by the appropriate expression.) (setq total (+ total row-number)) What this does is set the new value of `total' to be equal to the sum of adding the number of pebbles in the row to the previous total. After setting the value of `total', the conditions need to be established for the next repetition of the loop, if there is one. This is done by incrementing the value of the `row-number' variable, which serves as a counter. After the `row-number' variable has been incremented, the true-or-false-test at the beginning of the `while' loop tests whether its value is still less than or equal to the value of the `number-of-rows' and if it is, adds the new value of the `row-number' variable to the `total' of the previous repetition of the loop. The built-in Emacs Lisp function `1+' adds 1 to a number, so the `row-number' variable can be incremented with this expression: (setq row-number (1+ row-number))  File: eintr, Node: Inc Example altogether, Prev: Inc Example parts, Up: Incrementing Loop Details Putting the function definition together ........................................ We have created the parts for the function definition; now we need to put them together. First, the contents of the `while' expression: (while (<= row-number number-of-rows) ; true-or-false-test (setq total (+ total row-number)) (setq row-number (1+ row-number))) ; incrementer Along with the `let' expression varlist, this very nearly completes the body of the function definition. However, it requires one final element, the need for which is somewhat subtle. The final touch is to place the variable `total' on a line by itself after the `while' expression. Otherwise, the value returned by the whole function is the value of the last expression that is evaluated in the body of the `let', and this is the value returned by the `while', which is always `nil'. This may not be evident at first sight. It almost looks as if the incrementing expression is the last expression of the whole function. But that expression is part of the body of the `while'; it is the last element of the list that starts with the symbol `while'. Moreover, the whole of the `while' loop is a list within the body of the `let'. In outline, the function will look like this: (defun NAME-OF-FUNCTION (ARGUMENT-LIST) "DOCUMENTATION..." (let (VARLIST) (while (TRUE-OR-FALSE-TEST) BODY-OF-WHILE... ) ... )) ; Need final expression here. The result of evaluating the `let' is what is going to be returned by the `defun' since the `let' is not embedded within any containing list, except for the `defun' as a whole. However, if the `while' is the last element of the `let' expression, the function will always return `nil'. This is not what we want! Instead, what we want is the value of the variable `total'. This is returned by simply placing the symbol as the last element of the list starting with `let'. It gets evaluated after the preceding elements of the list are evaluated, which means it gets evaluated after it has been assigned the correct value for the total. It may be easier to see this by printing the list starting with `let' all on one line. This format makes it evident that the VARLIST and `while' expressions are the second and third elements of the list starting with `let', and the `total' is the last element: (let (VARLIST) (while (TRUE-OR-FALSE-TEST) BODY-OF-WHILE... ) total) Putting everything together, the `triangle' function definition looks like this: (defun triangle (number-of-rows) ; Version with ; incrementing counter. "Add up the number of pebbles in a triangle. The first row has one pebble, the second row two pebbles, the third row three pebbles, and so on. The argument is NUMBER-OF-ROWS." (let ((total 0) (row-number 1)) (while (<= row-number number-of-rows) (setq total (+ total row-number)) (setq row-number (1+ row-number))) total)) After you have installed `triangle' by evaluating the function, you can try it out. Here are two examples: (triangle 4) (triangle 7) The sum of the first four numbers is 10 and the sum of the first seven numbers is 28.  File: eintr, Node: Decrementing Loop, Prev: Incrementing Loop Details, Up: while 11.1.4 Loop with a Decrementing Counter --------------------------------------- Another common way to write a `while' loop is to write the test so that it determines whether a counter is greater than zero. So long as the counter is greater than zero, the loop is repeated. But when the counter is equal to or less than zero, the loop is stopped. For this to work, the counter has to start out greater than zero and then be made smaller and smaller by a form that is evaluated repeatedly. The test will be an expression such as `(> counter 0)' which returns `t' for true if the value of `counter' is greater than zero, and `nil' for false if the value of `counter' is equal to or less than zero. The expression that makes the number smaller and smaller can be a simple `setq' such as `(setq counter (1- counter))', where `1-' is a built-in function in Emacs Lisp that subtracts 1 from its argument. The template for a decrementing `while' loop looks like this: (while (> counter 0) ; true-or-false-test BODY... (setq counter (1- counter))) ; decrementer * Menu: * Decrementing Example:: More pebbles on the beach. * Dec Example parts:: The parts of the function definition. * Dec Example altogether:: Putting the function definition together.  File: eintr, Node: Decrementing Example, Next: Dec Example parts, Prev: Decrementing Loop, Up: Decrementing Loop Example with decrementing counter ................................. To illustrate a loop with a decrementing counter, we will rewrite the `triangle' function so the counter decreases to zero. This is the reverse of the earlier version of the function. In this case, to find out how many pebbles are needed to make a triangle with 3 rows, add the number of pebbles in the third row, 3, to the number in the preceding row, 2, and then add the total of those two rows to the row that precedes them, which is 1. Likewise, to find the number of pebbles in a triangle with 7 rows, add the number of pebbles in the seventh row, 7, to the number in the preceding row, which is 6, and then add the total of those two rows to the row that precedes them, which is 5, and so on. As in the previous example, each addition only involves adding two numbers, the total of the rows already added up and the number of pebbles in the row that is being added to the total. This process of adding two numbers is repeated again and again until there are no more pebbles to add. We know how many pebbles to start with: the number of pebbles in the last row is equal to the number of rows. If the triangle has seven rows, the number of pebbles in the last row is 7. Likewise, we know how many pebbles are in the preceding row: it is one less than the number in the row.  File: eintr, Node: Dec Example parts, Next: Dec Example altogether, Prev: Decrementing Example, Up: Decrementing Loop The parts of the function definition .................................... We start with three variables: the total number of rows in the triangle; the number of pebbles in a row; and the total number of pebbles, which is what we want to calculate. These variables can be named `number-of-rows', `number-of-pebbles-in-row', and `total', respectively. Both `total' and `number-of-pebbles-in-row' are used only inside the function and are declared with `let'. The initial value of `total' should, of course, be zero. However, the initial value of `number-of-pebbles-in-row' should be equal to the number of rows in the triangle, since the addition will start with the longest row. This means that the beginning of the `let' expression will look like this: (let ((total 0) (number-of-pebbles-in-row number-of-rows)) BODY...) The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found, that is, by repeatedly evaluating the following expression: (setq total (+ total number-of-pebbles-in-row)) After the `number-of-pebbles-in-row' is added to the `total', the `number-of-pebbles-in-row' should be decremented by one, since the next time the loop repeats, the preceding row will be added to the total. The number of pebbles in a preceding row is one less than the number of pebbles in a row, so the built-in Emacs Lisp function `1-' can be used to compute the number of pebbles in the preceding row. This can be done with the following expression: (setq number-of-pebbles-in-row (1- number-of-pebbles-in-row)) Finally, we know that the `while' loop should stop making repeated additions when there are no pebbles in a row. So the test for the `while' loop is simply: (while (> number-of-pebbles-in-row 0)  File: eintr, Node: Dec Example altogether, Prev: Dec Example parts, Up: Decrementing Loop Putting the function definition together ........................................ We can put these expressions together to create a function definition that works. However, on examination, we find that one of the local variables is unneeded! The function definition looks like this: ;;; First subtractive version. (defun triangle (number-of-rows) "Add up the number of pebbles in a triangle." (let ((total 0) (number-of-pebbles-in-row number-of-rows)) (while (> number-of-pebbles-in-row 0) (setq total (+ total number-of-pebbles-in-row)) (setq number-of-pebbles-in-row (1- number-of-pebbles-in-row))) total)) As written, this function works. However, we do not need `number-of-pebbles-in-row'. When the `triangle' function is evaluated, the symbol `number-of-rows' will be bound to a number, giving it an initial value. That number can be changed in the body of the function as if it were a local variable, without any fear that such a change will effect the value of the variable outside of the function. This is a very useful characteristic of Lisp; it means that the variable `number-of-rows' can be used anywhere in the function where `number-of-pebbles-in-row' is used. Here is a second version of the function written a bit more cleanly: (defun triangle (number) ; Second version. "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (setq number (1- number))) total)) In brief, a properly written `while' loop will consist of three parts: 1. A test that will return false after the loop has repeated itself the correct number of times. 2. An expression the evaluation of which will return the value desired after being repeatedly evaluated. 3. An expression to change the value passed to the true-or-false-test so that the test returns false after the loop has repeated itself the right number of times.  File: eintr, Node: dolist dotimes, Next: Recursion, Prev: while, Up: Loops & Recursion 11.2 Save your time: `dolist' and `dotimes' =========================================== In addition to `while', both `dolist' and `dotimes' provide for looping. Sometimes these are quicker to write than the equivalent `while' loop. Both are Lisp macros. (*Note Macros: (elisp)Macros. ) `dolist' works like a `while' loop that `CDRs down a list': `dolist' automatically shortens the list each time it loops--takes the CDR of the list--and binds the CAR of each shorter version of the list to the first of its arguments. `dotimes' loops a specific number of times: you specify the number. * Menu: * dolist:: * dotimes::  File: eintr, Node: dolist, Next: dotimes, Prev: dolist dotimes, Up: dolist dotimes The `dolist' Macro .................. Suppose, for example, you want to reverse a list, so that "first" "second" "third" becomes "third" "second" "first". In practice, you would use the `reverse' function, like this: (setq animals '(gazelle giraffe lion tiger)) (reverse animals) Here is how you could reverse the list using a `while' loop: (setq animals '(gazelle giraffe lion tiger)) (defun reverse-list-with-while (list) "Using while, reverse the order of LIST." (let (value) ; make sure list starts empty (while list (setq value (cons (car list) value)) (setq list (cdr list))) value)) (reverse-list-with-while animals) And here is how you could use the `dolist' macro: (setq animals '(gazelle giraffe lion tiger)) (defun reverse-list-with-dolist (list) "Using dolist, reverse the order of LIST." (let (value) ; make sure list starts empty (dolist (element list value) (setq value (cons element value))))) (reverse-list-with-dolist animals) In Info, you can place your cursor after the closing parenthesis of each expression and type `C-x C-e'; in each case, you should see (tiger lion giraffe gazelle) in the echo area. For this example, the existing `reverse' function is obviously best. The `while' loop is just like our first example (*note A `while' Loop and a List: Loop Example.). The `while' first checks whether the list has elements; if so, it constructs a new list by adding the first element of the list to the existing list (which in the first iteration of the loop is `nil'). Since the second element is prepended in front of the first element, and the third element is prepended in front of the second element, the list is reversed. In the expression using a `while' loop, the `(setq list (cdr list))' expression shortens the list, so the `while' loop eventually stops. In addition, it provides the `cons' expression with a new first element by creating a new and shorter list at each repetition of the loop. The `dolist' expression does very much the same as the `while' expression, except that the `dolist' macro does some of the work you have to do when writing a `while' expression. Like a `while' loop, a `dolist' loops. What is different is that it automatically shortens the list each time it loops -- it `CDRs down the list' on its own -- and it automatically binds the CAR of each shorter version of the list to the first of its arguments. In the example, the CAR of each shorter version of the list is referred to using the symbol `element', the list itself is called `list', and the value returned is called `value'. The remainder of the `dolist' expression is the body. The `dolist' expression binds the CAR of each shorter version of the list to `element' and then evaluates the body of the expression; and repeats the loop. The result is returned in `value'.  File: eintr, Node: dotimes, Prev: dolist, Up: dolist dotimes The `dotimes' Macro ................... The `dotimes' macro is similar to `dolist', except that it loops a specific number of times. The first argument to `dotimes' is assigned the numbers 0, 1, 2 and so forth each time around the loop, and the value of the third argument is returned. You need to provide the value of the second argument, which is how many times the macro loops. For example, the following binds the numbers from 0 up to, but not including, the number 3 to the first argument, NUMBER, and then constructs a list of the three numbers. (The first number is 0, the second number is 1, and the third number is 2; this makes a total of three numbers in all, starting with zero as the first number.) (let (value) ; otherwise a value is a void variable (dotimes (number 3 value) (setq value (cons number value)))) => (2 1 0) `dotimes' returns `value', so the way to use `dotimes' is to operate on some expression NUMBER number of times and then return the result, either as a list or an atom. Here is an example of a `defun' that uses `dotimes' to add up the number of pebbles in a triangle. (defun triangle-using-dotimes (number-of-rows) "Using dotimes, add up the number of pebbles in a triangle." (let ((total 0)) ; otherwise a total is a void variable (dotimes (number number-of-rows total) (setq total (+ total (1+ number)))))) (triangle-using-dotimes 4)  File: eintr, Node: Recursion, Next: Looping exercise, Prev: dolist dotimes, Up: Loops & Recursion 11.3 Recursion ============== A recursive function contains code that tells the Lisp interpreter to call a program that runs exactly like itself, but with slightly different arguments. The code runs exactly the same because it has the same name. However, even though the program has the same name, it is not the same entity. It is different. In the jargon, it is a different `instance'. Eventually, if the program is written correctly, the `slightly different arguments' will become sufficiently different from the first arguments that the final instance will stop. * Menu: * Building Robots:: Same model, different serial number ... * Recursive Definition Parts:: Walk until you stop ... * Recursion with list:: Using a list as the test whether to recurse. * Recursive triangle function:: * Recursion with cond:: * Recursive Patterns:: Often used templates. * No Deferment:: Don't store up work ... * No deferment solution::  File: eintr, Node: Building Robots, Next: Recursive Definition Parts, Prev: Recursion, Up: Recursion 11.3.1 Building Robots: Extending the Metaphor ---------------------------------------------- It is sometimes helpful to think of a running program as a robot that does a job. In doing its job, a recursive function calls on a second robot to help it. The second robot is identical to the first in every way, except that the second robot helps the first and has been passed different arguments than the first. In a recursive function, the second robot may call a third; and the third may call a fourth, and so on. Each of these is a different entity; but all are clones. Since each robot has slightly different instructions--the arguments will differ from one robot to the next--the last robot should know when to stop. Let's expand on the metaphor in which a computer program is a robot. A function definition provides the blueprints for a robot. When you install a function definition, that is, when you evaluate a `defun' special form, you install the necessary equipment to build robots. It is as if you were in a factory, setting up an assembly line. Robots with the same name are built according to the same blueprints. So they have, as it were, the same `model number', but a different `serial number'. We often say that a recursive function `calls itself'. What we mean is that the instructions in a recursive function cause the Lisp interpreter to run a different function that has the same name and does the same job as the first, but with different arguments. It is important that the arguments differ from one instance to the next; otherwise, the process will never stop.  File: eintr, Node: Recursive Definition Parts, Next: Recursion with list, Prev: Building Robots, Up: Recursion 11.3.2 The Parts of a Recursive Definition ------------------------------------------ A recursive function typically contains a conditional expression which has three parts: 1. A true-or-false-test that determines whether the function is called again, here called the "do-again-test". 2. The name of the function. When this name is called, a new instance of the function--a new robot, as it were--is created and told what to do. 3. An expression that returns a different value each time the function is called, here called the "next-step-expression". Consequently, the argument (or arguments) passed to the new instance of the function will be different from that passed to the previous instance. This causes the conditional expression, the "do-again-test", to test false after the correct number of repetitions. Recursive functions can be much simpler than any other kind of function. Indeed, when people first start to use them, they often look so mysteriously simple as to be incomprehensible. Like riding a bicycle, reading a recursive function definition takes a certain knack which is hard at first but then seems simple. There are several different common recursive patterns. A very simple pattern looks like this: (defun NAME-OF-RECURSIVE-FUNCTION (ARGUMENT-LIST) "DOCUMENTATION..." (if DO-AGAIN-TEST BODY... (NAME-OF-RECURSIVE-FUNCTION NEXT-STEP-EXPRESSION))) Each time a recursive function is evaluated, a new instance of it is created and told what to do. The arguments tell the instance what to do. An argument is bound to the value of the next-step-expression. Each instance runs with a different value of the next-step-expression. The value in the next-step-expression is used in the do-again-test. The value returned by the next-step-expression is passed to the new instance of the function, which evaluates it (or some transmogrification of it) to determine whether to continue or stop. The next-step-expression is designed so that the do-again-test returns false when the function should no longer be repeated. The do-again-test is sometimes called the "stop condition", since it stops the repetitions when it tests false.  File: eintr, Node: Recursion with list, Next: Recursive triangle function, Prev: Recursive Definition Parts, Up: Recursion 11.3.3 Recursion with a List ---------------------------- The example of a `while' loop that printed the elements of a list of numbers can be written recursively. Here is the code, including an expression to set the value of the variable `animals' to a list. If you are using GNU Emacs 20 or before, this example must be copied to the `*scratch*' buffer and each expression must be evaluated there. Use `C-u C-x C-e' to evaluate the `(print-elements-recursively animals)' expression so that the results are printed in the buffer; otherwise the Lisp interpreter will try to squeeze the results into the one line of the echo area. Also, place your cursor immediately after the last closing parenthesis of the `print-elements-recursively' function, before the comment. Otherwise, the Lisp interpreter will try to evaluate the comment. If you are using a more recent version of Emacs, you can evaluate this expression directly in Info. (setq animals '(gazelle giraffe lion tiger)) (defun print-elements-recursively (list) "Print each element of LIST on a line of its own. Uses recursion." (when list ; do-again-test (print (car list)) ; body (print-elements-recursively ; recursive call (cdr list)))) ; next-step-expression (print-elements-recursively animals) The `print-elements-recursively' function first tests whether there is any content in the list; if there is, the function prints the first element of the list, the CAR of the list. Then the function `invokes itself', but gives itself as its argument, not the whole list, but the second and subsequent elements of the list, the CDR of the list. Put another way, if the list is not empty, the function invokes another instance of code that is similar to the initial code, but is a different thread of execution, with different arguments than the first instance. Put in yet another way, if the list is not empty, the first robot assembles a second robot and tells it what to do; the second robot is a different individual from the first, but is the same model. When the second evaluation occurs, the `when' expression is evaluated and if true, prints the first element of the list it receives as its argument (which is the second element of the original list). Then the function `calls itself' with the CDR of the list it is invoked with, which (the second time around) is the CDR of the CDR of the original list. Note that although we say that the function `calls itself', what we mean is that the Lisp interpreter assembles and instructs a new instance of the program. The new instance is a clone of the first, but is a separate individual. Each time the function `invokes itself', it invokes itself on a shorter version of the original list. It creates a new instance that works on a shorter list. Eventually, the function invokes itself on an empty list. It creates a new instance whose argument is `nil'. The conditional expression tests the value of `list'. Since the value of `list' is `nil', the `when' expression tests false so the then-part is not evaluated. The function as a whole then returns `nil'. When you evaluate the expression `(print-elements-recursively animals)' in the `*scratch*' buffer, you see this result: gazelle giraffe lion tiger nil  File: eintr, Node: Recursive triangle function, Next: Recursion with cond, Prev: Recursion with list, Up: Recursion 11.3.4 Recursion in Place of a Counter -------------------------------------- The `triangle' function described in a previous section can also be written recursively. It looks like this: (defun triangle-recursively (number) "Return the sum of the numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) ; do-again-test 1 ; then-part (+ number ; else-part (triangle-recursively ; recursive call (1- number))))) ; next-step-expression (triangle-recursively 7) You can install this function by evaluating it and then try it by evaluating `(triangle-recursively 7)'. (Remember to put your cursor immediately after the last parenthesis of the function definition, before the comment.) The function evaluates to 28. To understand how this function works, let's consider what happens in the various cases when the function is passed 1, 2, 3, or 4 as the value of its argument. * Menu: * Recursive Example arg of 1 or 2:: * Recursive Example arg of 3 or 4::  File: eintr, Node: Recursive Example arg of 1 or 2, Next: Recursive Example arg of 3 or 4, Prev: Recursive triangle function, Up: Recursive triangle function An argument of 1 or 2 ..................... First, what happens if the value of the argument is 1? The function has an `if' expression after the documentation string. It tests whether the value of `number' is equal to 1; if so, Emacs evaluates the then-part of the `if' expression, which returns the number 1 as the value of the function. (A triangle with one row has one pebble in it.) Suppose, however, that the value of the argument is 2. In this case, Emacs evaluates the else-part of the `if' expression. The else-part consists of an addition, the recursive call to `triangle-recursively' and a decrementing action; and it looks like this: (+ number (triangle-recursively (1- number))) When Emacs evaluates this expression, the innermost expression is evaluated first; then the other parts in sequence. Here are the steps in detail: Step 1 Evaluate the innermost expression. The innermost expression is `(1- number)' so Emacs decrements the value of `number' from 2 to 1. Step 2 Evaluate the `triangle-recursively' function. The Lisp interpreter creates an individual instance of `triangle-recursively'. It does not matter that this function is contained within itself. Emacs passes the result Step 1 as the argument used by this instance of the `triangle-recursively' function In this case, Emacs evaluates `triangle-recursively' with an argument of 1. This means that this evaluation of `triangle-recursively' returns 1. Step 3 Evaluate the value of `number'. The variable `number' is the second element of the list that starts with `+'; its value is 2. Step 4 Evaluate the `+' expression. The `+' expression receives two arguments, the first from the evaluation of `number' (Step 3) and the second from the evaluation of `triangle-recursively' (Step 2). The result of the addition is the sum of 2 plus 1, and the number 3 is returned, which is correct. A triangle with two rows has three pebbles in it.  File: eintr, Node: Recursive Example arg of 3 or 4, Prev: Recursive Example arg of 1 or 2, Up: Recursive triangle function An argument of 3 or 4 ..................... Suppose that `triangle-recursively' is called with an argument of 3. Step 1 Evaluate the do-again-test. The `if' expression is evaluated first. This is the do-again test and returns false, so the else-part of the `if' expression is evaluated. (Note that in this example, the do-again-test causes the function to call itself when it tests false, not when it tests true.) Step 2 Evaluate the innermost expression of the else-part. The innermost expression of the else-part is evaluated, which decrements 3 to 2. This is the next-step-expression. Step 3 Evaluate the `triangle-recursively' function. The number 2 is passed to the `triangle-recursively' function. We already know what happens when Emacs evaluates `triangle-recursively' with an argument of 2. After going through the sequence of actions described earlier, it returns a value of 3. So that is what will happen here. Step 4 Evaluate the addition. 3 will be passed as an argument to the addition and will be added to the number with which the function was called, which is 3. The value returned by the function as a whole will be 6. Now that we know what will happen when `triangle-recursively' is called with an argument of 3, it is evident what will happen if it is called with an argument of 4: In the recursive call, the evaluation of (triangle-recursively (1- 4)) will return the value of evaluating (triangle-recursively 3) which is 6 and this value will be added to 4 by the addition in the third line. The value returned by the function as a whole will be 10. Each time `triangle-recursively' is evaluated, it evaluates a version of itself--a different instance of itself--with a smaller argument, until the argument is small enough so that it does not evaluate itself. Note that this particular design for a recursive function requires that operations be deferred. Before `(triangle-recursively 7)' can calculate its answer, it must call `(triangle-recursively 6)'; and before `(triangle-recursively 6)' can calculate its answer, it must call `(triangle-recursively 5)'; and so on. That is to say, the calculation that `(triangle-recursively 7)' makes must be deferred until `(triangle-recursively 6)' makes its calculation; and `(triangle-recursively 6)' must defer until `(triangle-recursively 5)' completes; and so on. If each of these instances of `triangle-recursively' are thought of as different robots, the first robot must wait for the second to complete its job, which must wait until the third completes, and so on. There is a way around this kind of waiting, which we will discuss in *note Recursion without Deferments: No Deferment.  File: eintr, Node: Recursion with cond, Next: Recursive Patterns, Prev: Recursive triangle function, Up: Recursion 11.3.5 Recursion Example Using `cond' ------------------------------------- The version of `triangle-recursively' described earlier is written with the `if' special form. It can also be written using another special form called `cond'. The name of the special form `cond' is an abbreviation of the word `conditional'. Although the `cond' special form is not used as often in the Emacs Lisp sources as `if', it is used often enough to justify explaining it. The template for a `cond' expression looks like this: (cond BODY...) where the BODY is a series of lists. Written out more fully, the template looks like this: (cond (FIRST-TRUE-OR-FALSE-TEST FIRST-CONSEQUENT) (SECOND-TRUE-OR-FALSE-TEST SECOND-CONSEQUENT) (THIRD-TRUE-OR-FALSE-TEST THIRD-CONSEQUENT) ...) When the Lisp interpreter evaluates the `cond' expression, it evaluates the first element (the CAR or true-or-false-test) of the first expression in a series of expressions within the body of the `cond'. If the true-or-false-test returns `nil' the rest of that expression, the consequent, is skipped and the true-or-false-test of the next expression is evaluated. When an expression is found whose true-or-false-test returns a value that is not `nil', the consequent of that expression is evaluated. The consequent can be one or more expressions. If the consequent consists of more than one expression, the expressions are evaluated in sequence and the value of the last one is returned. If the expression does not have a consequent, the value of the true-or-false-test is returned. If none of the true-or-false-tests test true, the `cond' expression returns `nil'. Written using `cond', the `triangle' function looks like this: (defun triangle-using-cond (number) (cond ((<= number 0) 0) ((= number 1) 1) ((> number 1) (+ number (triangle-using-cond (1- number)))))) In this example, the `cond' returns 0 if the number is less than or equal to 0, it returns 1 if the number is 1 and it evaluates `(+ number (triangle-using-cond (1- number)))' if the number is greater than 1.  File: eintr, Node: Recursive Patterns, Next: No Deferment, Prev: Recursion with cond, Up: Recursion 11.3.6 Recursive Patterns ------------------------- Here are three common recursive patterns. Each involves a list. Recursion does not need to involve lists, but Lisp is designed for lists and this provides a sense of its primal capabilities. * Menu: * Every:: * Accumulate:: * Keep::  File: eintr, Node: Every, Next: Accumulate, Prev: Recursive Patterns, Up: Recursive Patterns Recursive Pattern: _every_ .......................... In the `every' recursive pattern, an action is performed on every element of a list. The basic pattern is: * If a list be empty, return `nil'. * Else, act on the beginning of the list (the CAR of the list) - through a recursive call by the function on the rest (the CDR) of the list, - and, optionally, combine the acted-on element, using `cons', with the results of acting on the rest. Here is example: (defun square-each (numbers-list) "Square each of a NUMBERS LIST, recursively." (if (not numbers-list) ; do-again-test nil (cons (* (car numbers-list) (car numbers-list)) (square-each (cdr numbers-list))))) ; next-step-expression (square-each '(1 2 3)) => (1 4 9) If `numbers-list' is empty, do nothing. But if it has content, construct a list combining the square of the first number in the list with the result of the recursive call. (The example follows the pattern exactly: `nil' is returned if the numbers' list is empty. In practice, you would write the conditional so it carries out the action when the numbers' list is not empty.) The `print-elements-recursively' function (*note Recursion with a List: Recursion with list.) is another example of an `every' pattern, except in this case, rather than bring the results together using `cons', we print each element of output. The `print-elements-recursively' function looks like this: (setq animals '(gazelle giraffe lion tiger)) (defun print-elements-recursively (list) "Print each element of LIST on a line of its own. Uses recursion." (when list ; do-again-test (print (car list)) ; body (print-elements-recursively ; recursive call (cdr list)))) ; next-step-expression (print-elements-recursively animals) The pattern for `print-elements-recursively' is: * When the list is empty, do nothing. * But when the list has at least one element, - act on the beginning of the list (the CAR of the list), - and make a recursive call on the rest (the CDR) of the list.  File: eintr, Node: Accumulate, Next: Keep, Prev: Every, Up: Recursive Patterns Recursive Pattern: _accumulate_ ............................... Another recursive pattern is called the `accumulate' pattern. In the `accumulate' recursive pattern, an action is performed on every element of a list and the result of that action is accumulated with the results of performing the action on the other elements. This is very like the `every' pattern using `cons', except that `cons' is not used, but some other combiner. The pattern is: * If a list be empty, return zero or some other constant. * Else, act on the beginning of the list (the CAR of the list), - and combine that acted-on element, using `+' or some other combining function, with - a recursive call by the function on the rest (the CDR) of the list. Here is an example: (defun add-elements (numbers-list) "Add the elements of NUMBERS-LIST together." (if (not numbers-list) 0 (+ (car numbers-list) (add-elements (cdr numbers-list))))) (add-elements '(1 2 3 4)) => 10 *Note Making a List of Files: Files List, for an example of the accumulate pattern.  File: eintr, Node: Keep, Prev: Accumulate, Up: Recursive Patterns Recursive Pattern: _keep_ ......................... A third recursive pattern is called the `keep' pattern. In the `keep' recursive pattern, each element of a list is tested; the element is acted on and the results are kept only if the element meets a criterion. Again, this is very like the `every' pattern, except the element is skipped unless it meets a criterion. The pattern has three parts: * If a list be empty, return `nil'. * Else, if the beginning of the list (the CAR of the list) passes a test - act on that element and combine it, using `cons' with - a recursive call by the function on the rest (the CDR) of the list. * Otherwise, if the beginning of the list (the CAR of the list) fails the test - skip on that element, - and, recursively call the function on the rest (the CDR) of the list. Here is an example that uses `cond': (defun keep-three-letter-words (word-list) "Keep three letter words in WORD-LIST." (cond ;; First do-again-test: stop-condition ((not word-list) nil) ;; Second do-again-test: when to act ((eq 3 (length (symbol-name (car word-list)))) ;; combine acted-on element with recursive call on shorter list (cons (car word-list) (keep-three-letter-words (cdr word-list)))) ;; Third do-again-test: when to skip element; ;; recursively call shorter list with next-step expression (t (keep-three-letter-words (cdr word-list))))) (keep-three-letter-words '(one two three four five six)) => (one two six) It goes without saying that you need not use `nil' as the test for when to stop; and you can, of course, combine these patterns.  File: eintr, Node: No Deferment, Next: No deferment solution, Prev: Recursive Patterns, Up: Recursion 11.3.7 Recursion without Deferments ----------------------------------- Let's consider again what happens with the `triangle-recursively' function. We will find that the intermediate calculations are deferred until all can be done. Here is the function definition: (defun triangle-recursively (number) "Return the sum of the numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) ; do-again-test 1 ; then-part (+ number ; else-part (triangle-recursively ; recursive call (1- number))))) ; next-step-expression What happens when we call this function with a argument of 7? The first instance of the `triangle-recursively' function adds the number 7 to the value returned by a second instance of `triangle-recursively', an instance that has been passed an argument of 6. That is to say, the first calculation is: (+ 7 (triangle-recursively 6)) The first instance of `triangle-recursively'--you may want to think of it as a little robot--cannot complete its job. It must hand off the calculation for `(triangle-recursively 6)' to a second instance of the program, to a second robot. This second individual is completely different from the first one; it is, in the jargon, a `different instantiation'. Or, put another way, it is a different robot. It is the same model as the first; it calculates triangle numbers recursively; but it has a different serial number. And what does `(triangle-recursively 6)' return? It returns the number 6 added to the value returned by evaluating `triangle-recursively' with an argument of 5. Using the robot metaphor, it asks yet another robot to help it. Now the total is: (+ 7 6 (triangle-recursively 5)) And what happens next? (+ 7 6 5 (triangle-recursively 4)) Each time `triangle-recursively' is called, except for the last time, it creates another instance of the program--another robot--and asks it to make a calculation. Eventually, the full addition is set up and performed: (+ 7 6 5 4 3 2 1) This design for the function defers the calculation of the first step until the second can be done, and defers that until the third can be done, and so on. Each deferment means the computer must remember what is being waited on. This is not a problem when there are only a few steps, as in this example. But it can be a problem when there are more steps.  File: eintr, Node: No deferment solution, Prev: No Deferment, Up: Recursion 11.3.8 No Deferment Solution ---------------------------- The solution to the problem of deferred operations is to write in a manner that does not defer operations(1). This requires writing to a different pattern, often one that involves writing two function definitions, an `initialization' function and a `helper' function. The `initialization' function sets up the job; the `helper' function does the work. Here are the two function definitions for adding up numbers. They are so simple, I find them hard to understand. (defun triangle-initialization (number) "Return the sum of the numbers 1 through NUMBER inclusive. This is the `initialization' component of a two function duo that uses recursion." (triangle-recursive-helper 0 0 number)) (defun triangle-recursive-helper (sum counter number) "Return SUM, using COUNTER, through NUMBER inclusive. This is the `helper' component of a two function duo that uses recursion." (if (> counter number) sum (triangle-recursive-helper (+ sum counter) ; sum (1+ counter) ; counter number))) ; number Install both function definitions by evaluating them, then call `triangle-initialization' with 2 rows: (triangle-initialization 2) => 3 The `initialization' function calls the first instance of the `helper' function with three arguments: zero, zero, and a number which is the number of rows in the triangle. The first two arguments passed to the `helper' function are initialization values. These values are changed when `triangle-recursive-helper' invokes new instances.(2) Let's see what happens when we have a triangle that has one row. (This triangle will have one pebble in it!) `triangle-initialization' will call its helper with the arguments `0 0 1'. That function will run the conditional test whether `(> counter number)': (> 0 1) and find that the result is false, so it will invoke the else-part of the `if' clause: (triangle-recursive-helper (+ sum counter) ; sum plus counter => sum (1+ counter) ; increment counter => counter number) ; number stays the same which will first compute: (triangle-recursive-helper (+ 0 0) ; sum (1+ 0) ; counter 1) ; number which is: (triangle-recursive-helper 0 1 1) Again, `(> counter number)' will be false, so again, the Lisp interpreter will evaluate `triangle-recursive-helper', creating a new instance with new arguments. This new instance will be; (triangle-recursive-helper (+ sum counter) ; sum plus counter => sum (1+ counter) ; increment counter => counter number) ; number stays the same which is: (triangle-recursive-helper 1 2 1) In this case, the `(> counter number)' test will be true! So the instance will return the value of the sum, which will be 1, as expected. Now, let's pass `triangle-initialization' an argument of 2, to find out how many pebbles there are in a triangle with two rows. That function calls `(triangle-recursive-helper 0 0 2)'. In stages, the instances called will be: sum counter number (triangle-recursive-helper 0 1 2) (triangle-recursive-helper 1 2 2) (triangle-recursive-helper 3 3 2) When the last instance is called, the `(> counter number)' test will be true, so the instance will return the value of `sum', which will be 3. This kind of pattern helps when you are writing functions that can use many resources in a computer. ---------- Footnotes ---------- (1) The phrase "tail recursive" is used to describe such a process, one that uses `constant space'. (2) The jargon is mildly confusing: `triangle-recursive-helper' uses a process that is iterative in a procedure that is recursive. The process is called iterative because the computer need only record the three values, `sum', `counter', and `number'; the procedure is recursive because the function `calls itself'. On the other hand, both the process and the procedure used by `triangle-recursively' are called recursive. The word `recursive' has different meanings in the two contexts.  File: eintr, Node: Looping exercise, Prev: Recursion, Up: Loops & Recursion 11.4 Looping Exercise ===================== * Write a function similar to `triangle' in which each row has a value which is the square of the row number. Use a `while' loop. * Write a function similar to `triangle' that multiplies instead of adds the values. * Rewrite these two functions recursively. Rewrite these functions using `cond'. * Write a function for Texinfo mode that creates an index entry at the beginning of a paragraph for every `@dfn' within the paragraph. (In a Texinfo file, `@dfn' marks a definition. This book is written in Texinfo.) Many of the functions you will need are described in two of the previous chapters, *note Cutting and Storing Text: Cutting & Storing Text, and *note Yanking Text Back: Yanking. If you use `forward-paragraph' to put the index entry at the beginning of the paragraph, you will have to use `C-h f' (`describe-function') to find out how to make the command go backwards. For more information, see *note Indicating Definitions: (texinfo)Indicating.  File: eintr, Node: Regexp Search, Next: Counting Words, Prev: Loops & Recursion, Up: Top 12 Regular Expression Searches ****************************** Regular expression searches are used extensively in GNU Emacs. The two functions, `forward-sentence' and `forward-paragraph', illustrate these searches well. They use regular expressions to find where to move point. The phrase `regular expression' is often written as `regexp'. Regular expression searches are described in *note Regular Expression Search: (emacs)Regexp Search, as well as in *note Regular Expressions: (elisp)Regular Expressions. In writing this chapter, I am presuming that you have at least a mild acquaintance with them. The major point to remember is that regular expressions permit you to search for patterns as well as for literal strings of characters. For example, the code in `forward-sentence' searches for the pattern of possible characters that could mark the end of a sentence, and moves point to that spot. Before looking at the code for the `forward-sentence' function, it is worth considering what the pattern that marks the end of a sentence must be. The pattern is discussed in the next section; following that is a description of the regular expression search function, `re-search-forward'. The `forward-sentence' function is described in the section following. Finally, the `forward-paragraph' function is described in the last section of this chapter. `forward-paragraph' is a complex function that introduces several new features. * Menu: * sentence-end:: The regular expression for `sentence-end'. * re-search-forward:: Very similar to `search-forward'. * forward-sentence:: A straightforward example of regexp search. * forward-paragraph:: A somewhat complex example. * etags:: How to create your own `TAGS' table. * Regexp Review:: * re-search Exercises::  File: eintr, Node: sentence-end, Next: re-search-forward, Prev: Regexp Search, Up: Regexp Search 12.1 The Regular Expression for `sentence-end' ============================================== The symbol `sentence-end' is bound to the pattern that marks the end of a sentence. What should this regular expression be? Clearly, a sentence may be ended by a period, a question mark, or an exclamation mark. Indeed, in English, only clauses that end with one of those three characters should be considered the end of a sentence. This means that the pattern should include the character set: [.?!] However, we do not want `forward-sentence' merely to jump to a period, a question mark, or an exclamation mark, because such a character might be used in the middle of a sentence. A period, for example, is used after abbreviations. So other information is needed. According to convention, you type two spaces after every sentence, but only one space after a period, a question mark, or an exclamation mark in the body of a sentence. So a period, a question mark, or an exclamation mark followed by two spaces is a good indicator of an end of sentence. However, in a file, the two spaces may instead be a tab or the end of a line. This means that the regular expression should include these three items as alternatives. This group of alternatives will look like this: \\($\\| \\| \\) ^ ^^ TAB SPC Here, `$' indicates the end of the line, and I have pointed out where the tab and two spaces are inserted in the expression. Both are inserted by putting the actual characters into the expression. Two backslashes, `\\', are required before the parentheses and vertical bars: the first backslash quotes the following backslash in Emacs; and the second indicates that the following character, the parenthesis or the vertical bar, is special. Also, a sentence may be followed by one or more carriage returns, like this: [ ]* Like tabs and spaces, a carriage return is inserted into a regular expression by inserting it literally. The asterisk indicates that the is repeated zero or more times. But a sentence end does not consist only of a period, a question mark or an exclamation mark followed by appropriate space: a closing quotation mark or a closing brace of some kind may precede the space. Indeed more than one such mark or brace may precede the space. These require a expression that looks like this: []\"')}]* In this expression, the first `]' is the first character in the expression; the second character is `"', which is preceded by a `\' to tell Emacs the `"' is _not_ special. The last three characters are `'', `)', and `}'. All this suggests what the regular expression pattern for matching the end of a sentence should be; and, indeed, if we evaluate `sentence-end' we find that it returns the following value: sentence-end => "[.?!][]\"')}]*\\($\\| \\| \\)[ ]*" (Well, not in GNU Emacs 22; that is because of an effort to make the process simpler and to handle more glyphs and languages. When the value of `sentence-end' is `nil', then use the value defined by the function `sentence-end'. (Here is a use of the difference between a value and a function in Emacs Lisp.) The function returns a value constructed from the variables `sentence-end-base', `sentence-end-double-space', `sentence-end-without-period', and `sentence-end-without-space'. The critical variable is `sentence-end-base'; its global value is similar to the one described above but it also contains two additional quotation marks. These have differing degrees of curliness. The `sentence-end-without-period' variable, when true, tells Emacs that a sentence may end without a period, such as text in Thai.)  File: eintr, Node: re-search-forward, Next: forward-sentence, Prev: sentence-end, Up: Regexp Search 12.2 The `re-search-forward' Function ===================================== The `re-search-forward' function is very like the `search-forward' function. (*Note The `search-forward' Function: search-forward.) `re-search-forward' searches for a regular expression. If the search is successful, it leaves point immediately after the last character in the target. If the search is backwards, it leaves point just before the first character in the target. You may tell `re-search-forward' to return `t' for true. (Moving point is therefore a `side effect'.) Like `search-forward', the `re-search-forward' function takes four arguments: 1. The first argument is the regular expression that the function searches for. The regular expression will be a string between quotations marks. 2. The optional second argument limits how far the function will search; it is a bound, which is specified as a position in the buffer. 3. The optional third argument specifies how the function responds to failure: `nil' as the third argument causes the function to signal an error (and print a message) when the search fails; any other value causes it to return `nil' if the search fails and `t' if the search succeeds. 4. The optional fourth argument is the repeat count. A negative repeat count causes `re-search-forward' to search backwards. The template for `re-search-forward' looks like this: (re-search-forward "REGULAR-EXPRESSION" LIMIT-OF-SEARCH WHAT-TO-DO-IF-SEARCH-FAILS REPEAT-COUNT) The second, third, and fourth arguments are optional. However, if you want to pass a value to either or both of the last two arguments, you must also pass a value to all the preceding arguments. Otherwise, the Lisp interpreter will mistake which argument you are passing the value to. In the `forward-sentence' function, the regular expression will be the value of the variable `sentence-end'. In simple form, that is: "[.?!][]\"')}]*\\($\\| \\| \\)[ ]*" The limit of the search will be the end of the paragraph (since a sentence cannot go beyond a paragraph). If the search fails, the function will return `nil'; and the repeat count will be provided by the argument to the `forward-sentence' function.  File: eintr, Node: forward-sentence, Next: forward-paragraph, Prev: re-search-forward, Up: Regexp Search 12.3 `forward-sentence' ======================= The command to move the cursor forward a sentence is a straightforward illustration of how to use regular expression searches in Emacs Lisp. Indeed, the function looks longer and more complicated than it is; this is because the function is designed to go backwards as well as forwards; and, optionally, over more than one sentence. The function is usually bound to the key command `M-e'. * Menu: * Complete forward-sentence:: * fwd-sentence while loops:: Two `while' loops. * fwd-sentence re-search:: A regular expression search.  File: eintr, Node: Complete forward-sentence, Next: fwd-sentence while loops, Prev: forward-sentence, Up: forward-sentence Complete `forward-sentence' function definition ----------------------------------------------- Here is the code for `forward-sentence': (defun forward-sentence (&optional arg) "Move forward to next `sentence-end'. With argument, repeat. With negative argument, move backward repeatedly to `sentence-beginning'. The variable `sentence-end' is a regular expression that matches ends of sentences. Also, every paragraph boundary terminates sentences as well." (interactive "p") (or arg (setq arg 1)) (let ((opoint (point)) (sentence-end (sentence-end))) (while (< arg 0) (let ((pos (point)) (par-beg (save-excursion (start-of-paragraph-text) (point)))) (if (and (re-search-backward sentence-end par-beg t) (or (< (match-end 0) pos) (re-search-backward sentence-end par-beg t))) (goto-char (match-end 0)) (goto-char par-beg))) (setq arg (1+ arg))) (while (> arg 0) (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) (if (re-search-forward sentence-end par-end t) (skip-chars-backward " \t\n") (goto-char par-end))) (setq arg (1- arg))) (constrain-to-field nil opoint t))) The function looks long at first sight and it is best to look at its skeleton first, and then its muscle. The way to see the skeleton is to look at the expressions that start in the left-most columns: (defun forward-sentence (&optional arg) "DOCUMENTATION..." (interactive "p") (or arg (setq arg 1)) (let ((opoint (point)) (sentence-end (sentence-end))) (while (< arg 0) (let ((pos (point)) (par-beg (save-excursion (start-of-paragraph-text) (point)))) REST-OF-BODY-OF-WHILE-LOOP-WHEN-GOING-BACKWARDS (while (> arg 0) (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) REST-OF-BODY-OF-WHILE-LOOP-WHEN-GOING-FORWARDS HANDLE-FORMS-AND-EQUIVALENT This looks much simpler! The function definition consists of documentation, an `interactive' expression, an `or' expression, a `let' expression, and `while' loops. Let's look at each of these parts in turn. We note that the documentation is thorough and understandable. The function has an `interactive "p"' declaration. This means that the processed prefix argument, if any, is passed to the function as its argument. (This will be a number.) If the function is not passed an argument (it is optional) then the argument `arg' will be bound to 1. When `forward-sentence' is called non-interactively without an argument, `arg' is bound to `nil'. The `or' expression handles this. What it does is either leave the value of `arg' as it is, but only if `arg' is bound to a value; or it sets the value of `arg' to 1, in the case when `arg' is bound to `nil'. Next is a `let'. That specifies the values of two local variables, `point' and `sentence-end'. The local value of point, from before the search, is used in the `constrain-to-field' function which handles forms and equivalents. The `sentence-end' variable is set by the `sentence-end' function.  File: eintr, Node: fwd-sentence while loops, Next: fwd-sentence re-search, Prev: Complete forward-sentence, Up: forward-sentence The `while' loops ----------------- Two `while' loops follow. The first `while' has a true-or-false-test that tests true if the prefix argument for `forward-sentence' is a negative number. This is for going backwards. The body of this loop is similar to the body of the second `while' clause, but it is not exactly the same. We will skip this `while' loop and concentrate on the second `while' loop. The second `while' loop is for moving point forward. Its skeleton looks like this: (while (> arg 0) ; true-or-false-test (let VARLIST (if (TRUE-OR-FALSE-TEST) THEN-PART ELSE-PART (setq arg (1- arg)))) ; `while' loop decrementer The `while' loop is of the decrementing kind. (*Note A Loop with a Decrementing Counter: Decrementing Loop.) It has a true-or-false-test that tests true so long as the counter (in this case, the variable `arg') is greater than zero; and it has a decrementer that subtracts 1 from the value of the counter every time the loop repeats. If no prefix argument is given to `forward-sentence', which is the most common way the command is used, this `while' loop will run once, since the value of `arg' will be 1. The body of the `while' loop consists of a `let' expression, which creates and binds a local variable, and has, as its body, an `if' expression. The body of the `while' loop looks like this: (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) (if (re-search-forward sentence-end par-end t) (skip-chars-backward " \t\n") (goto-char par-end))) The `let' expression creates and binds the local variable `par-end'. As we shall see, this local variable is designed to provide a bound or limit to the regular expression search. If the search fails to find a proper sentence ending in the paragraph, it will stop on reaching the end of the paragraph. But first, let us examine how `par-end' is bound to the value of the end of the paragraph. What happens is that the `let' sets the value of `par-end' to the value returned when the Lisp interpreter evaluates the expression (save-excursion (end-of-paragraph-text) (point)) In this expression, `(end-of-paragraph-text)' moves point to the end of the paragraph, `(point)' returns the value of point, and then `save-excursion' restores point to its original position. Thus, the `let' binds `par-end' to the value returned by the `save-excursion' expression, which is the position of the end of the paragraph. (The `end-of-paragraph-text' function uses `forward-paragraph', which we will discuss shortly.) Emacs next evaluates the body of the `let', which is an `if' expression that looks like this: (if (re-search-forward sentence-end par-end t) ; if-part (skip-chars-backward " \t\n") ; then-part (goto-char par-end))) ; else-part The `if' tests whether its first argument is true and if so, evaluates its then-part; otherwise, the Emacs Lisp interpreter evaluates the else-part. The true-or-false-test of the `if' expression is the regular expression search. It may seem odd to have what looks like the `real work' of the `forward-sentence' function buried here, but this is a common way this kind of operation is carried out in Lisp.  File: eintr, Node: fwd-sentence re-search, Prev: fwd-sentence while loops, Up: forward-sentence The regular expression search ----------------------------- The `re-search-forward' function searches for the end of the sentence, that is, for the pattern defined by the `sentence-end' regular expression. If the pattern is found--if the end of the sentence is found--then the `re-search-forward' function does two things: 1. The `re-search-forward' function carries out a side effect, which is to move point to the end of the occurrence found. 2. The `re-search-forward' function returns a value of true. This is the value received by the `if', and means that the search was successful. The side effect, the movement of point, is completed before the `if' function is handed the value returned by the successful conclusion of the search. When the `if' function receives the value of true from a successful call to `re-search-forward', the `if' evaluates the then-part, which is the expression `(skip-chars-backward " \t\n")'. This expression moves backwards over any blank spaces, tabs or carriage returns until a printed character is found and then leaves point after the character. Since point has already been moved to the end of the pattern that marks the end of the sentence, this action leaves point right after the closing printed character of the sentence, which is usually a period. On the other hand, if the `re-search-forward' function fails to find a pattern marking the end of the sentence, the function returns false. The false then causes the `if' to evaluate its third argument, which is `(goto-char par-end)': it moves point to the end of the paragraph. (And if the text is in a form or equivalent, and point may not move fully, then the `constrain-to-field' function comes into play.) Regular expression searches are exceptionally useful and the pattern illustrated by `re-search-forward', in which the search is the test of an `if' expression, is handy. You will see or write code incorporating this pattern often.  File: eintr, Node: forward-paragraph, Next: etags, Prev: forward-sentence, Up: Regexp Search 12.4 `forward-paragraph': a Goldmine of Functions ================================================= The `forward-paragraph' function moves point forward to the end of the paragraph. It is usually bound to `M-}' and makes use of a number of functions that are important in themselves, including `let*', `match-beginning', and `looking-at'. The function definition for `forward-paragraph' is considerably longer than the function definition for `forward-sentence' because it works with a paragraph, each line of which may begin with a fill prefix. A fill prefix consists of a string of characters that are repeated at the beginning of each line. For example, in Lisp code, it is a convention to start each line of a paragraph-long comment with `;;; '. In Text mode, four blank spaces make up another common fill prefix, creating an indented paragraph. (*Note Fill Prefix: (emacs)Fill Prefix, for more information about fill prefixes.) The existence of a fill prefix means that in addition to being able to find the end of a paragraph whose lines begin on the left-most column, the `forward-paragraph' function must be able to find the end of a paragraph when all or many of the lines in the buffer begin with the fill prefix. Moreover, it is sometimes practical to ignore a fill prefix that exists, especially when blank lines separate paragraphs. This is an added complication. * Menu: * forward-paragraph in brief:: Key parts of the function definition. * fwd-para let:: The `let*' expression. * fwd-para while:: The forward motion `while' loop.  File: eintr, Node: forward-paragraph in brief, Next: fwd-para let, Prev: forward-paragraph, Up: forward-paragraph Shortened `forward-paragraph' function definition ------------------------------------------------- Rather than print all of the `forward-paragraph' function, we will only print parts of it. Read without preparation, the function can be daunting! In outline, the function looks like this: (defun forward-paragraph (&optional arg) "DOCUMENTATION..." (interactive "p") (or arg (setq arg 1)) (let* VARLIST (while (and (< arg 0) (not (bobp))) ; backward-moving-code ... (while (and (> arg 0) (not (eobp))) ; forward-moving-code ... The first parts of the function are routine: the function's argument list consists of one optional argument. Documentation follows. The lower case `p' in the `interactive' declaration means that the processed prefix argument, if any, is passed to the function. This will be a number, and is the repeat count of how many paragraphs point will move. The `or' expression in the next line handles the common case when no argument is passed to the function, which occurs if the function is called from other code rather than interactively. This case was described earlier. (*Note The `forward-sentence' function: forward-sentence.) Now we reach the end of the familiar part of this function.  File: eintr, Node: fwd-para let, Next: fwd-para while, Prev: forward-paragraph in brief, Up: forward-paragraph The `let*' expression --------------------- The next line of the `forward-paragraph' function begins a `let*' expression. This is a different than `let'. The symbol is `let*' not `let'. The `let*' special form is like `let' except that Emacs sets each variable in sequence, one after another, and variables in the latter part of the varlist can make use of the values to which Emacs set variables in the earlier part of the varlist. (*note `save-excursion' in `append-to-buffer': append save-excursion.) In the `let*' expression in this function, Emacs binds a total of seven variables: `opoint', `fill-prefix-regexp', `parstart', `parsep', `sp-parstart', `start', and `found-start'. The variable `parsep' appears twice, first, to remove instances of `^', and second, to handle fill prefixes. The variable `opoint' is just the value of `point'. As you can guess, it is used in a `constrain-to-field' expression, just as in `forward-sentence'. The variable `fill-prefix-regexp' is set to the value returned by evaluating the following list: (and fill-prefix (not (equal fill-prefix "")) (not paragraph-ignore-fill-prefix) (regexp-quote fill-prefix)) This is an expression whose first element is the `and' special form. As we learned earlier (*note The `kill-new' function: kill-new function.), the `and' special form evaluates each of its arguments until one of the arguments returns a value of `nil', in which case the `and' expression returns `nil'; however, if none of the arguments returns a value of `nil', the value resulting from evaluating the last argument is returned. (Since such a value is not `nil', it is considered true in Lisp.) In other words, an `and' expression returns a true value only if all its arguments are true. In this case, the variable `fill-prefix-regexp' is bound to a non-`nil' value only if the following four expressions produce a true (i.e., a non-`nil') value when they are evaluated; otherwise, `fill-prefix-regexp' is bound to `nil'. `fill-prefix' When this variable is evaluated, the value of the fill prefix, if any, is returned. If there is no fill prefix, this variable returns `nil'. `(not (equal fill-prefix "")' This expression checks whether an existing fill prefix is an empty string, that is, a string with no characters in it. An empty string is not a useful fill prefix. `(not paragraph-ignore-fill-prefix)' This expression returns `nil' if the variable `paragraph-ignore-fill-prefix' has been turned on by being set to a true value such as `t'. `(regexp-quote fill-prefix)' This is the last argument to the `and' special form. If all the arguments to the `and' are true, the value resulting from evaluating this expression will be returned by the `and' expression and bound to the variable `fill-prefix-regexp', The result of evaluating this `and' expression successfully is that `fill-prefix-regexp' will be bound to the value of `fill-prefix' as modified by the `regexp-quote' function. What `regexp-quote' does is read a string and return a regular expression that will exactly match the string and match nothing else. This means that `fill-prefix-regexp' will be set to a value that will exactly match the fill prefix if the fill prefix exists. Otherwise, the variable will be set to `nil'. The next two local variables in the `let*' expression are designed to remove instances of `^' from `parstart' and `parsep', the local variables which indicate the paragraph start and the paragraph separator. The next expression sets `parsep' again. That is to handle fill prefixes. This is the setting that requires the definition call `let*' rather than `let'. The true-or-false-test for the `if' depends on whether the variable `fill-prefix-regexp' evaluates to `nil' or some other value. If `fill-prefix-regexp' does not have a value, Emacs evaluates the else-part of the `if' expression and binds `parsep' to its local value. (`parsep' is a regular expression that matches what separates paragraphs.) But if `fill-prefix-regexp' does have a value, Emacs evaluates the then-part of the `if' expression and binds `parsep' to a regular expression that includes the `fill-prefix-regexp' as part of the pattern. Specifically, `parsep' is set to the original value of the paragraph separate regular expression concatenated with an alternative expression that consists of the `fill-prefix-regexp' followed by optional whitespace to the end of the line. The whitespace is defined by `"[ \t]*$"'.) The `\\|' defines this portion of the regexp as an alternative to `parsep'. According to a comment in the code, the next local variable, `sp-parstart', is used for searching, and then the final two, `start' and `found-start', are set to `nil'. Now we get into the body of the `let*'. The first part of the body of the `let*' deals with the case when the function is given a negative argument and is therefore moving backwards. We will skip this section.  File: eintr, Node: fwd-para while, Prev: fwd-para let, Up: forward-paragraph The forward motion `while' loop ------------------------------- The second part of the body of the `let*' deals with forward motion. It is a `while' loop that repeats itself so long as the value of `arg' is greater than zero. In the most common use of the function, the value of the argument is 1, so the body of the `while' loop is evaluated exactly once, and the cursor moves forward one paragraph. This part handles three situations: when point is between paragraphs, when there is a fill prefix and when there is no fill prefix. The `while' loop looks like this: ;; going forwards and not at the end of the buffer (while (and (> arg 0) (not (eobp))) ;; between paragraphs ;; Move forward over separator lines... (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (looking-at parsep)) (forward-line 1)) ;; This decrements the loop (unless (eobp) (setq arg (1- arg))) ;; ... and one more line. (forward-line 1) (if fill-prefix-regexp ;; There is a fill prefix; it overrides parstart; ;; we go forward line by line (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (not (looking-at parsep)) (looking-at fill-prefix-regexp)) (forward-line 1)) ;; There is no fill prefix; ;; we go forward character by character (while (and (re-search-forward sp-parstart nil 1) (progn (setq start (match-beginning 0)) (goto-char start) (not (eobp))) (progn (move-to-left-margin) (not (looking-at parsep))) (or (not (looking-at parstart)) (and use-hard-newlines (not (get-text-property (1- start) 'hard))))) (forward-char 1)) ;; and if there is no fill prefix and if we are not at the end, ;; go to whatever was found in the regular expression search ;; for sp-parstart (if (< (point) (point-max)) (goto-char start)))) We can see that this is a decrementing counter `while' loop, using the expression `(setq arg (1- arg))' as the decrementer. That expression is not far from the `while', but is hidden in another Lisp macro, an `unless' macro. Unless we are at the end of the buffer -- that is what the `eobp' function determines; it is an abbreviation of `End Of Buffer P' -- we decrease the value of `arg' by one. (If we are at the end of the buffer, we cannot go forward any more and the next loop of the `while' expression will test false since the test is an `and' with `(not (eobp))'. The `not' function means exactly as you expect; it is another name for `null', a function that returns true when its argument is false.) Interestingly, the loop count is not decremented until we leave the space between paragraphs, unless we come to the end of buffer or stop seeing the local value of the paragraph separator. That second `while' also has a `(move-to-left-margin)' expression. The function is self-explanatory. It is inside a `progn' expression and not the last element of its body, so it is only invoked for its side effect, which is to move point to the left margin of the current line. The `looking-at' function is also self-explanatory; it returns true if the text after point matches the regular expression given as its argument. The rest of the body of the loop looks difficult at first, but makes sense as you come to understand it. First consider what happens if there is a fill prefix: (if fill-prefix-regexp ;; There is a fill prefix; it overrides parstart; ;; we go forward line by line (while (and (not (eobp)) (progn (move-to-left-margin) (not (eobp))) (not (looking-at parsep)) (looking-at fill-prefix-regexp)) (forward-line 1)) This expression moves point forward line by line so long as four conditions are true: 1. Point is not at the end of the buffer. 2. We can move to the left margin of the text and are not at the end of the buffer. 3. The text following point does not separate paragraphs. 4. The pattern following point is the fill prefix regular expression. The last condition may be puzzling, until you remember that point was moved to the beginning of the line early in the `forward-paragraph' function. This means that if the text has a fill prefix, the `looking-at' function will see it. Consider what happens when there is no fill prefix. (while (and (re-search-forward sp-parstart nil 1) (progn (setq start (match-beginning 0)) (goto-char start) (not (eobp))) (progn (move-to-left-margin) (not (looking-at parsep))) (or (not (looking-at parstart)) (and use-hard-newlines (not (get-text-property (1- start) 'hard))))) (forward-char 1)) This `while' loop has us searching forward for `sp-parstart', which is the combination of possible whitespace with a the local value of the start of a paragraph or of a paragraph separator. (The latter two are within an expression starting `\(?:' so that they are not referenced by the `match-beginning' function.) The two expressions, (setq start (match-beginning 0)) (goto-char start) mean go to the start of the text matched by the regular expression search. The `(match-beginning 0)' expression is new. It returns a number specifying the location of the start of the text that was matched by the last search. The `match-beginning' function is used here because of a characteristic of a forward search: a successful forward search, regardless of whether it is a plain search or a regular expression search, moves point to the end of the text that is found. In this case, a successful search moves point to the end of the pattern for `sp-parstart'. However, we want to put point at the end of the current paragraph, not somewhere else. Indeed, since the search possibly includes the paragraph separator, point may end up at the beginning of the next one unless we use an expression that includes `match-beginning'. When given an argument of 0, `match-beginning' returns the position that is the start of the text matched by the most recent search. In this case, the most recent search looks for `sp-parstart'. The `(match-beginning 0)' expression returns the beginning position of that pattern, rather than the end position of that pattern. (Incidentally, when passed a positive number as an argument, the `match-beginning' function returns the location of point at that parenthesized expression in the last search unless that parenthesized expression begins with `\(?:'. I don't know why `\(?:' appears here since the argument is 0.) The last expression when there is no fill prefix is (if (< (point) (point-max)) (goto-char start)))) This says that if there is no fill prefix and if we are not at the end, point should move to the beginning of whatever was found by the regular expression search for `sp-parstart'. The full definition for the `forward-paragraph' function not only includes code for going forwards, but also code for going backwards. If you are reading this inside of GNU Emacs and you want to see the whole function, you can type `C-h f' (`describe-function') and the name of the function. This gives you the function documentation and the name of the library containing the function's source. Place point over the name of the library and press the RET key; you will be taken directly to the source. (Be sure to install your sources! Without them, you are like a person who tries to drive a car with his eyes shut!)  File: eintr, Node: etags, Next: Regexp Review, Prev: forward-paragraph, Up: Regexp Search 12.5 Create Your Own `TAGS' File ================================ Besides `C-h f' (`describe-function'), another way to see the source of a function is to type `M-.' (`find-tag') and the name of the function when prompted for it. This is a good habit to get into. The `M-.' (`find-tag') command takes you directly to the source for a function, variable, or node. The function depends on tags tables to tell it where to go. If the `find-tag' function first asks you for the name of a `TAGS' table, give it the name of a `TAGS' file such as `/usr/local/src/emacs/src/TAGS'. (The exact path to your `TAGS' file depends on how your copy of Emacs was installed. I just told you the location that provides both my C and my Emacs Lisp sources.) You can also create your own `TAGS' file for directories that lack one. You often need to build and install tags tables yourself. They are not built automatically. A tags table is called a `TAGS' file; the name is in upper case letters. You can create a `TAGS' file by calling the `etags' program that comes as a part of the Emacs distribution. Usually, `etags' is compiled and installed when Emacs is built. (`etags' is not an Emacs Lisp function or a part of Emacs; it is a C program.) To create a `TAGS' file, first switch to the directory in which you want to create the file. In Emacs you can do this with the `M-x cd' command, or by visiting a file in the directory, or by listing the directory with `C-x d' (`dired'). Then run the compile command, with `etags *.el' as the command to execute M-x compile RET etags *.el RET to create a `TAGS' file for Emacs Lisp. For example, if you have a large number of files in your `~/emacs' directory, as I do--I have 137 `.el' files in it, of which I load 12--you can create a `TAGS' file for the Emacs Lisp files in that directory. The `etags' program takes all the usual shell `wildcards'. For example, if you have two directories for which you want a single `TAGS' file, type `etags *.el ../elisp/*.el', where `../elisp/' is the second directory: M-x compile RET etags *.el ../elisp/*.el RET Type M-x compile RET etags --help RET to see a list of the options accepted by `etags' as well as a list of supported languages. The `etags' program handles more than 20 languages, including Emacs Lisp, Common Lisp, Scheme, C, C++, Ada, Fortran, HTML, Java, LaTeX, Pascal, Perl, Postscript, Python, TeX, Texinfo, makefiles, and most assemblers. The program has no switches for specifying the language; it recognizes the language in an input file according to its file name and contents. `etags' is very helpful when you are writing code yourself and want to refer back to functions you have already written. Just run `etags' again at intervals as you write new functions, so they become part of the `TAGS' file. If you think an appropriate `TAGS' file already exists for what you want, but do not know where it is, you can use the `locate' program to attempt to find it. Type `M-x locate TAGS ' and Emacs will list for you the full path names of all your `TAGS' files. On my system, this command lists 34 `TAGS' files. On the other hand, a `plain vanilla' system I recently installed did not contain any `TAGS' files. If the tags table you want has been created, you can use the `M-x visit-tags-table' command to specify it. Otherwise, you will need to create the tag table yourself and then use `M-x visit-tags-table'. Building Tags in the Emacs sources .................................. The GNU Emacs sources come with a `Makefile' that contains a sophisticated `etags' command that creates, collects, and merges tags tables from all over the Emacs sources and puts the information into one `TAGS' file in the `src/' directory. (The `src/' directory is below the top level of your Emacs directory.) To build this `TAGS' file, go to the top level of your Emacs source directory and run the compile command `make tags': M-x compile RET make tags RET (The `make tags' command works well with the GNU Emacs sources, as well as with some other source packages.) For more information, see *note Tag Tables: (emacs)Tags.  File: eintr, Node: Regexp Review, Next: re-search Exercises, Prev: etags, Up: Regexp Search 12.6 Review =========== Here is a brief summary of some recently introduced functions. `while' Repeatedly evaluate the body of the expression so long as the first element of the body tests true. Then return `nil'. (The expression is evaluated only for its side effects.) For example: (let ((foo 2)) (while (> foo 0) (insert (format "foo is %d.\n" foo)) (setq foo (1- foo)))) => foo is 2. foo is 1. nil (The `insert' function inserts its arguments at point; the `format' function returns a string formatted from its arguments the way `message' formats its arguments; `\n' produces a new line.) `re-search-forward' Search for a pattern, and if the pattern is found, move point to rest just after it. Takes four arguments, like `search-forward': 1. A regular expression that specifies the pattern to search for. (Remember to put quotation marks around this argument!) 2. Optionally, the limit of the search. 3. Optionally, what to do if the search fails, return `nil' or an error message. 4. Optionally, how many times to repeat the search; if negative, the search goes backwards. `let*' Bind some variables locally to particular values, and then evaluate the remaining arguments, returning the value of the last one. While binding the local variables, use the local values of variables bound earlier, if any. For example: (let* ((foo 7) (bar (* 3 foo))) (message "`bar' is %d." bar)) => `bar' is 21. `match-beginning' Return the position of the start of the text found by the last regular expression search. `looking-at' Return `t' for true if the text after point matches the argument, which should be a regular expression. `eobp' Return `t' for true if point is at the end of the accessible part of a buffer. The end of the accessible part is the end of the buffer if the buffer is not narrowed; it is the end of the narrowed part if the buffer is narrowed.  File: eintr, Node: re-search Exercises, Prev: Regexp Review, Up: Regexp Search 12.7 Exercises with `re-search-forward' ======================================= * Write a function to search for a regular expression that matches two or more blank lines in sequence. * Write a function to search for duplicated words, such as `the the'. *Note Syntax of Regular Expressions: (emacs)Regexps, for information on how to write a regexp (a regular expression) to match a string that is composed of two identical halves. You can devise several regexps; some are better than others. The function I use is described in an appendix, along with several regexps. *Note `the-the' Duplicated Words Function: the-the.  File: eintr, Node: Counting Words, Next: Words in a defun, Prev: Regexp Search, Up: Top 13 Counting: Repetition and Regexps *********************************** Repetition and regular expression searches are powerful tools that you often use when you write code in Emacs Lisp. This chapter illustrates the use of regular expression searches through the construction of word count commands using `while' loops and recursion. * Menu: * Why Count Words:: * count-words-region:: Use a regexp, but find a problem. * recursive-count-words:: Start with case of no words in region. * Counting Exercise::  File: eintr, Node: Why Count Words, Next: count-words-region, Prev: Counting Words, Up: Counting Words Counting words ============== The standard Emacs distribution contains a function for counting the number of lines within a region. However, there is no corresponding function for counting words. Certain types of writing ask you to count words. Thus, if you write an essay, you may be limited to 800 words; if you write a novel, you may discipline yourself to write 1000 words a day. It seems odd to me that Emacs lacks a word count command. Perhaps people use Emacs mostly for code or types of documentation that do not require word counts; or perhaps they restrict themselves to the operating system word count command, `wc'. Alternatively, people may follow the publishers' convention and compute a word count by dividing the number of characters in a document by five. In any event, here are commands to count words.  File: eintr, Node: count-words-region, Next: recursive-count-words, Prev: Why Count Words, Up: Counting Words 13.1 The `count-words-region' Function ====================================== A word count command could count words in a line, paragraph, region, or buffer. What should the command cover? You could design the command to count the number of words in a complete buffer. However, the Emacs tradition encourages flexibility--you may want to count words in just a section, rather than all of a buffer. So it makes more sense to design the command to count the number of words in a region. Once you have a `count-words-region' command, you can, if you wish, count words in a whole buffer by marking it with `C-x h' (`mark-whole-buffer'). Clearly, counting words is a repetitive act: starting from the beginning of the region, you count the first word, then the second word, then the third word, and so on, until you reach the end of the region. This means that word counting is ideally suited to recursion or to a `while' loop. * Menu: * Design count-words-region:: The definition using a `while' loop. * Whitespace Bug:: The Whitespace Bug in `count-words-region'.  File: eintr, Node: Design count-words-region, Next: Whitespace Bug, Prev: count-words-region, Up: count-words-region Designing `count-words-region' ------------------------------ First, we will implement the word count command with a `while' loop, then with recursion. The command will, of course, be interactive. The template for an interactive function definition is, as always: (defun NAME-OF-FUNCTION (ARGUMENT-LIST) "DOCUMENTATION..." (INTERACTIVE-EXPRESSION...) BODY...) What we need to do is fill in the slots. The name of the function should be self-explanatory and similar to the existing `count-lines-region' name. This makes the name easier to remember. `count-words-region' is a good choice. The function counts words within a region. This means that the argument list must contain symbols that are bound to the two positions, the beginning and end of the region. These two positions can be called `beginning' and `end' respectively. The first line of the documentation should be a single sentence, since that is all that is printed as documentation by a command such as `apropos'. The interactive expression will be of the form `(interactive "r")', since that will cause Emacs to pass the beginning and end of the region to the function's argument list. All this is routine. The body of the function needs to be written to do three tasks: first, to set up conditions under which the `while' loop can count words, second, to run the `while' loop, and third, to send a message to the user. When a user calls `count-words-region', point may be at the beginning or the end of the region. However, the counting process must start at the beginning of the region. This means we will want to put point there if it is not already there. Executing `(goto-char beginning)' ensures this. Of course, we will want to return point to its expected position when the function finishes its work. For this reason, the body must be enclosed in a `save-excursion' expression. The central part of the body of the function consists of a `while' loop in which one expression jumps point forward word by word, and another expression counts those jumps. The true-or-false-test of the `while' loop should test true so long as point should jump forward, and false when point is at the end of the region. We could use `(forward-word 1)' as the expression for moving point forward word by word, but it is easier to see what Emacs identifies as a `word' if we use a regular expression search. A regular expression search that finds the pattern for which it is searching leaves point after the last character matched. This means that a succession of successful word searches will move point forward word by word. As a practical matter, we want the regular expression search to jump over whitespace and punctuation between words as well as over the words themselves. A regexp that refuses to jump over interword whitespace would never jump more than one word! This means that the regexp should include the whitespace and punctuation that follows a word, if any, as well as the word itself. (A word may end a buffer and not have any following whitespace or punctuation, so that part of the regexp must be optional.) Thus, what we want for the regexp is a pattern defining one or more word constituent characters followed, optionally, by one or more characters that are not word constituents. The regular expression for this is: \w+\W* The buffer's syntax table determines which characters are and are not word constituents. (*Note What Constitutes a Word or Symbol?: Syntax, for more about syntax. Also, see *note Syntax: (emacs)Syntax, and *note Syntax Tables: (elisp)Syntax Tables.) The search expression looks like this: (re-search-forward "\\w+\\W*") (Note that paired backslashes precede the `w' and `W'. A single backslash has special meaning to the Emacs Lisp interpreter. It indicates that the following character is interpreted differently than usual. For example, the two characters, `\n', stand for `newline', rather than for a backslash followed by `n'. Two backslashes in a row stand for an ordinary, `unspecial' backslash, so Emacs Lisp interpreter ends of seeing a single backslash followed by a letter. So it discovers the letter is special.) We need a counter to count how many words there are; this variable must first be set to 0 and then incremented each time Emacs goes around the `while' loop. The incrementing expression is simply: (setq count (1+ count)) Finally, we want to tell the user how many words there are in the region. The `message' function is intended for presenting this kind of information to the user. The message has to be phrased so that it reads properly regardless of how many words there are in the region: we don't want to say that "there are 1 words in the region". The conflict between singular and plural is ungrammatical. We can solve this problem by using a conditional expression that evaluates different messages depending on the number of words in the region. There are three possibilities: no words in the region, one word in the region, and more than one word. This means that the `cond' special form is appropriate. All this leads to the following function definition: ;;; First version; has bugs! (defun count-words-region (beginning end) "Print number of words in the region. Words are defined as at least one word-constituent character followed by at least one character that is not a word-constituent. The buffer's syntax table determines which characters these are." (interactive "r") (message "Counting words in region ... ") ;;; 1. Set up appropriate conditions. (save-excursion (goto-char beginning) (let ((count 0)) ;;; 2. Run the while loop. (while (< (point) end) (re-search-forward "\\w+\\W*") (setq count (1+ count))) ;;; 3. Send a message to the user. (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count)))))) As written, the function works, but not in all circumstances.  File: eintr, Node: Whitespace Bug, Prev: Design count-words-region, Up: count-words-region 13.1.1 The Whitespace Bug in `count-words-region' ------------------------------------------------- The `count-words-region' command described in the preceding section has two bugs, or rather, one bug with two manifestations. First, if you mark a region containing only whitespace in the middle of some text, the `count-words-region' command tells you that the region contains one word! Second, if you mark a region containing only whitespace at the end of the buffer or the accessible portion of a narrowed buffer, the command displays an error message that looks like this: Search failed: "\\w+\\W*" If you are reading this in Info in GNU Emacs, you can test for these bugs yourself. First, evaluate the function in the usual manner to install it. Here is a copy of the definition. Place your cursor after the closing parenthesis and type `C-x C-e' to install it. ;; First version; has bugs! (defun count-words-region (beginning end) "Print number of words in the region. Words are defined as at least one word-constituent character followed by at least one character that is not a word-constituent. The buffer's syntax table determines which characters these are." (interactive "r") (message "Counting words in region ... ") ;;; 1. Set up appropriate conditions. (save-excursion (goto-char beginning) (let ((count 0)) ;;; 2. Run the while loop. (while (< (point) end) (re-search-forward "\\w+\\W*") (setq count (1+ count))) ;;; 3. Send a message to the user. (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count)))))) If you wish, you can also install this keybinding by evaluating it: (global-set-key "\C-c=" 'count-words-region) To conduct the first test, set mark and point to the beginning and end of the following line and then type `C-c =' (or `M-x count-words-region' if you have not bound `C-c ='): one two three Emacs will tell you, correctly, that the region has three words. Repeat the test, but place mark at the beginning of the line and place point just _before_ the word `one'. Again type the command `C-c =' (or `M-x count-words-region'). Emacs should tell you that the region has no words, since it is composed only of the whitespace at the beginning of the line. But instead Emacs tells you that the region has one word! For the third test, copy the sample line to the end of the `*scratch*' buffer and then type several spaces at the end of the line. Place mark right after the word `three' and point at the end of line. (The end of the line will be the end of the buffer.) Type `C-c =' (or `M-x count-words-region') as you did before. Again, Emacs should tell you that the region has no words, since it is composed only of the whitespace at the end of the line. Instead, Emacs displays an error message saying `Search failed'. The two bugs stem from the same problem. Consider the first manifestation of the bug, in which the command tells you that the whitespace at the beginning of the line contains one word. What happens is this: The `M-x count-words-region' command moves point to the beginning of the region. The `while' tests whether the value of point is smaller than the value of `end', which it is. Consequently, the regular expression search looks for and finds the first word. It leaves point after the word. `count' is set to one. The `while' loop repeats; but this time the value of point is larger than the value of `end', the loop is exited; and the function displays a message saying the number of words in the region is one. In brief, the regular expression search looks for and finds the word even though it is outside the marked region. In the second manifestation of the bug, the region is whitespace at the end of the buffer. Emacs says `Search failed'. What happens is that the true-or-false-test in the `while' loop tests true, so the search expression is executed. But since there are no more words in the buffer, the search fails. In both manifestations of the bug, the search extends or attempts to extend outside of the region. The solution is to limit the search to the region--this is a fairly simple action, but as you may have come to expect, it is not quite as simple as you might think. As we have seen, the `re-search-forward' function takes a search pattern as its first argument. But in addition to this first, mandatory argument, it accepts three optional arguments. The optional second argument bounds the search. The optional third argument, if `t', causes the function to return `nil' rather than signal an error if the search fails. The optional fourth argument is a repeat count. (In Emacs, you can see a function's documentation by typing `C-h f', the name of the function, and then .) In the `count-words-region' definition, the value of the end of the region is held by the variable `end' which is passed as an argument to the function. Thus, we can add `end' as an argument to the regular expression search expression: (re-search-forward "\\w+\\W*" end) However, if you make only this change to the `count-words-region' definition and then test the new version of the definition on a stretch of whitespace, you will receive an error message saying `Search failed'. What happens is this: the search is limited to the region, and fails as you expect because there are no word-constituent characters in the region. Since it fails, we receive an error message. But we do not want to receive an error message in this case; we want to receive the message that "The region does NOT have any words." The solution to this problem is to provide `re-search-forward' with a third argument of `t', which causes the function to return `nil' rather than signal an error if the search fails. However, if you make this change and try it, you will see the message "Counting words in region ... " and ... you will keep on seeing that message ..., until you type `C-g' (`keyboard-quit'). Here is what happens: the search is limited to the region, as before, and it fails because there are no word-constituent characters in the region, as expected. Consequently, the `re-search-forward' expression returns `nil'. It does nothing else. In particular, it does not move point, which it does as a side effect if it finds the search target. After the `re-search-forward' expression returns `nil', the next expression in the `while' loop is evaluated. This expression increments the count. Then the loop repeats. The true-or-false-test tests true because the value of point is still less than the value of end, since the `re-search-forward' expression did not move point. ... and the cycle repeats ... The `count-words-region' definition requires yet another modification, to cause the true-or-false-test of the `while' loop to test false if the search fails. Put another way, there are two conditions that must be satisfied in the true-or-false-test before the word count variable is incremented: point must still be within the region and the search expression must have found a word to count. Since both the first condition and the second condition must be true together, the two expressions, the region test and the search expression, can be joined with an `and' special form and embedded in the `while' loop as the true-or-false-test, like this: (and (< (point) end) (re-search-forward "\\w+\\W*" end t)) (*Note The `kill-new' function: kill-new function, for information about `and'.) The `re-search-forward' expression returns `t' if the search succeeds and as a side effect moves point. Consequently, as words are found, point is moved through the region. When the search expression fails to find another word, or when point reaches the end of the region, the true-or-false-test tests false, the `while' loop exits, and the `count-words-region' function displays one or other of its messages. After incorporating these final changes, the `count-words-region' works without bugs (or at least, without bugs that I have found!). Here is what it looks like: ;;; Final version: `while' (defun count-words-region (beginning end) "Print number of words in the region." (interactive "r") (message "Counting words in region ... ") ;;; 1. Set up appropriate conditions. (save-excursion (let ((count 0)) (goto-char beginning) ;;; 2. Run the while loop. (while (and (< (point) end) (re-search-forward "\\w+\\W*" end t)) (setq count (1+ count))) ;;; 3. Send a message to the user. (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count))))))  File: eintr, Node: recursive-count-words, Next: Counting Exercise, Prev: count-words-region, Up: Counting Words 13.2 Count Words Recursively ============================ You can write the function for counting words recursively as well as with a `while' loop. Let's see how this is done. First, we need to recognize that the `count-words-region' function has three jobs: it sets up the appropriate conditions for counting to occur; it counts the words in the region; and it sends a message to the user telling how many words there are. If we write a single recursive function to do everything, we will receive a message for every recursive call. If the region contains 13 words, we will receive thirteen messages, one right after the other. We don't want this! Instead, we must write two functions to do the job, one of which (the recursive function) will be used inside of the other. One function will set up the conditions and display the message; the other will return the word count. Let us start with the function that causes the message to be displayed. We can continue to call this `count-words-region'. This is the function that the user will call. It will be interactive. Indeed, it will be similar to our previous versions of this function, except that it will call `recursive-count-words' to determine how many words are in the region. We can readily construct a template for this function, based on our previous versions: ;; Recursive version; uses regular expression search (defun count-words-region (beginning end) "DOCUMENTATION..." (INTERACTIVE-EXPRESSION...) ;;; 1. Set up appropriate conditions. (EXPLANATORY MESSAGE) (SET-UP FUNCTIONS... ;;; 2. Count the words. RECURSIVE CALL ;;; 3. Send a message to the user. MESSAGE PROVIDING WORD COUNT)) The definition looks straightforward, except that somehow the count returned by the recursive call must be passed to the message displaying the word count. A little thought suggests that this can be done by making use of a `let' expression: we can bind a variable in the varlist of a `let' expression to the number of words in the region, as returned by the recursive call; and then the `cond' expression, using binding, can display the value to the user. Often, one thinks of the binding within a `let' expression as somehow secondary to the `primary' work of a function. But in this case, what you might consider the `primary' job of the function, counting words, is done within the `let' expression. Using `let', the function definition looks like this: (defun count-words-region (beginning end) "Print number of words in the region." (interactive "r") ;;; 1. Set up appropriate conditions. (message "Counting words in region ... ") (save-excursion (goto-char beginning) ;;; 2. Count the words. (let ((count (recursive-count-words end))) ;;; 3. Send a message to the user. (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count)))))) Next, we need to write the recursive counting function. A recursive function has at least three parts: the `do-again-test', the `next-step-expression', and the recursive call. The do-again-test determines whether the function will or will not be called again. Since we are counting words in a region and can use a function that moves point forward for every word, the do-again-test can check whether point is still within the region. The do-again-test should find the value of point and determine whether point is before, at, or after the value of the end of the region. We can use the `point' function to locate point. Clearly, we must pass the value of the end of the region to the recursive counting function as an argument. In addition, the do-again-test should also test whether the search finds a word. If it does not, the function should not call itself again. The next-step-expression changes a value so that when the recursive function is supposed to stop calling itself, it stops. More precisely, the next-step-expression changes a value so that at the right time, the do-again-test stops the recursive function from calling itself again. In this case, the next-step-expression can be the expression that moves point forward, word by word. The third part of a recursive function is the recursive call. Somewhere, also, we also need a part that does the `work' of the function, a part that does the counting. A vital part! But already, we have an outline of the recursive counting function: (defun recursive-count-words (region-end) "DOCUMENTATION..." DO-AGAIN-TEST NEXT-STEP-EXPRESSION RECURSIVE CALL) Now we need to fill in the slots. Let's start with the simplest cases first: if point is at or beyond the end of the region, there cannot be any words in the region, so the function should return zero. Likewise, if the search fails, there are no words to count, so the function should return zero. On the other hand, if point is within the region and the search succeeds, the function should call itself again. Thus, the do-again-test should look like this: (and (< (point) region-end) (re-search-forward "\\w+\\W*" region-end t)) Note that the search expression is part of the do-again-test--the function returns `t' if its search succeeds and `nil' if it fails. (*Note The Whitespace Bug in `count-words-region': Whitespace Bug, for an explanation of how `re-search-forward' works.) The do-again-test is the true-or-false test of an `if' clause. Clearly, if the do-again-test succeeds, the then-part of the `if' clause should call the function again; but if it fails, the else-part should return zero since either point is outside the region or the search failed because there were no words to find. But before considering the recursive call, we need to consider the next-step-expression. What is it? Interestingly, it is the search part of the do-again-test. In addition to returning `t' or `nil' for the do-again-test, `re-search-forward' moves point forward as a side effect of a successful search. This is the action that changes the value of point so that the recursive function stops calling itself when point completes its movement through the region. Consequently, the `re-search-forward' expression is the next-step-expression. In outline, then, the body of the `recursive-count-words' function looks like this: (if DO-AGAIN-TEST-AND-NEXT-STEP-COMBINED ;; then RECURSIVE-CALL-RETURNING-COUNT ;; else RETURN-ZERO) How to incorporate the mechanism that counts? If you are not used to writing recursive functions, a question like this can be troublesome. But it can and should be approached systematically. We know that the counting mechanism should be associated in some way with the recursive call. Indeed, since the next-step-expression moves point forward by one word, and since a recursive call is made for each word, the counting mechanism must be an expression that adds one to the value returned by a call to `recursive-count-words'. Consider several cases: * If there are two words in the region, the function should return a value resulting from adding one to the value returned when it counts the first word, plus the number returned when it counts the remaining words in the region, which in this case is one. * If there is one word in the region, the function should return a value resulting from adding one to the value returned when it counts that word, plus the number returned when it counts the remaining words in the region, which in this case is zero. * If there are no words in the region, the function should return zero. From the sketch we can see that the else-part of the `if' returns zero for the case of no words. This means that the then-part of the `if' must return a value resulting from adding one to the value returned from a count of the remaining words. The expression will look like this, where `1+' is a function that adds one to its argument. (1+ (recursive-count-words region-end)) The whole `recursive-count-words' function will then look like this: (defun recursive-count-words (region-end) "DOCUMENTATION..." ;;; 1. do-again-test (if (and (< (point) region-end) (re-search-forward "\\w+\\W*" region-end t)) ;;; 2. then-part: the recursive call (1+ (recursive-count-words region-end)) ;;; 3. else-part 0)) Let's examine how this works: If there are no words in the region, the else part of the `if' expression is evaluated and consequently the function returns zero. If there is one word in the region, the value of point is less than the value of `region-end' and the search succeeds. In this case, the true-or-false-test of the `if' expression tests true, and the then-part of the `if' expression is evaluated. The counting expression is evaluated. This expression returns a value (which will be the value returned by the whole function) that is the sum of one added to the value returned by a recursive call. Meanwhile, the next-step-expression has caused point to jump over the first (and in this case only) word in the region. This means that when `(recursive-count-words region-end)' is evaluated a second time, as a result of the recursive call, the value of point will be equal to or greater than the value of region end. So this time, `recursive-count-words' will return zero. The zero will be added to one, and the original evaluation of `recursive-count-words' will return one plus zero, which is one, which is the correct amount. Clearly, if there are two words in the region, the first call to `recursive-count-words' returns one added to the value returned by calling `recursive-count-words' on a region containing the remaining word--that is, it adds one to one, producing two, which is the correct amount. Similarly, if there are three words in the region, the first call to `recursive-count-words' returns one added to the value returned by calling `recursive-count-words' on a region containing the remaining two words--and so on and so on. With full documentation the two functions look like this: The recursive function: (defun recursive-count-words (region-end) "Number of words between point and REGION-END." ;;; 1. do-again-test (if (and (< (point) region-end) (re-search-forward "\\w+\\W*" region-end t)) ;;; 2. then-part: the recursive call (1+ (recursive-count-words region-end)) ;;; 3. else-part 0)) The wrapper: ;;; Recursive version (defun count-words-region (beginning end) "Print number of words in the region. Words are defined as at least one word-constituent character followed by at least one character that is not a word-constituent. The buffer's syntax table determines which characters these are." (interactive "r") (message "Counting words in region ... ") (save-excursion (goto-char beginning) (let ((count (recursive-count-words end))) (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count))))))  File: eintr, Node: Counting Exercise, Prev: recursive-count-words, Up: Counting Words 13.3 Exercise: Counting Punctuation =================================== Using a `while' loop, write a function to count the number of punctuation marks in a region--period, comma, semicolon, colon, exclamation mark, and question mark. Do the same using recursion.  File: eintr, Node: Words in a defun, Next: Readying a Graph, Prev: Counting Words, Up: Top 14 Counting Words in a `defun' ****************************** Our next project is to count the number of words in a function definition. Clearly, this can be done using some variant of `count-word-region'. *Note Counting Words: Repetition and Regexps: Counting Words. If we are just going to count the words in one definition, it is easy enough to mark the definition with the `C-M-h' (`mark-defun') command, and then call `count-word-region'. However, I am more ambitious: I want to count the words and symbols in every definition in the Emacs sources and then print a graph that shows how many functions there are of each length: how many contain 40 to 49 words or symbols, how many contain 50 to 59 words or symbols, and so on. I have often been curious how long a typical function is, and this will tell. * Menu: * Divide and Conquer:: * Words and Symbols:: What to count? * Syntax:: What constitutes a word or symbol? * count-words-in-defun:: Very like `count-words'. * Several defuns:: Counting several defuns in a file. * Find a File:: Do you want to look at a file? * lengths-list-file:: A list of the lengths of many definitions. * Several files:: Counting in definitions in different files. * Several files recursively:: Recursively counting in different files. * Prepare the data:: Prepare the data for display in a graph.  File: eintr, Node: Divide and Conquer, Next: Words and Symbols, Prev: Words in a defun, Up: Words in a defun Divide and Conquer ================== Described in one phrase, the histogram project is daunting; but divided into numerous small steps, each of which we can take one at a time, the project becomes less fearsome. Let us consider what the steps must be: * First, write a function to count the words in one definition. This includes the problem of handling symbols as well as words. * Second, write a function to list the numbers of words in each function in a file. This function can use the `count-words-in-defun' function. * Third, write a function to list the numbers of words in each function in each of several files. This entails automatically finding the various files, switching to them, and counting the words in the definitions within them. * Fourth, write a function to convert the list of numbers that we created in step three to a form that will be suitable for printing as a graph. * Fifth, write a function to print the results as a graph. This is quite a project! But if we take each step slowly, it will not be difficult.  File: eintr, Node: Words and Symbols, Next: Syntax, Prev: Divide and Conquer, Up: Words in a defun 14.1 What to Count? =================== When we first start thinking about how to count the words in a function definition, the first question is (or ought to be) what are we going to count? When we speak of `words' with respect to a Lisp function definition, we are actually speaking, in large part, of `symbols'. For example, the following `multiply-by-seven' function contains the five symbols `defun', `multiply-by-seven', `number', `*', and `7'. In addition, in the documentation string, it contains the four words `Multiply', `NUMBER', `by', and `seven'. The symbol `number' is repeated, so the definition contains a total of ten words and symbols. (defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number)) However, if we mark the `multiply-by-seven' definition with `C-M-h' (`mark-defun'), and then call `count-words-region' on it, we will find that `count-words-region' claims the definition has eleven words, not ten! Something is wrong! The problem is twofold: `count-words-region' does not count the `*' as a word, and it counts the single symbol, `multiply-by-seven', as containing three words. The hyphens are treated as if they were interword spaces rather than intraword connectors: `multiply-by-seven' is counted as if it were written `multiply by seven'. The cause of this confusion is the regular expression search within the `count-words-region' definition that moves point forward word by word. In the canonical version of `count-words-region', the regexp is: "\\w+\\W*" This regular expression is a pattern defining one or more word constituent characters possibly followed by one or more characters that are not word constituents. What is meant by `word constituent characters' brings us to the issue of syntax, which is worth a section of its own.  File: eintr, Node: Syntax, Next: count-words-in-defun, Prev: Words and Symbols, Up: Words in a defun 14.2 What Constitutes a Word or Symbol? ======================================= Emacs treats different characters as belonging to different "syntax categories". For example, the regular expression, `\\w+', is a pattern specifying one or more _word constituent_ characters. Word constituent characters are members of one syntax category. Other syntax categories include the class of punctuation characters, such as the period and the comma, and the class of whitespace characters, such as the blank space and the tab character. (For more information, see *note Syntax: (emacs)Syntax, and *note Syntax Tables: (elisp)Syntax Tables.) Syntax tables specify which characters belong to which categories. Usually, a hyphen is not specified as a `word constituent character'. Instead, it is specified as being in the `class of characters that are part of symbol names but not words.' This means that the `count-words-region' function treats it in the same way it treats an interword white space, which is why `count-words-region' counts `multiply-by-seven' as three words. There are two ways to cause Emacs to count `multiply-by-seven' as one symbol: modify the syntax table or modify the regular expression. We could redefine a hyphen as a word constituent character by modifying the syntax table that Emacs keeps for each mode. This action would serve our purpose, except that a hyphen is merely the most common character within symbols that is not typically a word constituent character; there are others, too. Alternatively, we can redefine the regular expression used in the `count-words' definition so as to include symbols. This procedure has the merit of clarity, but the task is a little tricky. The first part is simple enough: the pattern must match "at least one character that is a word or symbol constituent". Thus: "\\(\\w\\|\\s_\\)+" The `\\(' is the first part of the grouping construct that includes the `\\w' and the `\\s_' as alternatives, separated by the `\\|'. The `\\w' matches any word-constituent character and the `\\s_' matches any character that is part of a symbol name but not a word-constituent character. The `+' following the group indicates that the word or symbol constituent characters must be matched at least once. However, the second part of the regexp is more difficult to design. What we want is to follow the first part with "optionally one or more characters that are not constituents of a word or symbol". At first, I thought I could define this with the following: "\\(\\W\\|\\S_\\)*" The upper case `W' and `S' match characters that are _not_ word or symbol constituents. Unfortunately, this expression matches any character that is either not a word constituent or not a symbol constituent. This matches any character! I then noticed that every word or symbol in my test region was followed by white space (blank space, tab, or newline). So I tried placing a pattern to match one or more blank spaces after the pattern for one or more word or symbol constituents. This failed, too. Words and symbols are often separated by whitespace, but in actual code parentheses may follow symbols and punctuation may follow words. So finally, I designed a pattern in which the word or symbol constituents are followed optionally by characters that are not white space and then followed optionally by white space. Here is the full regular expression: "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"  File: eintr, Node: count-words-in-defun, Next: Several defuns, Prev: Syntax, Up: Words in a defun 14.3 The `count-words-in-defun' Function ======================================== We have seen that there are several ways to write a `count-word-region' function. To write a `count-words-in-defun', we need merely adapt one of these versions. The version that uses a `while' loop is easy to understand, so I am going to adapt that. Because `count-words-in-defun' will be part of a more complex program, it need not be interactive and it need not display a message but just return the count. These considerations simplify the definition a little. On the other hand, `count-words-in-defun' will be used within a buffer that contains function definitions. Consequently, it is reasonable to ask that the function determine whether it is called when point is within a function definition, and if it is, to return the count for that definition. This adds complexity to the definition, but saves us from needing to pass arguments to the function. These considerations lead us to prepare the following template: (defun count-words-in-defun () "DOCUMENTATION..." (SET UP... (WHILE LOOP...) RETURN COUNT) As usual, our job is to fill in the slots. First, the set up. We are presuming that this function will be called within a buffer containing function definitions. Point will either be within a function definition or not. For `count-words-in-defun' to work, point must move to the beginning of the definition, a counter must start at zero, and the counting loop must stop when point reaches the end of the definition. The `beginning-of-defun' function searches backwards for an opening delimiter such as a `(' at the beginning of a line, and moves point to that position, or else to the limit of the search. In practice, this means that `beginning-of-defun' moves point to the beginning of an enclosing or preceding function definition, or else to the beginning of the buffer. We can use `beginning-of-defun' to place point where we wish to start. The `while' loop requires a counter to keep track of the words or symbols being counted. A `let' expression can be used to create a local variable for this purpose, and bind it to an initial value of zero. The `end-of-defun' function works like `beginning-of-defun' except that it moves point to the end of the definition. `end-of-defun' can be used as part of an expression that determines the position of the end of the definition. The set up for `count-words-in-defun' takes shape rapidly: first we move point to the beginning of the definition, then we create a local variable to hold the count, and finally, we record the position of the end of the definition so the `while' loop will know when to stop looping. The code looks like this: (beginning-of-defun) (let ((count 0) (end (save-excursion (end-of-defun) (point)))) The code is simple. The only slight complication is likely to concern `end': it is bound to the position of the end of the definition by a `save-excursion' expression that returns the value of point after `end-of-defun' temporarily moves it to the end of the definition. The second part of the `count-words-in-defun', after the set up, is the `while' loop. The loop must contain an expression that jumps point forward word by word and symbol by symbol, and another expression that counts the jumps. The true-or-false-test for the `while' loop should test true so long as point should jump forward, and false when point is at the end of the definition. We have already redefined the regular expression for this (*note Syntax::), so the loop is straightforward: (while (and (< (point) end) (re-search-forward "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t) (setq count (1+ count))) The third part of the function definition returns the count of words and symbols. This part is the last expression within the body of the `let' expression, and can be, very simply, the local variable `count', which when evaluated returns the count. Put together, the `count-words-in-defun' definition looks like this: (defun count-words-in-defun () "Return the number of words and symbols in a defun." (beginning-of-defun) (let ((count 0) (end (save-excursion (end-of-defun) (point)))) (while (and (< (point) end) (re-search-forward "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)) (setq count (1+ count))) count)) How to test this? The function is not interactive, but it is easy to put a wrapper around the function to make it interactive; we can use almost the same code as for the recursive version of `count-words-region': ;;; Interactive version. (defun count-words-defun () "Number of words and symbols in a function definition." (interactive) (message "Counting words and symbols in function definition ... ") (let ((count (count-words-in-defun))) (cond ((zerop count) (message "The definition does NOT have any words or symbols.")) ((= 1 count) (message "The definition has 1 word or symbol.")) (t (message "The definition has %d words or symbols." count))))) Let's re-use `C-c =' as a convenient keybinding: (global-set-key "\C-c=" 'count-words-defun) Now we can try out `count-words-defun': install both `count-words-in-defun' and `count-words-defun', and set the keybinding, and then place the cursor within the following definition: (defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number)) => 10 Success! The definition has 10 words and symbols. The next problem is to count the numbers of words and symbols in several definitions within a single file.  File: eintr, Node: Several defuns, Next: Find a File, Prev: count-words-in-defun, Up: Words in a defun 14.4 Count Several `defuns' Within a File ========================================= A file such as `simple.el' may have a hundred or more function definitions within it. Our long term goal is to collect statistics on many files, but as a first step, our immediate goal is to collect statistics on one file. The information will be a series of numbers, each number being the length of a function definition. We can store the numbers in a list. We know that we will want to incorporate the information regarding one file with information about many other files; this means that the function for counting definition lengths within one file need only return the list of lengths. It need not and should not display any messages. The word count commands contain one expression to jump point forward word by word and another expression to count the jumps. The function to return the lengths of definitions can be designed to work the same way, with one expression to jump point forward definition by definition and another expression to construct the lengths' list. This statement of the problem makes it elementary to write the function definition. Clearly, we will start the count at the beginning of the file, so the first command will be `(goto-char (point-min))'. Next, we start the `while' loop; and the true-or-false test of the loop can be a regular expression search for the next function definition--so long as the search succeeds, point is moved forward and then the body of the loop is evaluated. The body needs an expression that constructs the lengths' list. `cons', the list construction command, can be used to create the list. That is almost all there is to it. Here is what this fragment of code looks like: (goto-char (point-min)) (while (re-search-forward "^(defun" nil t) (setq lengths-list (cons (count-words-in-defun) lengths-list))) What we have left out is the mechanism for finding the file that contains the function definitions. In previous examples, we either used this, the Info file, or we switched back and forth to some other buffer, such as the `*scratch*' buffer. Finding a file is a new process that we have not yet discussed.  File: eintr, Node: Find a File, Next: lengths-list-file, Prev: Several defuns, Up: Words in a defun 14.5 Find a File ================ To find a file in Emacs, you use the `C-x C-f' (`find-file') command. This command is almost, but not quite right for the lengths problem. Let's look at the source for `find-file': (defun find-file (filename) "Edit file FILENAME. Switch to a buffer visiting file FILENAME, creating one if none already exists." (interactive "FFind file: ") (switch-to-buffer (find-file-noselect filename))) (The most recent version of the `find-file' function definition permits you to specify optional wildcards to visit multiple files; that makes the definition more complex and we will not discuss it here, since it is not relevant. You can see its source using either `M-.' (`find-tag') or `C-h f' (`describe-function').) The definition I am showing possesses short but complete documentation and an interactive specification that prompts you for a file name when you use the command interactively. The body of the definition contains two functions, `find-file-noselect' and `switch-to-buffer'. According to its documentation as shown by `C-h f' (the `describe-function' command), the `find-file-noselect' function reads the named file into a buffer and returns the buffer. (Its most recent version includes an optional wildcards argument, too, as well as another to read a file literally and an other you suppress warning messages. These optional arguments are irrelevant.) However, the `find-file-noselect' function does not select the buffer in which it puts the file. Emacs does not switch its attention (or yours if you are using `find-file-noselect') to the selected buffer. That is what `switch-to-buffer' does: it switches the buffer to which Emacs attention is directed; and it switches the buffer displayed in the window to the new buffer. We have discussed buffer switching elsewhere. (*Note Switching Buffers::.) In this histogram project, we do not need to display each file on the screen as the program determines the length of each definition within it. Instead of employing `switch-to-buffer', we can work with `set-buffer', which redirects the attention of the computer program to a different buffer but does not redisplay it on the screen. So instead of calling on `find-file' to do the job, we must write our own expression. The task is easy: use `find-file-noselect' and `set-buffer'.  File: eintr, Node: lengths-list-file, Next: Several files, Prev: Find a File, Up: Words in a defun 14.6 `lengths-list-file' in Detail ================================== The core of the `lengths-list-file' function is a `while' loop containing a function to move point forward `defun by defun' and a function to count the number of words and symbols in each defun. This core must be surrounded by functions that do various other tasks, including finding the file, and ensuring that point starts out at the beginning of the file. The function definition looks like this: (defun lengths-list-file (filename) "Return list of definitions' lengths within FILE. The returned list is a list of numbers. Each number is the number of words or symbols in one function definition." (message "Working on `%s' ... " filename) (save-excursion (let ((buffer (find-file-noselect filename)) (lengths-list)) (set-buffer buffer) (setq buffer-read-only t) (widen) (goto-char (point-min)) (while (re-search-forward "^(defun" nil t) (setq lengths-list (cons (count-words-in-defun) lengths-list))) (kill-buffer buffer) lengths-list))) The function is passed one argument, the name of the file on which it will work. It has four lines of documentation, but no interactive specification. Since people worry that a computer is broken if they don't see anything going on, the first line of the body is a message. The next line contains a `save-excursion' that returns Emacs' attention to the current buffer when the function completes. This is useful in case you embed this function in another function that presumes point is restored to the original buffer. In the varlist of the `let' expression, Emacs finds the file and binds the local variable `buffer' to the buffer containing the file. At the same time, Emacs creates `lengths-list' as a local variable. Next, Emacs switches its attention to the buffer. In the following line, Emacs makes the buffer read-only. Ideally, this line is not necessary. None of the functions for counting words and symbols in a function definition should change the buffer. Besides, the buffer is not going to be saved, even if it were changed. This line is entirely the consequence of great, perhaps excessive, caution. The reason for the caution is that this function and those it calls work on the sources for Emacs and it is inconvenient if they are inadvertently modified. It goes without saying that I did not realize a need for this line until an experiment went awry and started to modify my Emacs source files ... Next comes a call to widen the buffer if it is narrowed. This function is usually not needed--Emacs creates a fresh buffer if none already exists; but if a buffer visiting the file already exists Emacs returns that one. In this case, the buffer may be narrowed and must be widened. If we wanted to be fully `user-friendly', we would arrange to save the restriction and the location of point, but we won't. The `(goto-char (point-min))' expression moves point to the beginning of the buffer. Then comes a `while' loop in which the `work' of the function is carried out. In the loop, Emacs determines the length of each definition and constructs a lengths' list containing the information. Emacs kills the buffer after working through it. This is to save space inside of Emacs. My version of GNU Emacs 19 contained over 300 source files of interest; GNU Emacs 22 contains over a thousand source files. Another function will apply `lengths-list-file' to each of the files. Finally, the last expression within the `let' expression is the `lengths-list' variable; its value is returned as the value of the whole function. You can try this function by installing it in the usual fashion. Then place your cursor after the following expression and type `C-x C-e' (`eval-last-sexp'). (lengths-list-file "/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el") (You may need to change the pathname of the file; the one here is for GNU Emacs version 22.1.1. To change the expression, copy it to the `*scratch*' buffer and edit it. (Also, to see the full length of the list, rather than a truncated version, you may have to evaluate the following: (custom-set-variables '(eval-expression-print-length nil)) (*Note Specifying Variables using `defcustom': defcustom. Then evaluate the `lengths-list-file' expression.) The lengths' list for `debug.el' takes less than a second to produce and looks like this in GNU Emacs 22: (83 113 105 144 289 22 30 97 48 89 25 52 52 88 28 29 77 49 43 290 232 587) (Using my old machine, the version 19 lengths' list for `debug.el' took seven seconds to produce and looked like this: (75 41 80 62 20 45 44 68 45 12 34 235) (The newer version of `debug.el' contains more defuns than the earlier one; and my new machine is much faster than the old one.) Note that the length of the last definition in the file is first in the list.  File: eintr, Node: Several files, Next: Several files recursively, Prev: lengths-list-file, Up: Words in a defun 14.7 Count Words in `defuns' in Different Files =============================================== In the previous section, we created a function that returns a list of the lengths of each definition in a file. Now, we want to define a function to return a master list of the lengths of the definitions in a list of files. Working on each of a list of files is a repetitious act, so we can use either a `while' loop or recursion. * Menu: * lengths-list-many-files:: Return a list of the lengths of defuns. * append:: Attach one list to another.  File: eintr, Node: lengths-list-many-files, Next: append, Prev: Several files, Up: Several files Determine the lengths of `defuns' --------------------------------- The design using a `while' loop is routine. The argument passed the function is a list of files. As we saw earlier (*note Loop Example::), you can write a `while' loop so that the body of the loop is evaluated if such a list contains elements, but to exit the loop if the list is empty. For this design to work, the body of the loop must contain an expression that shortens the list each time the body is evaluated, so that eventually the list is empty. The usual technique is to set the value of the list to the value of the CDR of the list each time the body is evaluated. The template looks like this: (while TEST-WHETHER-LIST-IS-EMPTY BODY... SET-LIST-TO-CDR-OF-LIST) Also, we remember that a `while' loop returns `nil' (the result of evaluating the true-or-false-test), not the result of any evaluation within its body. (The evaluations within the body of the loop are done for their side effects.) However, the expression that sets the lengths' list is part of the body--and that is the value that we want returned by the function as a whole. To do this, we enclose the `while' loop within a `let' expression, and arrange that the last element of the `let' expression contains the value of the lengths' list. (*Note Loop Example with an Incrementing Counter: Incrementing Example.) These considerations lead us directly to the function itself: ;;; Use `while' loop. (defun lengths-list-many-files (list-of-files) "Return list of lengths of defuns in LIST-OF-FILES." (let (lengths-list) ;;; true-or-false-test (while list-of-files (setq lengths-list (append lengths-list ;;; Generate a lengths' list. (lengths-list-file (expand-file-name (car list-of-files))))) ;;; Make files' list shorter. (setq list-of-files (cdr list-of-files))) ;;; Return final value of lengths' list. lengths-list)) `expand-file-name' is a built-in function that converts a file name to the absolute, long, path name form. The function employs the name of the directory in which the function is called. Thus, if `expand-file-name' is called on `debug.el' when Emacs is visiting the `/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/' directory, debug.el becomes /usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el The only other new element of this function definition is the as yet unstudied function `append', which merits a short section for itself.  File: eintr, Node: append, Prev: lengths-list-many-files, Up: Several files 14.7.1 The `append' Function ---------------------------- The `append' function attaches one list to another. Thus, (append '(1 2 3 4) '(5 6 7 8)) produces the list (1 2 3 4 5 6 7 8) This is exactly how we want to attach two lengths' lists produced by `lengths-list-file' to each other. The results contrast with `cons', (cons '(1 2 3 4) '(5 6 7 8)) which constructs a new list in which the first argument to `cons' becomes the first element of the new list: ((1 2 3 4) 5 6 7 8)  File: eintr, Node: Several files recursively, Next: Prepare the data, Prev: Several files, Up: Words in a defun 14.8 Recursively Count Words in Different Files =============================================== Besides a `while' loop, you can work on each of a list of files with recursion. A recursive version of `lengths-list-many-files' is short and simple. The recursive function has the usual parts: the `do-again-test', the `next-step-expression', and the recursive call. The `do-again-test' determines whether the function should call itself again, which it will do if the `list-of-files' contains any remaining elements; the `next-step-expression' resets the `list-of-files' to the CDR of itself, so eventually the list will be empty; and the recursive call calls itself on the shorter list. The complete function is shorter than this description! (defun recursive-lengths-list-many-files (list-of-files) "Return list of lengths of each defun in LIST-OF-FILES." (if list-of-files ; do-again-test (append (lengths-list-file (expand-file-name (car list-of-files))) (recursive-lengths-list-many-files (cdr list-of-files))))) In a sentence, the function returns the lengths' list for the first of the `list-of-files' appended to the result of calling itself on the rest of the `list-of-files'. Here is a test of `recursive-lengths-list-many-files', along with the results of running `lengths-list-file' on each of the files individually. Install `recursive-lengths-list-many-files' and `lengths-list-file', if necessary, and then evaluate the following expressions. You may need to change the files' pathnames; those here work when this Info file and the Emacs sources are located in their customary places. To change the expressions, copy them to the `*scratch*' buffer, edit them, and then evaluate them. The results are shown after the `=>'. (These results are for files from Emacs version 22.1.1; files from other versions of Emacs may produce different results.) (cd "/usr/local/share/emacs/22.1.1/") (lengths-list-file "./lisp/macros.el") => (283 263 480 90) (lengths-list-file "./lisp/mail/mailalias.el") => (38 32 29 95 178 180 321 218 324) (lengths-list-file "./lisp/makesum.el") => (85 181) (recursive-lengths-list-many-files '("./lisp/macros.el" "./lisp/mail/mailalias.el" "./lisp/makesum.el")) => (283 263 480 90 38 32 29 95 178 180 321 218 324 85 181) The `recursive-lengths-list-many-files' function produces the output we want. The next step is to prepare the data in the list for display in a graph.  File: eintr, Node: Prepare the data, Prev: Several files recursively, Up: Words in a defun 14.9 Prepare the Data for Display in a Graph ============================================ The `recursive-lengths-list-many-files' function returns a list of numbers. Each number records the length of a function definition. What we need to do now is transform this data into a list of numbers suitable for generating a graph. The new list will tell how many functions definitions contain less than 10 words and symbols, how many contain between 10 and 19 words and symbols, how many contain between 20 and 29 words and symbols, and so on. In brief, we need to go through the lengths' list produced by the `recursive-lengths-list-many-files' function and count the number of defuns within each range of lengths, and produce a list of those numbers. * Menu: * Data for Display in Detail:: * Sorting:: Sorting lists. * Files List:: Making a list of files. * Counting function definitions::  File: eintr, Node: Data for Display in Detail, Next: Sorting, Prev: Prepare the data, Up: Prepare the data The Data for Display in Detail ------------------------------ Based on what we have done before, we can readily foresee that it should not be too hard to write a function that `CDRs' down the lengths' list, looks at each element, determines which length range it is in, and increments a counter for that range. However, before beginning to write such a function, we should consider the advantages of sorting the lengths' list first, so the numbers are ordered from smallest to largest. First, sorting will make it easier to count the numbers in each range, since two adjacent numbers will either be in the same length range or in adjacent ranges. Second, by inspecting a sorted list, we can discover the highest and lowest number, and thereby determine the largest and smallest length range that we will need.  File: eintr, Node: Sorting, Next: Files List, Prev: Data for Display in Detail, Up: Prepare the data 14.9.1 Sorting Lists -------------------- Emacs contains a function to sort lists, called (as you might guess) `sort'. The `sort' function takes two arguments, the list to be sorted, and a predicate that determines whether the first of two list elements is "less" than the second. As we saw earlier (*note Using the Wrong Type Object as an Argument: Wrong Type of Argument.), a predicate is a function that determines whether some property is true or false. The `sort' function will reorder a list according to whatever property the predicate uses; this means that `sort' can be used to sort non-numeric lists by non-numeric criteria--it can, for example, alphabetize a list. The `<' function is used when sorting a numeric list. For example, (sort '(4 8 21 17 33 7 21 7) '<) produces this: (4 7 7 8 17 21 21 33) (Note that in this example, both the arguments are quoted so that the symbols are not evaluated before being passed to `sort' as arguments.) Sorting the list returned by the `recursive-lengths-list-many-files' function is straightforward; it uses the `<' function: (sort (recursive-lengths-list-many-files '("./lisp/macros.el" "./lisp/mailalias.el" "./lisp/makesum.el")) '<) which produces: (29 32 38 85 90 95 178 180 181 218 263 283 321 324 480) (Note that in this example, the first argument to `sort' is not quoted, since the expression must be evaluated so as to produce the list that is passed to `sort'.)  File: eintr, Node: Files List, Next: Counting function definitions, Prev: Sorting, Up: Prepare the data 14.9.2 Making a List of Files ----------------------------- The `recursive-lengths-list-many-files' function requires a list of files as its argument. For our test examples, we constructed such a list by hand; but the Emacs Lisp source directory is too large for us to do for that. Instead, we will write a function to do the job for us. In this function, we will use both a `while' loop and a recursive call. We did not have to write a function like this for older versions of GNU Emacs, since they placed all the `.el' files in one directory. Instead, we were able to use the `directory-files' function, which lists the names of files that match a specified pattern within a single directory. However, recent versions of Emacs place Emacs Lisp files in sub-directories of the top level `lisp' directory. This re-arrangement eases navigation. For example, all the mail related files are in a `lisp' sub-directory called `mail'. But at the same time, this arrangement forces us to create a file listing function that descends into the sub-directories. We can create this function, called `files-in-below-directory', using familiar functions such as `car', `nthcdr', and `substring' in conjunction with an existing function called `directory-files-and-attributes'. This latter function not only lists all the filenames in a directory, including the names of sub-directories, but also their attributes. To restate our goal: to create a function that will enable us to feed filenames to `recursive-lengths-list-many-files' as a list that looks like this (but with more elements): ("./lisp/macros.el" "./lisp/mail/rmail.el" "./lisp/makesum.el") The `directory-files-and-attributes' function returns a list of lists. Each of the lists within the main list consists of 13 elements. The first element is a string that contains the name of the file - which, in GNU/Linux, may be a `directory file', that is to say, a file with the special attributes of a directory. The second element of the list is `t' for a directory, a string for symbolic link (the string is the name linked to), or `nil'. For example, the first `.el' file in the `lisp/' directory is `abbrev.el'. Its name is `/usr/local/share/emacs/22.1.1/lisp/abbrev.el' and it is not a directory or a symbolic link. This is how `directory-files-and-attributes' lists that file and its attributes: ("abbrev.el" nil 1 1000 100 (17733 259) (17491 28834) (17596 62124) 13157 "-rw-rw-r--" nil 2971624 773) On the other hand, `mail/' is a directory within the `lisp/' directory. The beginning of its listing looks like this: ("mail" t ... ) (To learn about the different attributes, look at the documentation of `file-attributes'. Bear in mind that the `file-attributes' function does not list the filename, so its first element is `directory-files-and-attributes''s second element.) We will want our new function, `files-in-below-directory', to list the `.el' files in the directory it is told to check, and in any directories below that directory. This gives us a hint on how to construct `files-in-below-directory': within a directory, the function should add `.el' filenames to a list; and if, within a directory, the function comes upon a sub-directory, it should go into that sub-directory and repeat its actions. However, we should note that every directory contains a name that refers to itself, called `.', ("dot") and a name that refers to its parent directory, called `..' ("double dot"). (In `/', the root directory, `..' refers to itself, since `/' has no parent.) Clearly, we do not want our `files-in-below-directory' function to enter those directories, since they always lead us, directly or indirectly, to the current directory. Consequently, our `files-in-below-directory' function must do several tasks: * Check to see whether it is looking at a filename that ends in `.el'; and if so, add its name to a list. * Check to see whether it is looking at a filename that is the name of a directory; and if so, - Check to see whether it is looking at `.' or `..'; and if so skip it. - Or else, go into that directory and repeat the process. Let's write a function definition to do these tasks. We will use a `while' loop to move from one filename to another within a directory, checking what needs to be done; and we will use a recursive call to repeat the actions on each sub-directory. The recursive pattern is `accumulate' (*note Recursive Pattern: _accumulate_: Accumulate.), using `append' as the combiner. Here is the function: (defun files-in-below-directory (directory) "List the .el files in DIRECTORY and in its sub-directories." ;; Although the function will be used non-interactively, ;; it will be easier to test if we make it interactive. ;; The directory will have a name such as ;; "/usr/local/share/emacs/22.1.1/lisp/" (interactive "DDirectory name: ") (let (el-files-list (current-directory-list (directory-files-and-attributes directory t))) ;; while we are in the current directory (while current-directory-list (cond ;; check to see whether filename ends in `.el' ;; and if so, append its name to a list. ((equal ".el" (substring (car (car current-directory-list)) -3)) (setq el-files-list (cons (car (car current-directory-list)) el-files-list))) ;; check whether filename is that of a directory ((eq t (car (cdr (car current-directory-list)))) ;; decide whether to skip or recurse (if (equal "." (substring (car (car current-directory-list)) -1)) ;; then do nothing since filename is that of ;; current directory or parent, "." or ".." () ;; else descend into the directory and repeat the process (setq el-files-list (append (files-in-below-directory (car (car current-directory-list))) el-files-list))))) ;; move to the next filename in the list; this also ;; shortens the list so the while loop eventually comes to an end (setq current-directory-list (cdr current-directory-list))) ;; return the filenames el-files-list)) The `files-in-below-directory' `directory-files' function takes one argument, the name of a directory. Thus, on my system, (length (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/")) tells me that in and below my Lisp sources directory are 1031 `.el' files. `files-in-below-directory' returns a list in reverse alphabetical order. An expression to sort the list in alphabetical order looks like this: (sort (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/") 'string-lessp)  File: eintr, Node: Counting function definitions, Prev: Files List, Up: Prepare the data 14.9.3 Counting function definitions ------------------------------------ Our immediate goal is to generate a list that tells us how many function definitions contain fewer than 10 words and symbols, how many contain between 10 and 19 words and symbols, how many contain between 20 and 29 words and symbols, and so on. With a sorted list of numbers, this is easy: count how many elements of the list are smaller than 10, then, after moving past the numbers just counted, count how many are smaller than 20, then, after moving past the numbers just counted, count how many are smaller than 30, and so on. Each of the numbers, 10, 20, 30, 40, and the like, is one larger than the top of that range. We can call the list of such numbers the `top-of-ranges' list. If we wished, we could generate this list automatically, but it is simpler to write a list manually. Here it is: (defvar top-of-ranges '(10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300) "List specifying ranges for `defuns-per-range'.") To change the ranges, we edit this list. Next, we need to write the function that creates the list of the number of definitions within each range. Clearly, this function must take the `sorted-lengths' and the `top-of-ranges' lists as arguments. The `defuns-per-range' function must do two things again and again: it must count the number of definitions within a range specified by the current top-of-range value; and it must shift to the next higher value in the `top-of-ranges' list after counting the number of definitions in the current range. Since each of these actions is repetitive, we can use `while' loops for the job. One loop counts the number of definitions in the range defined by the current top-of-range value, and the other loop selects each of the top-of-range values in turn. Several entries of the `sorted-lengths' list are counted for each range; this means that the loop for the `sorted-lengths' list will be inside the loop for the `top-of-ranges' list, like a small gear inside a big gear. The inner loop counts the number of definitions within the range. It is a simple counting loop of the type we have seen before. (*Note A loop with an incrementing counter: Incrementing Loop.) The true-or-false test of the loop tests whether the value from the `sorted-lengths' list is smaller than the current value of the top of the range. If it is, the function increments the counter and tests the next value from the `sorted-lengths' list. The inner loop looks like this: (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths))) The outer loop must start with the lowest value of the `top-of-ranges' list, and then be set to each of the succeeding higher values in turn. This can be done with a loop like this: (while top-of-ranges BODY-OF-LOOP... (setq top-of-ranges (cdr top-of-ranges))) Put together, the two loops look like this: (while top-of-ranges ;; Count the number of elements within the current range. (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths))) ;; Move to next range. (setq top-of-ranges (cdr top-of-ranges))) In addition, in each circuit of the outer loop, Emacs should record the number of definitions within that range (the value of `number-within-range') in a list. We can use `cons' for this purpose. (*Note `cons': cons.) The `cons' function works fine, except that the list it constructs will contain the number of definitions for the highest range at its beginning and the number of definitions for the lowest range at its end. This is because `cons' attaches new elements of the list to the beginning of the list, and since the two loops are working their way through the lengths' list from the lower end first, the `defuns-per-range-list' will end up largest number first. But we will want to print our graph with smallest values first and the larger later. The solution is to reverse the order of the `defuns-per-range-list'. We can do this using the `nreverse' function, which reverses the order of a list. For example, (nreverse '(1 2 3 4)) produces: (4 3 2 1) Note that the `nreverse' function is "destructive"--that is, it changes the list to which it is applied; this contrasts with the `car' and `cdr' functions, which are non-destructive. In this case, we do not want the original `defuns-per-range-list', so it does not matter that it is destroyed. (The `reverse' function provides a reversed copy of a list, leaving the original list as is.) Put all together, the `defuns-per-range' looks like this: (defun defuns-per-range (sorted-lengths top-of-ranges) "SORTED-LENGTHS defuns in each TOP-OF-RANGES range." (let ((top-of-range (car top-of-ranges)) (number-within-range 0) defuns-per-range-list) ;; Outer loop. (while top-of-ranges ;; Inner loop. (while (and ;; Need number for numeric test. (car sorted-lengths) (< (car sorted-lengths) top-of-range)) ;; Count number of definitions within current range. (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths))) ;; Exit inner loop but remain within outer loop. (setq defuns-per-range-list (cons number-within-range defuns-per-range-list)) (setq number-within-range 0) ; Reset count to zero. ;; Move to next range. (setq top-of-ranges (cdr top-of-ranges)) ;; Specify next top of range value. (setq top-of-range (car top-of-ranges))) ;; Exit outer loop and count the number of defuns larger than ;; the largest top-of-range value. (setq defuns-per-range-list (cons (length sorted-lengths) defuns-per-range-list)) ;; Return a list of the number of definitions within each range, ;; smallest to largest. (nreverse defuns-per-range-list))) The function is straightforward except for one subtle feature. The true-or-false test of the inner loop looks like this: (and (car sorted-lengths) (< (car sorted-lengths) top-of-range)) instead of like this: (< (car sorted-lengths) top-of-range) The purpose of the test is to determine whether the first item in the `sorted-lengths' list is less than the value of the top of the range. The simple version of the test works fine unless the `sorted-lengths' list has a `nil' value. In that case, the `(car sorted-lengths)' expression function returns `nil'. The `<' function cannot compare a number to `nil', which is an empty list, so Emacs signals an error and stops the function from attempting to continue to execute. The `sorted-lengths' list always becomes `nil' when the counter reaches the end of the list. This means that any attempt to use the `defuns-per-range' function with the simple version of the test will fail. We solve the problem by using the `(car sorted-lengths)' expression in conjunction with the `and' expression. The `(car sorted-lengths)' expression returns a non-`nil' value so long as the list has at least one number within it, but returns `nil' if the list is empty. The `and' expression first evaluates the `(car sorted-lengths)' expression, and if it is `nil', returns false _without_ evaluating the `<' expression. But if the `(car sorted-lengths)' expression returns a non-`nil' value, the `and' expression evaluates the `<' expression, and returns that value as the value of the `and' expression. This way, we avoid an error. (*Note The `kill-new' function: kill-new function, for information about `and'.) Here is a short test of the `defuns-per-range' function. First, evaluate the expression that binds (a shortened) `top-of-ranges' list to the list of values, then evaluate the expression for binding the `sorted-lengths' list, and then evaluate the `defuns-per-range' function. ;; (Shorter list than we will use later.) (setq top-of-ranges '(110 120 130 140 150 160 170 180 190 200)) (setq sorted-lengths '(85 86 110 116 122 129 154 176 179 200 265 300 300)) (defuns-per-range sorted-lengths top-of-ranges) The list returned looks like this: (2 2 2 0 0 1 0 2 0 0 4) Indeed, there are two elements of the `sorted-lengths' list smaller than 110, two elements between 110 and 119, two elements between 120 and 129, and so on. There are four elements with a value of 200 or larger.  File: eintr, Node: Readying a Graph, Next: Emacs Initialization, Prev: Words in a defun, Up: Top 15 Readying a Graph ******************* Our goal is to construct a graph showing the numbers of function definitions of various lengths in the Emacs lisp sources. As a practical matter, if you were creating a graph, you would probably use a program such as `gnuplot' to do the job. (`gnuplot' is nicely integrated into GNU Emacs.) In this case, however, we create one from scratch, and in the process we will re-acquaint ourselves with some of what we learned before and learn more. In this chapter, we will first write a simple graph printing function. This first definition will be a "prototype", a rapidly written function that enables us to reconnoiter this unknown graph-making territory. We will discover dragons, or find that they are myth. After scouting the terrain, we will feel more confident and enhance the function to label the axes automatically. * Menu: * Columns of a graph:: * graph-body-print:: How to print the body of a graph. * recursive-graph-body-print:: * Printed Axes:: * Line Graph Exercise::  File: eintr, Node: Columns of a graph, Next: graph-body-print, Prev: Readying a Graph, Up: Readying a Graph Printing the Columns of a Graph =============================== Since Emacs is designed to be flexible and work with all kinds of terminals, including character-only terminals, the graph will need to be made from one of the `typewriter' symbols. An asterisk will do; as we enhance the graph-printing function, we can make the choice of symbol a user option. We can call this function `graph-body-print'; it will take a `numbers-list' as its only argument. At this stage, we will not label the graph, but only print its body. The `graph-body-print' function inserts a vertical column of asterisks for each element in the `numbers-list'. The height of each line is determined by the value of that element of the `numbers-list'. Inserting columns is a repetitive act; that means that this function can be written either with a `while' loop or recursively. Our first challenge is to discover how to print a column of asterisks. Usually, in Emacs, we print characters onto a screen horizontally, line by line, by typing. We have two routes we can follow: write our own column-insertion function or discover whether one exists in Emacs. To see whether there is one in Emacs, we can use the `M-x apropos' command. This command is like the `C-h a' (`command-apropos') command, except that the latter finds only those functions that are commands. The `M-x apropos' command lists all symbols that match a regular expression, including functions that are not interactive. What we want to look for is some command that prints or inserts columns. Very likely, the name of the function will contain either the word `print' or the word `insert' or the word `column'. Therefore, we can simply type `M-x apropos RET print\|insert\|column RET' and look at the result. On my system, this command once too takes quite some time, and then produced a list of 79 functions and variables. Now it does not take much time at all and produces a list of 211 functions and variables. Scanning down the list, the only function that looks as if it might do the job is `insert-rectangle'. Indeed, this is the function we want; its documentation says: insert-rectangle: Insert text of RECTANGLE with upper left corner at point. RECTANGLE's first line is inserted at point, its second line is inserted at a point vertically under point, etc. RECTANGLE should be a list of strings. After this command, the mark is at the upper left corner and point is at the lower right corner. We can run a quick test, to make sure it does what we expect of it. Here is the result of placing the cursor after the `insert-rectangle' expression and typing `C-u C-x C-e' (`eval-last-sexp'). The function inserts the strings `"first"', `"second"', and `"third"' at and below point. Also the function returns `nil'. (insert-rectangle '("first" "second" "third"))first second thirdnil Of course, we won't be inserting the text of the `insert-rectangle' expression itself into the buffer in which we are making the graph, but will call the function from our program. We shall, however, have to make sure that point is in the buffer at the place where the `insert-rectangle' function will insert its column of strings. If you are reading this in Info, you can see how this works by switching to another buffer, such as the `*scratch*' buffer, placing point somewhere in the buffer, typing `M-:', typing the `insert-rectangle' expression into the minibuffer at the prompt, and then typing . This causes Emacs to evaluate the expression in the minibuffer, but to use as the value of point the position of point in the `*scratch*' buffer. (`M-:' is the keybinding for `eval-expression'. Also, `nil' does not appear in the `*scratch*' buffer since the expression is evaluated in the minibuffer.) We find when we do this that point ends up at the end of the last inserted line--that is to say, this function moves point as a side-effect. If we were to repeat the command, with point at this position, the next insertion would be below and to the right of the previous insertion. We don't want this! If we are going to make a bar graph, the columns need to be beside each other. So we discover that each cycle of the column-inserting `while' loop must reposition point to the place we want it, and that place will be at the top, not the bottom, of the column. Moreover, we remember that when we print a graph, we do not expect all the columns to be the same height. This means that the top of each column may be at a different height from the previous one. We cannot simply reposition point to the same line each time, but moved over to the right--or perhaps we can... We are planning to make the columns of the bar graph out of asterisks. The number of asterisks in the column is the number specified by the current element of the `numbers-list'. We need to construct a list of asterisks of the right length for each call to `insert-rectangle'. If this list consists solely of the requisite number of asterisks, then we will have position point the right number of lines above the base for the graph to print correctly. This could be difficult. Alternatively, if we can figure out some way to pass `insert-rectangle' a list of the same length each time, then we can place point on the same line each time, but move it over one column to the right for each new column. If we do this, however, some of the entries in the list passed to `insert-rectangle' must be blanks rather than asterisks. For example, if the maximum height of the graph is 5, but the height of the column is 3, then `insert-rectangle' requires an argument that looks like this: (" " " " "*" "*" "*") This last proposal is not so difficult, so long as we can determine the column height. There are two ways for us to specify the column height: we can arbitrarily state what it will be, which would work fine for graphs of that height; or we can search through the list of numbers and use the maximum height of the list as the maximum height of the graph. If the latter operation were difficult, then the former procedure would be easiest, but there is a function built into Emacs that determines the maximum of its arguments. We can use that function. The function is called `max' and it returns the largest of all its arguments, which must be numbers. Thus, for example, (max 3 4 6 5 7 3) returns 7. (A corresponding function called `min' returns the smallest of all its arguments.) However, we cannot simply call `max' on the `numbers-list'; the `max' function expects numbers as its argument, not a list of numbers. Thus, the following expression, (max '(3 4 6 5 7 3)) produces the following error message; Wrong type of argument: number-or-marker-p, (3 4 6 5 7 3) We need a function that passes a list of arguments to a function. This function is `apply'. This function `applies' its first argument (a function) to its remaining arguments, the last of which may be a list. For example, (apply 'max 3 4 7 3 '(4 8 5)) returns 8. (Incidentally, I don't know how you would learn of this function without a book such as this. It is possible to discover other functions, like `search-forward' or `insert-rectangle', by guessing at a part of their names and then using `apropos'. Even though its base in metaphor is clear--`apply' its first argument to the rest--I doubt a novice would come up with that particular word when using `apropos' or other aid. Of course, I could be wrong; after all, the function was first named by someone who had to invent it.) The second and subsequent arguments to `apply' are optional, so we can use `apply' to call a function and pass the elements of a list to it, like this, which also returns 8: (apply 'max '(4 8 5)) This latter way is how we will use `apply'. The `recursive-lengths-list-many-files' function returns a numbers' list to which we can apply `max' (we could also apply `max' to the sorted numbers' list; it does not matter whether the list is sorted or not.) Hence, the operation for finding the maximum height of the graph is this: (setq max-graph-height (apply 'max numbers-list)) Now we can return to the question of how to create a list of strings for a column of the graph. Told the maximum height of the graph and the number of asterisks that should appear in the column, the function should return a list of strings for the `insert-rectangle' command to insert. Each column is made up of asterisks or blanks. Since the function is passed the value of the height of the column and the number of asterisks in the column, the number of blanks can be found by subtracting the number of asterisks from the height of the column. Given the number of blanks and the number of asterisks, two `while' loops can be used to construct the list: ;;; First version. (defun column-of-graph (max-graph-height actual-height) "Return list of strings that is one column of a graph." (let ((insert-list nil) (number-of-top-blanks (- max-graph-height actual-height))) ;; Fill in asterisks. (while (> actual-height 0) (setq insert-list (cons "*" insert-list)) (setq actual-height (1- actual-height))) ;; Fill in blanks. (while (> number-of-top-blanks 0) (setq insert-list (cons " " insert-list)) (setq number-of-top-blanks (1- number-of-top-blanks))) ;; Return whole list. insert-list)) If you install this function and then evaluate the following expression you will see that it returns the list as desired: (column-of-graph 5 3) returns (" " " " "*" "*" "*") As written, `column-of-graph' contains a major flaw: the symbols used for the blank and for the marked entries in the column are `hard-coded' as a space and asterisk. This is fine for a prototype, but you, or another user, may wish to use other symbols. For example, in testing the graph function, you many want to use a period in place of the space, to make sure the point is being repositioned properly each time the `insert-rectangle' function is called; or you might want to substitute a `+' sign or other symbol for the asterisk. You might even want to make a graph-column that is more than one display column wide. The program should be more flexible. The way to do that is to replace the blank and the asterisk with two variables that we can call `graph-blank' and `graph-symbol' and define those variables separately. Also, the documentation is not well written. These considerations lead us to the second version of the function: (defvar graph-symbol "*" "String used as symbol in graph, usually an asterisk.") (defvar graph-blank " " "String used as blank in graph, usually a blank space. graph-blank must be the same number of columns wide as graph-symbol.") (For an explanation of `defvar', see *note Initializing a Variable with `defvar': defvar.) ;;; Second version. (defun column-of-graph (max-graph-height actual-height) "Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols. The graph-symbols are contiguous entries at the end of the list. The list will be inserted as one column of a graph. The strings are either graph-blank or graph-symbol." (let ((insert-list nil) (number-of-top-blanks (- max-graph-height actual-height))) ;; Fill in `graph-symbols'. (while (> actual-height 0) (setq insert-list (cons graph-symbol insert-list)) (setq actual-height (1- actual-height))) ;; Fill in `graph-blanks'. (while (> number-of-top-blanks 0) (setq insert-list (cons graph-blank insert-list)) (setq number-of-top-blanks (1- number-of-top-blanks))) ;; Return whole list. insert-list)) If we wished, we could rewrite `column-of-graph' a third time to provide optionally for a line graph as well as for a bar graph. This would not be hard to do. One way to think of a line graph is that it is no more than a bar graph in which the part of each bar that is below the top is blank. To construct a column for a line graph, the function first constructs a list of blanks that is one shorter than the value, then it uses `cons' to attach a graph symbol to the list; then it uses `cons' again to attach the `top blanks' to the list. It is easy to see how to write such a function, but since we don't need it, we will not do it. But the job could be done, and if it were done, it would be done with `column-of-graph'. Even more important, it is worth noting that few changes would have to be made anywhere else. The enhancement, if we ever wish to make it, is simple. Now, finally, we come to our first actual graph printing function. This prints the body of a graph, not the labels for the vertical and horizontal axes, so we can call this `graph-body-print'.  File: eintr, Node: graph-body-print, Next: recursive-graph-body-print, Prev: Columns of a graph, Up: Readying a Graph 15.1 The `graph-body-print' Function ==================================== After our preparation in the preceding section, the `graph-body-print' function is straightforward. The function will print column after column of asterisks and blanks, using the elements of a numbers' list to specify the number of asterisks in each column. This is a repetitive act, which means we can use a decrementing `while' loop or recursive function for the job. In this section, we will write the definition using a `while' loop. The `column-of-graph' function requires the height of the graph as an argument, so we should determine and record that as a local variable. This leads us to the following template for the `while' loop version of this function: (defun graph-body-print (numbers-list) "DOCUMENTATION..." (let ((height ... ...)) (while numbers-list INSERT-COLUMNS-AND-REPOSITION-POINT (setq numbers-list (cdr numbers-list))))) We need to fill in the slots of the template. Clearly, we can use the `(apply 'max numbers-list)' expression to determine the height of the graph. The `while' loop will cycle through the `numbers-list' one element at a time. As it is shortened by the `(setq numbers-list (cdr numbers-list))' expression, the CAR of each instance of the list is the value of the argument for `column-of-graph'. At each cycle of the `while' loop, the `insert-rectangle' function inserts the list returned by `column-of-graph'. Since the `insert-rectangle' function moves point to the lower right of the inserted rectangle, we need to save the location of point at the time the rectangle is inserted, move back to that position after the rectangle is inserted, and then move horizontally to the next place from which `insert-rectangle' is called. If the inserted columns are one character wide, as they will be if single blanks and asterisks are used, the repositioning command is simply `(forward-char 1)'; however, the width of a column may be greater than one. This means that the repositioning command should be written `(forward-char symbol-width)'. The `symbol-width' itself is the length of a `graph-blank' and can be found using the expression `(length graph-blank)'. The best place to bind the `symbol-width' variable to the value of the width of graph column is in the varlist of the `let' expression. These considerations lead to the following function definition: (defun graph-body-print (numbers-list) "Print a bar graph of the NUMBERS-LIST. The numbers-list consists of the Y-axis values." (let ((height (apply 'max numbers-list)) (symbol-width (length graph-blank)) from-position) (while numbers-list (setq from-position (point)) (insert-rectangle (column-of-graph height (car numbers-list))) (goto-char from-position) (forward-char symbol-width) ;; Draw graph column by column. (sit-for 0) (setq numbers-list (cdr numbers-list))) ;; Place point for X axis labels. (forward-line height) (insert "\n") )) The one unexpected expression in this function is the `(sit-for 0)' expression in the `while' loop. This expression makes the graph printing operation more interesting to watch than it would be otherwise. The expression causes Emacs to `sit' or do nothing for a zero length of time and then redraw the screen. Placed here, it causes Emacs to redraw the screen column by column. Without it, Emacs would not redraw the screen until the function exits. We can test `graph-body-print' with a short list of numbers. 1. Install `graph-symbol', `graph-blank', `column-of-graph', which are in *note Columns of a graph::, and `graph-body-print'. 2. Copy the following expression: (graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3)) 3. Switch to the `*scratch*' buffer and place the cursor where you want the graph to start. 4. Type `M-:' (`eval-expression'). 5. Yank the `graph-body-print' expression into the minibuffer with `C-y' (`yank)'. 6. Press to evaluate the `graph-body-print' expression. Emacs will print a graph like this: * * ** * **** *** **** ********* * ************ *************  File: eintr, Node: recursive-graph-body-print, Next: Printed Axes, Prev: graph-body-print, Up: Readying a Graph 15.2 The `recursive-graph-body-print' Function ============================================== The `graph-body-print' function may also be written recursively. The recursive solution is divided into two parts: an outside `wrapper' that uses a `let' expression to determine the values of several variables that need only be found once, such as the maximum height of the graph, and an inside function that is called recursively to print the graph. The `wrapper' is uncomplicated: (defun recursive-graph-body-print (numbers-list) "Print a bar graph of the NUMBERS-LIST. The numbers-list consists of the Y-axis values." (let ((height (apply 'max numbers-list)) (symbol-width (length graph-blank)) from-position) (recursive-graph-body-print-internal numbers-list height symbol-width))) The recursive function is a little more difficult. It has four parts: the `do-again-test', the printing code, the recursive call, and the `next-step-expression'. The `do-again-test' is a `when' expression that determines whether the `numbers-list' contains any remaining elements; if it does, the function prints one column of the graph using the printing code and calls itself again. The function calls itself again according to the value produced by the `next-step-expression' which causes the call to act on a shorter version of the `numbers-list'. (defun recursive-graph-body-print-internal (numbers-list height symbol-width) "Print a bar graph. Used within recursive-graph-body-print function." (when numbers-list (setq from-position (point)) (insert-rectangle (column-of-graph height (car numbers-list))) (goto-char from-position) (forward-char symbol-width) (sit-for 0) ; Draw graph column by column. (recursive-graph-body-print-internal (cdr numbers-list) height symbol-width))) After installation, this expression can be tested; here is a sample: (recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1)) Here is what `recursive-graph-body-print' produces: * ** * **** * **** *** * ********* ************ ************* Either of these two functions, `graph-body-print' or `recursive-graph-body-print', create the body of a graph.  File: eintr, Node: Printed Axes, Next: Line Graph Exercise, Prev: recursive-graph-body-print, Up: Readying a Graph 15.3 Need for Printed Axes ========================== A graph needs printed axes, so you can orient yourself. For a do-once project, it may be reasonable to draw the axes by hand using Emacs' Picture mode; but a graph drawing function may be used more than once. For this reason, I have written enhancements to the basic `print-graph-body' function that automatically print labels for the horizontal and vertical axes. Since the label printing functions do not contain much new material, I have placed their description in an appendix. *Note A Graph with Labelled Axes: Full Graph.  File: eintr, Node: Line Graph Exercise, Prev: Printed Axes, Up: Readying a Graph 15.4 Exercise ============= Write a line graph version of the graph printing functions.  File: eintr, Node: Emacs Initialization, Next: Debugging, Prev: Readying a Graph, Up: Top 16 Your `.emacs' File ********************* "You don't have to like Emacs to like it" - this seemingly paradoxical statement is the secret of GNU Emacs. The plain, `out of the box' Emacs is a generic tool. Most people who use it, customize it to suit themselves. GNU Emacs is mostly written in Emacs Lisp; this means that by writing expressions in Emacs Lisp you can change or extend Emacs. * Menu: * Default Configuration:: * Site-wide Init:: You can write site-wide init files. * defcustom:: Emacs will write code for you. * Beginning a .emacs File:: How to write a `.emacs file'. * Text and Auto-fill:: Automatically wrap lines. * Mail Aliases:: Use abbreviations for email addresses. * Indent Tabs Mode:: Don't use tabs with TeX * Keybindings:: Create some personal keybindings. * Keymaps:: More about key binding. * Loading Files:: Load (i.e., evaluate) files automatically. * Autoload:: Make functions available. * Simple Extension:: Define a function; bind it to a key. * X11 Colors:: Colors in X. * Miscellaneous:: * Mode Line:: How to customize your mode line.  File: eintr, Node: Default Configuration, Next: Site-wide Init, Prev: Emacs Initialization, Up: Emacs Initialization Emacs' Default Configuration ============================ There are those who appreciate Emacs' default configuration. After all, Emacs starts you in C mode when you edit a C file, starts you in Fortran mode when you edit a Fortran file, and starts you in Fundamental mode when you edit an unadorned file. This all makes sense, if you do not know who is going to use Emacs. Who knows what a person hopes to do with an unadorned file? Fundamental mode is the right default for such a file, just as C mode is the right default for editing C code. (Enough programming languages have syntaxes that enable them to share or nearly share features, so C mode is now provided by CC mode, the `C Collection'.) But when you do know who is going to use Emacs--you, yourself--then it makes sense to customize Emacs. For example, I seldom want Fundamental mode when I edit an otherwise undistinguished file; I want Text mode. This is why I customize Emacs: so it suits me. You can customize and extend Emacs by writing or adapting a `~/.emacs' file. This is your personal initialization file; its contents, written in Emacs Lisp, tell Emacs what to do.(1) A `~/.emacs' file contains Emacs Lisp code. You can write this code yourself; or you can use Emacs' `customize' feature to write the code for you. You can combine your own expressions and auto-written Customize expressions in your `.emacs' file. (I myself prefer to write my own expressions, except for those, particularly fonts, that I find easier to manipulate using the `customize' command. I combine the two methods.) Most of this chapter is about writing expressions yourself. It describes a simple `.emacs' file; for more information, see *note The Init File: (emacs)Init File, and *note The Init File: (elisp)Init File. ---------- Footnotes ---------- (1) You may also add `.el' to `~/.emacs' and call it a `~/.emacs.el' file. In the past, you were forbidden to type the extra keystrokes that the name `~/.emacs.el' requires, but now you may. The new format is consistent with the Emacs Lisp file naming conventions; the old format saves typing.  File: eintr, Node: Site-wide Init, Next: defcustom, Prev: Default Configuration, Up: Emacs Initialization 16.1 Site-wide Initialization Files =================================== In addition to your personal initialization file, Emacs automatically loads various site-wide initialization files, if they exist. These have the same form as your `.emacs' file, but are loaded by everyone. Two site-wide initialization files, `site-load.el' and `site-init.el', are loaded into Emacs and then `dumped' if a `dumped' version of Emacs is created, as is most common. (Dumped copies of Emacs load more quickly. However, once a file is loaded and dumped, a change to it does not lead to a change in Emacs unless you load it yourself or re-dump Emacs. *Note Building Emacs: (elisp)Building Emacs, and the `INSTALL' file.) Three other site-wide initialization files are loaded automatically each time you start Emacs, if they exist. These are `site-start.el', which is loaded _before_ your `.emacs' file, and `default.el', and the terminal type file, which are both loaded _after_ your `.emacs' file. Settings and definitions in your `.emacs' file will overwrite conflicting settings and definitions in a `site-start.el' file, if it exists; but the settings and definitions in a `default.el' or terminal type file will overwrite those in your `.emacs' file. (You can prevent interference from a terminal type file by setting `term-file-prefix' to `nil'. *Note A Simple Extension: Simple Extension.) The `INSTALL' file that comes in the distribution contains descriptions of the `site-init.el' and `site-load.el' files. The `loadup.el', `startup.el', and `loaddefs.el' files control loading. These files are in the `lisp' directory of the Emacs distribution and are worth perusing. The `loaddefs.el' file contains a good many suggestions as to what to put into your own `.emacs' file, or into a site-wide initialization file.  File: eintr, Node: defcustom, Next: Beginning a .emacs File, Prev: Site-wide Init, Up: Emacs Initialization 16.2 Specifying Variables using `defcustom' =========================================== You can specify variables using `defcustom' so that you and others can then use Emacs' `customize' feature to set their values. (You cannot use `customize' to write function definitions; but you can write `defuns' in your `.emacs' file. Indeed, you can write any Lisp expression in your `.emacs' file.) The `customize' feature depends on the `defcustom' special form. Although you can use `defvar' or `setq' for variables that users set, the `defcustom' special form is designed for the job. You can use your knowledge of `defvar' for writing the first three arguments for `defcustom'. The first argument to `defcustom' is the name of the variable. The second argument is the variable's initial value, if any; and this value is set only if the value has not already been set. The third argument is the documentation. The fourth and subsequent arguments to `defcustom' specify types and options; these are not featured in `defvar'. (These arguments are optional.) Each of these arguments consists of a keyword followed by a value. Each keyword starts with the colon character `:'. For example, the customizable user option variable `text-mode-hook' looks like this: (defcustom text-mode-hook nil "Normal hook run when entering Text mode and many related modes." :type 'hook :options '(turn-on-auto-fill flyspell-mode) :group 'data) The name of the variable is `text-mode-hook'; it has no default value; and its documentation string tells you what it does. The `:type' keyword tells Emacs the kind of data to which `text-mode-hook' should be set and how to display the value in a Customization buffer. The `:options' keyword specifies a suggested list of values for the variable. Usually, `:options' applies to a hook. The list is only a suggestion; it is not exclusive; a person who sets the variable may set it to other values; the list shown following the `:options' keyword is intended to offer convenient choices to a user. Finally, the `:group' keyword tells the Emacs Customization command in which group the variable is located. This tells where to find it. The `defcustom' function recognizes more than a dozen keywords. For more information, see *note Writing Customization Definitions: (elisp)Customization. Consider `text-mode-hook' as an example. There are two ways to customize this variable. You can use the customization command or write the appropriate expressions yourself. Using the customization command, you can type: M-x customize and find that the group for editing files of data is called `data'. Enter that group. Text Mode Hook is the first member. You can click on its various options, such as `turn-on-auto-fill', to set the values. After you click on the button to Save for Future Sessions Emacs will write an expression into your `.emacs' file. It will look like this: (custom-set-variables ;; custom-set-variables was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won't work right. '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify)))) (The `text-mode-hook-identify' function tells `toggle-text-mode-auto-fill' which buffers are in Text mode. It comes on automatically.) The `custom-set-variables' function works somewhat differently than a `setq'. While I have never learned the differences, I modify the `custom-set-variables' expressions in my `.emacs' file by hand: I make the changes in what appears to me to be a reasonable manner and have not had any problems. Others prefer to use the Customization command and let Emacs do the work for them. Another `custom-set-...' function is `custom-set-faces'. This function sets the various font faces. Over time, I have set a considerable number of faces. Some of the time, I re-set them using `customize'; other times, I simply edit the `custom-set-faces' expression in my `.emacs' file itself. The second way to customize your `text-mode-hook' is to set it yourself in your `.emacs' file using code that has nothing to do with the `custom-set-...' functions. When you do this, and later use `customize', you will see a message that says CHANGED outside Customize; operating on it here may be unreliable. This message is only a warning. If you click on the button to Save for Future Sessions Emacs will write a `custom-set-...' expression near the end of your `.emacs' file that will be evaluated after your hand-written expression. It will, therefore, overrule your hand-written expression. No harm will be done. When you do this, however, be careful to remember which expression is active; if you forget, you may confuse yourself. So long as you remember where the values are set, you will have no trouble. In any event, the values are always set in your initialization file, which is usually called `.emacs'. I myself use `customize' for hardly anything. Mostly, I write expressions myself. Incidentally, to be more complete concerning defines: `defsubst' defines an inline function. The syntax is just like that of `defun'. `defconst' defines a symbol as a constant. The intent is that neither programs nor users should ever change a value set by `defconst'. (You can change it; the value set is a variable; but please do not.)  File: eintr, Node: Beginning a .emacs File, Next: Text and Auto-fill, Prev: defcustom, Up: Emacs Initialization 16.3 Beginning a `.emacs' File ============================== When you start Emacs, it loads your `.emacs' file unless you tell it not to by specifying `-q' on the command line. (The `emacs -q' command gives you a plain, out-of-the-box Emacs.) A `.emacs' file contains Lisp expressions. Often, these are no more than expressions to set values; sometimes they are function definitions. *Note The Init File `~/.emacs': (emacs)Init File, for a short description of initialization files. This chapter goes over some of the same ground, but is a walk among extracts from a complete, long-used `.emacs' file--my own. The first part of the file consists of comments: reminders to myself. By now, of course, I remember these things, but when I started, I did not. ;;;; Bob's .emacs file ; Robert J. Chassell ; 26 September 1985 Look at that date! I started this file a long time ago. I have been adding to it ever since. ; Each section in this file is introduced by a ; line beginning with four semicolons; and each ; entry is introduced by a line beginning with ; three semicolons. This describes the usual conventions for comments in Emacs Lisp. Everything on a line that follows a semicolon is a comment. Two, three, and four semicolons are used as subsection and section markers. (*Note Comments: (elisp)Comments, for more about comments.) ;;;; The Help Key ; Control-h is the help key; ; after typing control-h, type a letter to ; indicate the subject about which you want help. ; For an explanation of the help facility, ; type control-h two times in a row. Just remember: type `C-h' two times for help. ; To find out about any mode, type control-h m ; while in that mode. For example, to find out ; about mail mode, enter mail mode and then type ; control-h m. `Mode help', as I call this, is very helpful. Usually, it tells you all you need to know. Of course, you don't need to include comments like these in your `.emacs' file. I included them in mine because I kept forgetting about Mode help or the conventions for comments--but I was able to remember to look here to remind myself.  File: eintr, Node: Text and Auto-fill, Next: Mail Aliases, Prev: Beginning a .emacs File, Up: Emacs Initialization 16.4 Text and Auto Fill Mode ============================ Now we come to the part that `turns on' Text mode and Auto Fill mode. ;;; Text mode and Auto Fill mode ;; The next two lines put Emacs into Text mode ;; and Auto Fill mode, and are for writers who ;; want to start writing prose rather than code. (setq-default major-mode 'text-mode) (add-hook 'text-mode-hook 'turn-on-auto-fill) Here is the first part of this `.emacs' file that does something besides remind a forgetful human! The first of the two lines in parentheses tells Emacs to turn on Text mode when you find a file, _unless_ that file should go into some other mode, such as C mode. When Emacs reads a file, it looks at the extension to the file name, if any. (The extension is the part that comes after a `.'.) If the file ends with a `.c' or `.h' extension then Emacs turns on C mode. Also, Emacs looks at first nonblank line of the file; if the line says `-*- C -*-', Emacs turns on C mode. Emacs possesses a list of extensions and specifications that it uses automatically. In addition, Emacs looks near the last page for a per-buffer, "local variables list", if any. *Note How Major Modes are Chosen: (emacs)Choosing Modes. *Note Local Variables in Files: (emacs)File Variables. Now, back to the `.emacs' file. Here is the line again; how does it work? (setq major-mode 'text-mode) This line is a short, but complete Emacs Lisp expression. We are already familiar with `setq'. It sets the following variable, `major-mode', to the subsequent value, which is `text-mode'. The single quote mark before `text-mode' tells Emacs to deal directly with the `text-mode' symbol, not with whatever it might stand for. *Note Setting the Value of a Variable: set & setq, for a reminder of how `setq' works. The main point is that there is no difference between the procedure you use to set a value in your `.emacs' file and the procedure you use anywhere else in Emacs. Here is the next line: (add-hook 'text-mode-hook 'turn-on-auto-fill) In this line, the `add-hook' command adds `turn-on-auto-fill' to the variable. `turn-on-auto-fill' is the name of a program, that, you guessed it!, turns on Auto Fill mode. Every time Emacs turns on Text mode, Emacs runs the commands `hooked' onto Text mode. So every time Emacs turns on Text mode, Emacs also turns on Auto Fill mode. In brief, the first line causes Emacs to enter Text mode when you edit a file, unless the file name extension, a first non-blank line, or local variables to tell Emacs otherwise. Text mode among other actions, sets the syntax table to work conveniently for writers. In Text mode, Emacs considers an apostrophe as part of a word like a letter; but Emacs does not consider a period or a space as part of a word. Thus, `M-f' moves you over `it's'. On the other hand, in C mode, `M-f' stops just after the `t' of `it's'. The second line causes Emacs to turn on Auto Fill mode when it turns on Text mode. In Auto Fill mode, Emacs automatically breaks a line that is too wide and brings the excessively wide part of the line down to the next line. Emacs breaks lines between words, not within them. When Auto Fill mode is turned off, lines continue to the right as you type them. Depending on how you set the value of `truncate-lines', the words you type either disappear off the right side of the screen, or else are shown, in a rather ugly and unreadable manner, as a continuation line on the screen. In addition, in this part of my `.emacs' file, I tell the Emacs fill commands to insert two spaces after a colon: (setq colon-double-space t)  File: eintr, Node: Mail Aliases, Next: Indent Tabs Mode, Prev: Text and Auto-fill, Up: Emacs Initialization 16.5 Mail Aliases ================= Here is a `setq' that `turns on' mail aliases, along with more reminders. ;;; Mail mode ; To enter mail mode, type `C-x m' ; To enter RMAIL (for reading mail), ; type `M-x rmail' (setq mail-aliases t) This `setq' command sets the value of the variable `mail-aliases' to `t'. Since `t' means true, the line says, in effect, "Yes, use mail aliases." Mail aliases are convenient short names for long email addresses or for lists of email addresses. The file where you keep your `aliases' is `~/.mailrc'. You write an alias like this: alias geo george@foobar.wiz.edu When you write a message to George, address it to `geo'; the mailer will automatically expand `geo' to the full address.  File: eintr, Node: Indent Tabs Mode, Next: Keybindings, Prev: Mail Aliases, Up: Emacs Initialization 16.6 Indent Tabs Mode ===================== By default, Emacs inserts tabs in place of multiple spaces when it formats a region. (For example, you might indent many lines of text all at once with the `indent-region' command.) Tabs look fine on a terminal or with ordinary printing, but they produce badly indented output when you use TeX or Texinfo since TeX ignores tabs. The following turns off Indent Tabs mode: ;;; Prevent Extraneous Tabs (setq-default indent-tabs-mode nil) Note that this line uses `setq-default' rather than the `setq' command that we have seen before. The `setq-default' command sets values only in buffers that do not have their own local values for the variable. *Note Tabs vs. Spaces: (emacs)Just Spaces. *Note Local Variables in Files: (emacs)File Variables.  File: eintr, Node: Keybindings, Next: Keymaps, Prev: Indent Tabs Mode, Up: Emacs Initialization 16.7 Some Keybindings ===================== Now for some personal keybindings: ;;; Compare windows (global-set-key "\C-cw" 'compare-windows) `compare-windows' is a nifty command that compares the text in your current window with text in the next window. It makes the comparison by starting at point in each window, moving over text in each window as far as they match. I use this command all the time. This also shows how to set a key globally, for all modes. The command is `global-set-key'. It is followed by the keybinding. In a `.emacs' file, the keybinding is written as shown: `\C-c' stands for `control-c', which means `press the control key and the key at the same time'. The `w' means `press the key'. The keybinding is surrounded by double quotation marks. In documentation, you would write this as `C-c w'. (If you were binding a key, such as `M-c', rather than a key, you would write `\M-c' in your `.emacs' file. *Note Rebinding Keys in Your Init File: (emacs)Init Rebinding, for details.) The command invoked by the keys is `compare-windows'. Note that `compare-windows' is preceded by a single quote; otherwise, Emacs would first try to evaluate the symbol to determine its value. These three things, the double quotation marks, the backslash before the `C', and the single quote mark are necessary parts of keybinding that I tend to forget. Fortunately, I have come to remember that I should look at my existing `.emacs' file, and adapt what is there. As for the keybinding itself: `C-c w'. This combines the prefix key, `C-c', with a single character, in this case, `w'. This set of keys, `C-c' followed by a single character, is strictly reserved for individuals' own use. (I call these `own' keys, since these are for my own use.) You should always be able to create such a keybinding for your own use without stomping on someone else's keybinding. If you ever write an extension to Emacs, please avoid taking any of these keys for public use. Create a key like `C-c C-w' instead. Otherwise, we will run out of `own' keys. Here is another keybinding, with a comment: ;;; Keybinding for `occur' ; I use occur a lot, so let's bind it to a key: (global-set-key "\C-co" 'occur) The `occur' command shows all the lines in the current buffer that contain a match for a regular expression. Matching lines are shown in a buffer called `*Occur*'. That buffer serves as a menu to jump to occurrences. Here is how to unbind a key, so it does not work: ;;; Unbind `C-x f' (global-unset-key "\C-xf") There is a reason for this unbinding: I found I inadvertently typed `C-x f' when I meant to type `C-x C-f'. Rather than find a file, as I intended, I accidentally set the width for filled text, almost always to a width I did not want. Since I hardly ever reset my default width, I simply unbound the key. The following rebinds an existing key: ;;; Rebind `C-x C-b' for `buffer-menu' (global-set-key "\C-x\C-b" 'buffer-menu) By default, `C-x C-b' runs the `list-buffers' command. This command lists your buffers in _another_ window. Since I almost always want to do something in that window, I prefer the `buffer-menu' command, which not only lists the buffers, but moves point into that window.  File: eintr, Node: Keymaps, Next: Loading Files, Prev: Keybindings, Up: Emacs Initialization 16.8 Keymaps ============ Emacs uses "keymaps" to record which keys call which commands. When you use `global-set-key' to set the keybinding for a single command in all parts of Emacs, you are specifying the keybinding in `current-global-map'. Specific modes, such as C mode or Text mode, have their own keymaps; the mode-specific keymaps override the global map that is shared by all buffers. The `global-set-key' function binds, or rebinds, the global keymap. For example, the following binds the key `C-x C-b' to the function `buffer-menu': (global-set-key "\C-x\C-b" 'buffer-menu) Mode-specific keymaps are bound using the `define-key' function, which takes a specific keymap as an argument, as well as the key and the command. For example, my `.emacs' file contains the following expression to bind the `texinfo-insert-@group' command to `C-c C-c g': (define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group) The `texinfo-insert-@group' function itself is a little extension to Texinfo mode that inserts `@group' into a Texinfo file. I use this command all the time and prefer to type the three strokes `C-c C-c g' rather than the six strokes `@ g r o u p'. (`@group' and its matching `@end group' are commands that keep all enclosed text together on one page; many multi-line examples in this book are surrounded by `@group ... @end group'.) Here is the `texinfo-insert-@group' function definition: (defun texinfo-insert-@group () "Insert the string @group in a Texinfo buffer." (interactive) (beginning-of-line) (insert "@group\n")) (Of course, I could have used Abbrev mode to save typing, rather than write a function to insert a word; but I prefer key strokes consistent with other Texinfo mode key bindings.) You will see numerous `define-key' expressions in `loaddefs.el' as well as in the various mode libraries, such as `cc-mode.el' and `lisp-mode.el'. *Note Customizing Key Bindings: (emacs)Key Bindings, and *note Keymaps: (elisp)Keymaps, for more information about keymaps.  File: eintr, Node: Loading Files, Next: Autoload, Prev: Keymaps, Up: Emacs Initialization 16.9 Loading Files ================== Many people in the GNU Emacs community have written extensions to Emacs. As time goes by, these extensions are often included in new releases. For example, the Calendar and Diary packages are now part of the standard GNU Emacs, as is Calc. You can use a `load' command to evaluate a complete file and thereby install all the functions and variables in the file into Emacs. For example: (load "~/emacs/slowsplit") This evaluates, i.e. loads, the `slowsplit.el' file or if it exists, the faster, byte compiled `slowsplit.elc' file from the `emacs' sub-directory of your home directory. The file contains the function `split-window-quietly', which John Robinson wrote in 1989. The `split-window-quietly' function splits a window with the minimum of redisplay. I installed it in 1989 because it worked well with the slow 1200 baud terminals I was then using. Nowadays, I only occasionally come across such a slow connection, but I continue to use the function because I like the way it leaves the bottom half of a buffer in the lower of the new windows and the top half in the upper window. To replace the key binding for the default `split-window-vertically', you must also unset that key and bind the keys to `split-window-quietly', like this: (global-unset-key "\C-x2") (global-set-key "\C-x2" 'split-window-quietly) If you load many extensions, as I do, then instead of specifying the exact location of the extension file, as shown above, you can specify that directory as part of Emacs' `load-path'. Then, when Emacs loads a file, it will search that directory as well as its default list of directories. (The default list is specified in `paths.h' when Emacs is built.) The following command adds your `~/emacs' directory to the existing load path: ;;; Emacs Load Path (setq load-path (cons "~/emacs" load-path)) Incidentally, `load-library' is an interactive interface to the `load' function. The complete function looks like this: (defun load-library (library) "Load the library named LIBRARY. This is an interface to the function `load'." (interactive (list (completing-read "Load library: " (apply-partially 'locate-file-completion-table load-path (get-load-suffixes))))) (load library)) The name of the function, `load-library', comes from the use of `library' as a conventional synonym for `file'. The source for the `load-library' command is in the `files.el' library. Another interactive command that does a slightly different job is `load-file'. *Note Libraries of Lisp Code for Emacs: (emacs)Lisp Libraries, for information on the distinction between `load-library' and this command.  File: eintr, Node: Autoload, Next: Simple Extension, Prev: Loading Files, Up: Emacs Initialization 16.10 Autoloading ================= Instead of installing a function by loading the file that contains it, or by evaluating the function definition, you can make the function available but not actually install it until it is first called. This is called "autoloading". When you execute an autoloaded function, Emacs automatically evaluates the file that contains the definition, and then calls the function. Emacs starts quicker with autoloaded functions, since their libraries are not loaded right away; but you need to wait a moment when you first use such a function, while its containing file is evaluated. Rarely used functions are frequently autoloaded. The `loaddefs.el' library contains hundreds of autoloaded functions, from `bookmark-set' to `wordstar-mode'. Of course, you may come to use a `rare' function frequently. When you do, you should load that function's file with a `load' expression in your `.emacs' file. In my `.emacs' file, I load 14 libraries that contain functions that would otherwise be autoloaded. (Actually, it would have been better to include these files in my `dumped' Emacs, but I forgot. *Note Building Emacs: (elisp)Building Emacs, and the `INSTALL' file for more about dumping.) You may also want to include autoloaded expressions in your `.emacs' file. `autoload' is a built-in function that takes up to five arguments, the final three of which are optional. The first argument is the name of the function to be autoloaded; the second is the name of the file to be loaded. The third argument is documentation for the function, and the fourth tells whether the function can be called interactively. The fifth argument tells what type of object--`autoload' can handle a keymap or macro as well as a function (the default is a function). Here is a typical example: (autoload 'html-helper-mode "html-helper-mode" "Edit HTML documents" t) (`html-helper-mode' is an older alternative to `html-mode', which is a standard part of the distribution.) This expression autoloads the `html-helper-mode' function. It takes it from the `html-helper-mode.el' file (or from the byte compiled version `html-helper-mode.elc', if that exists.) The file must be located in a directory specified by `load-path'. The documentation says that this is a mode to help you edit documents written in the HyperText Markup Language. You can call this mode interactively by typing `M-x html-helper-mode'. (You need to duplicate the function's regular documentation in the autoload expression because the regular function is not yet loaded, so its documentation is not available.) *Note Autoload: (elisp)Autoload, for more information.  File: eintr, Node: Simple Extension, Next: X11 Colors, Prev: Autoload, Up: Emacs Initialization 16.11 A Simple Extension: `line-to-top-of-window' ================================================= Here is a simple extension to Emacs that moves the line point is on to the top of the window. I use this all the time, to make text easier to read. You can put the following code into a separate file and then load it from your `.emacs' file, or you can include it within your `.emacs' file. Here is the definition: ;;; Line to top of window; ;;; replace three keystroke sequence C-u 0 C-l (defun line-to-top-of-window () "Move the line point is on to top of window." (interactive) (recenter 0)) Now for the keybinding. Nowadays, function keys as well as mouse button events and non-ASCII characters are written within square brackets, without quotation marks. (In Emacs version 18 and before, you had to write different function key bindings for each different make of terminal.) I bind `line-to-top-of-window' to my function key like this: (global-set-key [f6] 'line-to-top-of-window) For more information, see *note Rebinding Keys in Your Init File: (emacs)Init Rebinding. If you run two versions of GNU Emacs, such as versions 22 and 23, and use one `.emacs' file, you can select which code to evaluate with the following conditional: (cond ((= 22 emacs-major-version) ;; evaluate version 22 code ( ... )) ((= 23 emacs-major-version) ;; evaluate version 23 code ( ... ))) For example, in contrast to version 20, more recent versions blink their cursors by default. I hate such blinking, as well as other features, so I placed the following in my `.emacs' file(1): (when (>= emacs-major-version 21) (blink-cursor-mode 0) ;; Insert newline when you press `C-n' (next-line) ;; at the end of the buffer (setq next-line-add-newlines t) ;; Turn on image viewing (auto-image-file-mode t) ;; Turn on menu bar (this bar has text) ;; (Use numeric argument to turn on) (menu-bar-mode 1) ;; Turn off tool bar (this bar has icons) ;; (Use numeric argument to turn on) (tool-bar-mode nil) ;; Turn off tooltip mode for tool bar ;; (This mode causes icon explanations to pop up) ;; (Use numeric argument to turn on) (tooltip-mode nil) ;; If tooltips turned on, make tips appear promptly (setq tooltip-delay 0.1) ; default is 0.7 second ) ---------- Footnotes ---------- (1) When I start instances of Emacs that do not load my `.emacs' file or any site file, I also turn off blinking: emacs -q --no-site-file -eval '(blink-cursor-mode nil)' Or nowadays, using an even more sophisticated set of options, emacs -Q - D  File: eintr, Node: X11 Colors, Next: Miscellaneous, Prev: Simple Extension, Up: Emacs Initialization 16.12 X11 Colors ================ You can specify colors when you use Emacs with the MIT X Windowing system. I dislike the default colors and specify my own. Here are the expressions in my `.emacs' file that set values: ;; Set cursor color (set-cursor-color "white") ;; Set mouse color (set-mouse-color "white") ;; Set foreground and background (set-foreground-color "white") (set-background-color "darkblue") ;;; Set highlighting colors for isearch and drag (set-face-foreground 'highlight "white") (set-face-background 'highlight "blue") (set-face-foreground 'region "cyan") (set-face-background 'region "blue") (set-face-foreground 'secondary-selection "skyblue") (set-face-background 'secondary-selection "darkblue") ;; Set calendar highlighting colors (setq calendar-load-hook '(lambda () (set-face-foreground 'diary-face "skyblue") (set-face-background 'holiday-face "slate blue") (set-face-foreground 'holiday-face "white"))) The various shades of blue soothe my eye and prevent me from seeing the screen flicker. Alternatively, I could have set my specifications in various X initialization files. For example, I could set the foreground, background, cursor, and pointer (i.e., mouse) colors in my `~/.Xresources' file like this: Emacs*foreground: white Emacs*background: darkblue Emacs*cursorColor: white Emacs*pointerColor: white In any event, since it is not part of Emacs, I set the root color of my X window in my `~/.xinitrc' file, like this(1): xsetroot -solid Navy -fg white & ---------- Footnotes ---------- (1) I also run more modern window managers, such as Enlightenment, Gnome, or KDE; in those cases, I often specify an image rather than a plain color.  File: eintr, Node: Miscellaneous, Next: Mode Line, Prev: X11 Colors, Up: Emacs Initialization 16.13 Miscellaneous Settings for a `.emacs' File ================================================ Here are a few miscellaneous settings: - Set the shape and color of the mouse cursor: ; Cursor shapes are defined in ; `/usr/include/X11/cursorfont.h'; ; for example, the `target' cursor is number 128; ; the `top_left_arrow' cursor is number 132. (let ((mpointer (x-get-resource "*mpointer" "*emacs*mpointer"))) ;; If you have not set your mouse pointer ;; then set it, otherwise leave as is: (if (eq mpointer nil) (setq mpointer "132")) ; top_left_arrow (setq x-pointer-shape (string-to-int mpointer)) (set-mouse-color "white")) - Or you can set the values of a variety of features in an alist, like this: (setq-default default-frame-alist '((cursor-color . "white") (mouse-color . "white") (foreground-color . "white") (background-color . "DodgerBlue4") ;; (cursor-type . bar) (cursor-type . box) (tool-bar-lines . 0) (menu-bar-lines . 1) (width . 80) (height . 58) (font . "-Misc-Fixed-Medium-R-Normal--20-200-75-75-C-100-ISO8859-1") )) - Convert `-h' into and into `-h'. (Some older keyboards needed this, although I have not seen the problem recently.) ;; Translate `C-h' to . ; (keyboard-translate ?\C-h ?\C-?) ;; Translate to `C-h'. (keyboard-translate ?\C-? ?\C-h) - Turn off a blinking cursor! (if (fboundp 'blink-cursor-mode) (blink-cursor-mode -1)) or start GNU Emacs with the command `emacs -nbc'. - When using `grep' `-i' Ignore case distinctions `-n' Prefix each line of output with line number `-H' Print the filename for each match. `-e' Protect patterns beginning with a hyphen character, `-' (setq grep-command "grep -i -nH -e ") - Find an existing buffer, even if it has a different name This avoids problems with symbolic links. (setq find-file-existing-other-name t) - Set your language environment and default input method (set-language-environment "latin-1") ;; Remember you can enable or disable multilingual text input ;; with the `toggle-input-method'' (C-\) command (setq default-input-method "latin-1-prefix") If you want to write with Chinese `GB' characters, set this instead: (set-language-environment "Chinese-GB") (setq default-input-method "chinese-tonepy") Fixing Unpleasant Key Bindings .............................. Some systems bind keys unpleasantly. Sometimes, for example, the key appears in an awkward spot rather than at the far left of the home row. Usually, when people fix these sorts of keybindings, they do not change their `~/.emacs' file. Instead, they bind the proper keys on their consoles with the `loadkeys' or `install-keymap' commands in their boot script and then include `xmodmap' commands in their `.xinitrc' or `.Xsession' file for X Windows. For a boot script: loadkeys /usr/share/keymaps/i386/qwerty/emacs2.kmap.gz or install-keymap emacs2 For a `.xinitrc' or `.Xsession' file when the key is at the far left of the home row: # Bind the key labeled `Caps Lock' to `Control' # (Such a broken user interface suggests that keyboard manufacturers # think that computers are typewriters from 1885.) xmodmap -e "clear Lock" xmodmap -e "add Control = Caps_Lock" In a `.xinitrc' or `.Xsession' file, to convert an key to a key: # Some ill designed keyboards have a key labeled ALT and no Meta xmodmap -e "keysym Alt_L = Meta_L Alt_L"  File: eintr, Node: Mode Line, Prev: Miscellaneous, Up: Emacs Initialization 16.14 A Modified Mode Line ========================== Finally, a feature I really like: a modified mode line. When I work over a network, I forget which machine I am using. Also, I tend to I lose track of where I am, and which line point is on. So I reset my mode line to look like this: -:-- foo.texi rattlesnake:/home/bob/ Line 1 (Texinfo Fill) Top I am visiting a file called `foo.texi', on my machine `rattlesnake' in my `/home/bob' buffer. I am on line 1, in Texinfo mode, and am at the top of the buffer. My `.emacs' file has a section that looks like this: ;; Set a Mode Line that tells me which machine, which directory, ;; and which line I am on, plus the other customary information. (setq-default mode-line-format (quote (#("-" 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) mode-line-mule-info mode-line-modified mode-line-frame-identification " " mode-line-buffer-identification " " (:eval (substring (system-name) 0 (string-match "\\..+" (system-name)))) ":" default-directory #(" " 0 1 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (line-number-mode " Line %l ") global-mode-string #(" %[(" 0 6 (help-echo "mouse-1: select window, mouse-2: delete others ...")) (:eval (mode-line-mode-name)) mode-line-process minor-mode-alist #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...))) ")%] " (-3 . "%P") ;; "-%-" ))) Here, I redefine the default mode line. Most of the parts are from the original; but I make a few changes. I set the _default_ mode line format so as to permit various modes, such as Info, to override it. Many elements in the list are self-explanatory: `mode-line-modified' is a variable that tells whether the buffer has been modified, `mode-name' tells the name of the mode, and so on. However, the format looks complicated because of two features we have not discussed. The first string in the mode line is a dash or hyphen, `-'. In the old days, it would have been specified simply as `"-"'. But nowadays, Emacs can add properties to a string, such as highlighting or, as in this case, a help feature. If you place your mouse cursor over the hyphen, some help information appears (By default, you must wait seven-tenths of a second before the information appears. You can change that timing by changing the value of `tooltip-delay'.) The new string format has a special syntax: #("-" 0 1 (help-echo "mouse-1: select window, ...")) The `#(' begins a list. The first element of the list is the string itself, just one `-'. The second and third elements specify the range over which the fourth element applies. A range starts _after_ a character, so a zero means the range starts just before the first character; a 1 means that the range ends just after the first character. The third element is the property for the range. It consists of a property list, a property name, in this case, `help-echo', followed by a value, in this case, a string. The second, third, and fourth elements of this new string format can be repeated. *Note Text Properties: (elisp)Text Properties, and see *note Mode Line Format: (elisp)Mode Line Format, for more information. `mode-line-buffer-identification' displays the current buffer name. It is a list beginning `(#("%12b" 0 4 ...'. The `#(' begins the list. The `"%12b"' displays the current buffer name, using the `buffer-name' function with which we are familiar; the `12' specifies the maximum number of characters that will be displayed. When a name has fewer characters, whitespace is added to fill out to this number. (Buffer names can and often should be longer than 12 characters; this length works well in a typical 80 column wide window.) `:eval' says to evaluate the following form and use the result as a string to display. In this case, the expression displays the first component of the full system name. The end of the first component is a `.' (`period'), so I use the `string-match' function to tell me the length of the first component. The substring from the zeroth character to that length is the name of the machine. This is the expression: (:eval (substring (system-name) 0 (string-match "\\..+" (system-name)))) `%[' and `%]' cause a pair of square brackets to appear for each recursive editing level. `%n' says `Narrow' when narrowing is in effect. `%P' tells you the percentage of the buffer that is above the bottom of the window, or `Top', `Bottom', or `All'. (A lower case `p' tell you the percentage above the _top_ of the window.) `%-' inserts enough dashes to fill out the line. Remember, "You don't have to like Emacs to like it" -- your own Emacs can have different colors, different commands, and different keys than a default Emacs. On the other hand, if you want to bring up a plain `out of the box' Emacs, with no customization, type: emacs -q This will start an Emacs that does _not_ load your `~/.emacs' initialization file. A plain, default Emacs. Nothing more.  File: eintr, Node: Debugging, Next: Conclusion, Prev: Emacs Initialization, Up: Top 17 Debugging ************ GNU Emacs has two debuggers, `debug' and `edebug'. The first is built into the internals of Emacs and is always with you; the second requires that you instrument a function before you can use it. Both debuggers are described extensively in *note Debugging Lisp Programs: (elisp)Debugging. In this chapter, I will walk through a short example of each. * Menu: * debug:: How to use the built-in debugger. * debug-on-entry:: Start debugging when you call a function. * debug-on-quit:: Start debugging when you quit with C-g. * edebug:: How to use Edebug, a source level debugger. * Debugging Exercises::  File: eintr, Node: debug, Next: debug-on-entry, Prev: Debugging, Up: Debugging 17.1 `debug' ============ Suppose you have written a function definition that is intended to return the sum of the numbers 1 through a given number. (This is the `triangle' function discussed earlier. *Note Example with Decrementing Counter: Decrementing Example, for a discussion.) However, your function definition has a bug. You have mistyped `1=' for `1-'. Here is the broken definition: (defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) ; Error here. total)) If you are reading this in Info, you can evaluate this definition in the normal fashion. You will see `triangle-bugged' appear in the echo area. Now evaluate the `triangle-bugged' function with an argument of 4: (triangle-bugged 4) In a recent GNU Emacs, you will create and enter a `*Backtrace*' buffer that says: ---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) (1= number) (setq number (1= number)) (while (> number 0) (setq total (+ total number)) (setq number (1= number))) (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) triangle-bugged(4) eval((triangle-bugged 4)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- (I have reformatted this example slightly; the debugger does not fold long lines. As usual, you can quit the debugger by typing `q' in the `*Backtrace*' buffer.) In practice, for a bug as simple as this, the `Lisp error' line will tell you what you need to know to correct the definition. The function `1=' is `void'. However, suppose you are not quite certain what is going on? You can read the complete backtrace. In this case, you need to run a recent GNU Emacs, which automatically starts the debugger that puts you in the `*Backtrace*' buffer; or else, you need to start the debugger manually as described below. Read the `*Backtrace*' buffer from the bottom up; it tells you what Emacs did that led to the error. Emacs made an interactive call to `C-x C-e' (`eval-last-sexp'), which led to the evaluation of the `triangle-bugged' expression. Each line above tells you what the Lisp interpreter evaluated next. The third line from the top of the buffer is (setq number (1= number)) Emacs tried to evaluate this expression; in order to do so, it tried to evaluate the inner expression shown on the second line from the top: (1= number) This is where the error occurred; as the top line says: Debugger entered--Lisp error: (void-function 1=) You can correct the mistake, re-evaluate the function definition, and then run your test again.  File: eintr, Node: debug-on-entry, Next: debug-on-quit, Prev: debug, Up: Debugging 17.2 `debug-on-entry' ===================== A recent GNU Emacs starts the debugger automatically when your function has an error. Incidentally, you can start the debugger manually for all versions of Emacs; the advantage is that the debugger runs even if you do not have a bug in your code. Sometimes your code will be free of bugs! You can enter the debugger when you call the function by calling `debug-on-entry'. Type: M-x debug-on-entry RET triangle-bugged RET Now, evaluate the following: (triangle-bugged 5) All versions of Emacs will create a `*Backtrace*' buffer and tell you that it is beginning to evaluate the `triangle-bugged' function: ---------- Buffer: *Backtrace* ---------- Debugger entered--entering a function: * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- In the `*Backtrace*' buffer, type `d'. Emacs will evaluate the first expression in `triangle-bugged'; the buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- Now, type `d' again, eight times, slowly. Each time you type `d', Emacs will evaluate another expression in the function definition. Eventually, the buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--beginning evaluation of function call form: * (setq number (1= number)) * (while (> number 0) (setq total (+ total number)) (setq number (1= number))) * (let ((total 0)) (while (> number 0) (setq total ...) (setq number ...)) total) * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ---------- Finally, after you type `d' two more times, Emacs will reach the error, and the top two lines of the `*Backtrace*' buffer will look like this: ---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function 1=) * (1= number) ... ---------- Buffer: *Backtrace* ---------- By typing `d', you were able to step through the function. You can quit a `*Backtrace*' buffer by typing `q' in it; this quits the trace, but does not cancel `debug-on-entry'. To cancel the effect of `debug-on-entry', call `cancel-debug-on-entry' and the name of the function, like this: M-x cancel-debug-on-entry RET triangle-bugged RET (If you are reading this in Info, cancel `debug-on-entry' now.)  File: eintr, Node: debug-on-quit, Next: edebug, Prev: debug-on-entry, Up: Debugging 17.3 `debug-on-quit' and `(debug)' ================================== In addition to setting `debug-on-error' or calling `debug-on-entry', there are two other ways to start `debug'. You can start `debug' whenever you type `C-g' (`keyboard-quit') by setting the variable `debug-on-quit' to `t'. This is useful for debugging infinite loops. Or, you can insert a line that says `(debug)' into your code where you want the debugger to start, like this: (defun triangle-bugged (number) "Return sum of numbers 1 through NUMBER inclusive." (let ((total 0)) (while (> number 0) (setq total (+ total number)) (debug) ; Start debugger. (setq number (1= number))) ; Error here. total)) The `debug' function is described in detail in *note The Lisp Debugger: (elisp)Debugger.  File: eintr, Node: edebug, Next: Debugging Exercises, Prev: debug-on-quit, Up: Debugging 17.4 The `edebug' Source Level Debugger ======================================= Edebug is a source level debugger. Edebug normally displays the source of the code you are debugging, with an arrow at the left that shows which line you are currently executing. You can walk through the execution of a function, line by line, or run quickly until reaching a "breakpoint" where execution stops. Edebug is described in *note Edebug: (elisp)edebug. Here is a bugged function definition for `triangle-recursively'. *Note Recursion in place of a counter: Recursive triangle function, for a review of it. (defun triangle-recursively-bugged (number) "Return sum of numbers 1 through NUMBER inclusive. Uses recursion." (if (= number 1) 1 (+ number (triangle-recursively-bugged (1= number))))) ; Error here. Normally, you would install this definition by positioning your cursor after the function's closing parenthesis and typing `C-x C-e' (`eval-last-sexp') or else by positioning your cursor within the definition and typing `C-M-x' (`eval-defun'). (By default, the `eval-defun' command works only in Emacs Lisp mode or in Lisp Interaction mode.) However, to prepare this function definition for Edebug, you must first "instrument" the code using a different command. You can do this by positioning your cursor within or just after the definition and typing M-x edebug-defun RET This will cause Emacs to load Edebug automatically if it is not already loaded, and properly instrument the function. After instrumenting the function, place your cursor after the following expression and type `C-x C-e' (`eval-last-sexp'): (triangle-recursively-bugged 3) You will be jumped back to the source for `triangle-recursively-bugged' and the cursor positioned at the beginning of the `if' line of the function. Also, you will see an arrowhead at the left hand side of that line. The arrowhead marks the line where the function is executing. (In the following examples, we show the arrowhead with `=>'; in a windowing system, you may see the arrowhead as a solid triangle in the window `fringe'.) =>-!-(if (= number 1) In the example, the location of point is displayed as `-!-' (in a printed book, it is displayed with a five pointed star). If you now press , point will move to the next expression to be executed; the line will look like this: =>(if -!-(= number 1) As you continue to press , point will move from expression to expression. At the same time, whenever an expression returns a value, that value will be displayed in the echo area. For example, after you move point past `number', you will see the following: Result: 3 (#o3, #x3, ?\C-c) This means the value of `number' is 3, which is octal three, hexadecimal three, and ASCII `control-c' (the third letter of the alphabet, in case you need to know this information). You can continue moving through the code until you reach the line with the error. Before evaluation, that line looks like this: => -!-(1= number))))) ; Error here. When you press once again, you will produce an error message that says: Symbol's function definition is void: 1= This is the bug. Press `q' to quit Edebug. To remove instrumentation from a function definition, simply re-evaluate it with a command that does not instrument it. For example, you could place your cursor after the definition's closing parenthesis and type `C-x C-e'. Edebug does a great deal more than walk with you through a function. You can set it so it races through on its own, stopping only at an error or at specified stopping points; you can cause it to display the changing values of various expressions; you can find out how many times a function is called, and more. Edebug is described in *note Edebug: (elisp)edebug.