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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001, | |
6ed161e1 | 4 | @c 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/lists |
b8d4c8d0 GM |
7 | @node Lists, Sequences Arrays Vectors, Strings and Characters, Top |
8 | @chapter Lists | |
9 | @cindex lists | |
10 | @cindex element (of list) | |
11 | ||
12 | A @dfn{list} represents a sequence of zero or more elements (which may | |
13 | be any Lisp objects). The important difference between lists and | |
14 | vectors is that two or more lists can share part of their structure; in | |
15 | addition, you can insert or delete elements in a list without copying | |
16 | the whole list. | |
17 | ||
18 | @menu | |
19 | * Cons Cells:: How lists are made out of cons cells. | |
20 | * List-related Predicates:: Is this object a list? Comparing two lists. | |
21 | * List Elements:: Extracting the pieces of a list. | |
22 | * Building Lists:: Creating list structure. | |
23 | * List Variables:: Modifying lists stored in variables. | |
24 | * Modifying Lists:: Storing new pieces into an existing list. | |
25 | * Sets And Lists:: A list can represent a finite mathematical set. | |
26 | * Association Lists:: A list can represent a finite relation or mapping. | |
27 | * Rings:: Managing a fixed-size ring of objects. | |
28 | @end menu | |
29 | ||
30 | @node Cons Cells | |
31 | @section Lists and Cons Cells | |
32 | @cindex lists and cons cells | |
33 | ||
34 | Lists in Lisp are not a primitive data type; they are built up from | |
35 | @dfn{cons cells}. A cons cell is a data object that represents an | |
36 | ordered pair. That is, it has two slots, and each slot @dfn{holds}, or | |
37 | @dfn{refers to}, some Lisp object. One slot is known as the @sc{car}, | |
38 | and the other is known as the @sc{cdr}. (These names are traditional; | |
39 | see @ref{Cons Cell Type}.) @sc{cdr} is pronounced ``could-er.'' | |
40 | ||
41 | We say that ``the @sc{car} of this cons cell is'' whatever object | |
42 | its @sc{car} slot currently holds, and likewise for the @sc{cdr}. | |
43 | ||
44 | A list is a series of cons cells ``chained together,'' so that each | |
45 | cell refers to the next one. There is one cons cell for each element of | |
46 | the list. By convention, the @sc{car}s of the cons cells hold the | |
47 | elements of the list, and the @sc{cdr}s are used to chain the list: the | |
48 | @sc{cdr} slot of each cons cell refers to the following cons cell. The | |
49 | @sc{cdr} of the last cons cell is @code{nil}. This asymmetry between | |
50 | the @sc{car} and the @sc{cdr} is entirely a matter of convention; at the | |
51 | level of cons cells, the @sc{car} and @sc{cdr} slots have the same | |
52 | characteristics. | |
53 | ||
54 | @cindex true list | |
55 | Since @code{nil} is the conventional value to put in the @sc{cdr} of | |
56 | the last cons cell in the list, we call that case a @dfn{true list}. | |
57 | ||
58 | In Lisp, we consider the symbol @code{nil} a list as well as a | |
59 | symbol; it is the list with no elements. For convenience, the symbol | |
60 | @code{nil} is considered to have @code{nil} as its @sc{cdr} (and also | |
61 | as its @sc{car}). Therefore, the @sc{cdr} of a true list is always a | |
62 | true list. | |
63 | ||
64 | @cindex dotted list | |
65 | @cindex circular list | |
66 | If the @sc{cdr} of a list's last cons cell is some other value, | |
67 | neither @code{nil} nor another cons cell, we call the structure a | |
68 | @dfn{dotted list}, since its printed representation would use | |
69 | @samp{.}. There is one other possibility: some cons cell's @sc{cdr} | |
70 | could point to one of the previous cons cells in the list. We call | |
71 | that structure a @dfn{circular list}. | |
72 | ||
73 | For some purposes, it does not matter whether a list is true, | |
74 | circular or dotted. If the program doesn't look far enough down the | |
75 | list to see the @sc{cdr} of the final cons cell, it won't care. | |
76 | However, some functions that operate on lists demand true lists and | |
77 | signal errors if given a dotted list. Most functions that try to find | |
78 | the end of a list enter infinite loops if given a circular list. | |
79 | ||
80 | @cindex list structure | |
81 | Because most cons cells are used as part of lists, the phrase | |
82 | @dfn{list structure} has come to mean any structure made out of cons | |
83 | cells. | |
84 | ||
85 | The @sc{cdr} of any nonempty true list @var{l} is a list containing all the | |
86 | elements of @var{l} except the first. | |
87 | ||
88 | @xref{Cons Cell Type}, for the read and print syntax of cons cells and | |
89 | lists, and for ``box and arrow'' illustrations of lists. | |
90 | ||
91 | @node List-related Predicates | |
92 | @section Predicates on Lists | |
93 | ||
94 | The following predicates test whether a Lisp object is an atom, | |
95 | whether it is a cons cell or is a list, or whether it is the | |
96 | distinguished object @code{nil}. (Many of these predicates can be | |
97 | defined in terms of the others, but they are used so often that it is | |
98 | worth having all of them.) | |
99 | ||
100 | @defun consp object | |
101 | This function returns @code{t} if @var{object} is a cons cell, @code{nil} | |
102 | otherwise. @code{nil} is not a cons cell, although it @emph{is} a list. | |
103 | @end defun | |
104 | ||
105 | @defun atom object | |
106 | This function returns @code{t} if @var{object} is an atom, @code{nil} | |
107 | otherwise. All objects except cons cells are atoms. The symbol | |
108 | @code{nil} is an atom and is also a list; it is the only Lisp object | |
109 | that is both. | |
110 | ||
111 | @example | |
112 | (atom @var{object}) @equiv{} (not (consp @var{object})) | |
113 | @end example | |
114 | @end defun | |
115 | ||
116 | @defun listp object | |
117 | This function returns @code{t} if @var{object} is a cons cell or | |
118 | @code{nil}. Otherwise, it returns @code{nil}. | |
119 | ||
120 | @example | |
121 | @group | |
122 | (listp '(1)) | |
123 | @result{} t | |
124 | @end group | |
125 | @group | |
126 | (listp '()) | |
127 | @result{} t | |
128 | @end group | |
129 | @end example | |
130 | @end defun | |
131 | ||
132 | @defun nlistp object | |
133 | This function is the opposite of @code{listp}: it returns @code{t} if | |
134 | @var{object} is not a list. Otherwise, it returns @code{nil}. | |
135 | ||
136 | @example | |
137 | (listp @var{object}) @equiv{} (not (nlistp @var{object})) | |
138 | @end example | |
139 | @end defun | |
140 | ||
141 | @defun null object | |
142 | This function returns @code{t} if @var{object} is @code{nil}, and | |
143 | returns @code{nil} otherwise. This function is identical to @code{not}, | |
144 | but as a matter of clarity we use @code{null} when @var{object} is | |
145 | considered a list and @code{not} when it is considered a truth value | |
146 | (see @code{not} in @ref{Combining Conditions}). | |
147 | ||
148 | @example | |
149 | @group | |
150 | (null '(1)) | |
151 | @result{} nil | |
152 | @end group | |
153 | @group | |
154 | (null '()) | |
155 | @result{} t | |
156 | @end group | |
157 | @end example | |
158 | @end defun | |
159 | ||
160 | ||
161 | @node List Elements | |
162 | @section Accessing Elements of Lists | |
163 | @cindex list elements | |
164 | ||
165 | @defun car cons-cell | |
166 | This function returns the value referred to by the first slot of the | |
167 | cons cell @var{cons-cell}. Expressed another way, this function | |
168 | returns the @sc{car} of @var{cons-cell}. | |
169 | ||
170 | As a special case, if @var{cons-cell} is @code{nil}, then @code{car} | |
171 | is defined to return @code{nil}; therefore, any list is a valid argument | |
172 | for @code{car}. An error is signaled if the argument is not a cons cell | |
173 | or @code{nil}. | |
174 | ||
175 | @example | |
176 | @group | |
177 | (car '(a b c)) | |
178 | @result{} a | |
179 | @end group | |
180 | @group | |
181 | (car '()) | |
182 | @result{} nil | |
183 | @end group | |
184 | @end example | |
185 | @end defun | |
186 | ||
187 | @defun cdr cons-cell | |
188 | This function returns the value referred to by the second slot of | |
189 | the cons cell @var{cons-cell}. Expressed another way, this function | |
190 | returns the @sc{cdr} of @var{cons-cell}. | |
191 | ||
192 | As a special case, if @var{cons-cell} is @code{nil}, then @code{cdr} | |
193 | is defined to return @code{nil}; therefore, any list is a valid argument | |
194 | for @code{cdr}. An error is signaled if the argument is not a cons cell | |
195 | or @code{nil}. | |
196 | ||
197 | @example | |
198 | @group | |
199 | (cdr '(a b c)) | |
200 | @result{} (b c) | |
201 | @end group | |
202 | @group | |
203 | (cdr '()) | |
204 | @result{} nil | |
205 | @end group | |
206 | @end example | |
207 | @end defun | |
208 | ||
209 | @defun car-safe object | |
210 | This function lets you take the @sc{car} of a cons cell while avoiding | |
211 | errors for other data types. It returns the @sc{car} of @var{object} if | |
212 | @var{object} is a cons cell, @code{nil} otherwise. This is in contrast | |
213 | to @code{car}, which signals an error if @var{object} is not a list. | |
214 | ||
215 | @example | |
216 | @group | |
217 | (car-safe @var{object}) | |
218 | @equiv{} | |
219 | (let ((x @var{object})) | |
220 | (if (consp x) | |
221 | (car x) | |
222 | nil)) | |
223 | @end group | |
224 | @end example | |
225 | @end defun | |
226 | ||
227 | @defun cdr-safe object | |
228 | This function lets you take the @sc{cdr} of a cons cell while | |
229 | avoiding errors for other data types. It returns the @sc{cdr} of | |
230 | @var{object} if @var{object} is a cons cell, @code{nil} otherwise. | |
231 | This is in contrast to @code{cdr}, which signals an error if | |
232 | @var{object} is not a list. | |
233 | ||
234 | @example | |
235 | @group | |
236 | (cdr-safe @var{object}) | |
237 | @equiv{} | |
238 | (let ((x @var{object})) | |
239 | (if (consp x) | |
240 | (cdr x) | |
241 | nil)) | |
242 | @end group | |
243 | @end example | |
244 | @end defun | |
245 | ||
246 | @defmac pop listname | |
247 | This macro is a way of examining the @sc{car} of a list, | |
248 | and taking it off the list, all at once. | |
249 | ||
250 | It operates on the list which is stored in the symbol @var{listname}. | |
251 | It removes this element from the list by setting @var{listname} | |
252 | to the @sc{cdr} of its old value---but it also returns the @sc{car} | |
253 | of that list, which is the element being removed. | |
254 | ||
255 | @example | |
256 | x | |
257 | @result{} (a b c) | |
258 | (pop x) | |
259 | @result{} a | |
260 | x | |
261 | @result{} (b c) | |
262 | @end example | |
263 | @end defmac | |
264 | ||
265 | @defun nth n list | |
266 | @anchor{Definition of nth} | |
267 | This function returns the @var{n}th element of @var{list}. Elements | |
268 | are numbered starting with zero, so the @sc{car} of @var{list} is | |
269 | element number zero. If the length of @var{list} is @var{n} or less, | |
270 | the value is @code{nil}. | |
271 | ||
272 | If @var{n} is negative, @code{nth} returns the first element of | |
273 | @var{list}. | |
274 | ||
275 | @example | |
276 | @group | |
277 | (nth 2 '(1 2 3 4)) | |
278 | @result{} 3 | |
279 | @end group | |
280 | @group | |
281 | (nth 10 '(1 2 3 4)) | |
282 | @result{} nil | |
283 | @end group | |
284 | @group | |
285 | (nth -3 '(1 2 3 4)) | |
286 | @result{} 1 | |
287 | ||
288 | (nth n x) @equiv{} (car (nthcdr n x)) | |
289 | @end group | |
290 | @end example | |
291 | ||
292 | The function @code{elt} is similar, but applies to any kind of sequence. | |
293 | For historical reasons, it takes its arguments in the opposite order. | |
294 | @xref{Sequence Functions}. | |
295 | @end defun | |
296 | ||
297 | @defun nthcdr n list | |
298 | This function returns the @var{n}th @sc{cdr} of @var{list}. In other | |
299 | words, it skips past the first @var{n} links of @var{list} and returns | |
300 | what follows. | |
301 | ||
302 | If @var{n} is zero or negative, @code{nthcdr} returns all of | |
303 | @var{list}. If the length of @var{list} is @var{n} or less, | |
304 | @code{nthcdr} returns @code{nil}. | |
305 | ||
306 | @example | |
307 | @group | |
308 | (nthcdr 1 '(1 2 3 4)) | |
309 | @result{} (2 3 4) | |
310 | @end group | |
311 | @group | |
312 | (nthcdr 10 '(1 2 3 4)) | |
313 | @result{} nil | |
314 | @end group | |
315 | @group | |
316 | (nthcdr -3 '(1 2 3 4)) | |
317 | @result{} (1 2 3 4) | |
318 | @end group | |
319 | @end example | |
320 | @end defun | |
321 | ||
322 | @defun last list &optional n | |
323 | This function returns the last link of @var{list}. The @code{car} of | |
324 | this link is the list's last element. If @var{list} is null, | |
325 | @code{nil} is returned. If @var{n} is non-@code{nil}, the | |
326 | @var{n}th-to-last link is returned instead, or the whole of @var{list} | |
327 | if @var{n} is bigger than @var{list}'s length. | |
328 | @end defun | |
329 | ||
330 | @defun safe-length list | |
331 | @anchor{Definition of safe-length} | |
332 | This function returns the length of @var{list}, with no risk of either | |
333 | an error or an infinite loop. It generally returns the number of | |
334 | distinct cons cells in the list. However, for circular lists, | |
335 | the value is just an upper bound; it is often too large. | |
336 | ||
337 | If @var{list} is not @code{nil} or a cons cell, @code{safe-length} | |
338 | returns 0. | |
339 | @end defun | |
340 | ||
341 | The most common way to compute the length of a list, when you are not | |
342 | worried that it may be circular, is with @code{length}. @xref{Sequence | |
343 | Functions}. | |
344 | ||
345 | @defun caar cons-cell | |
346 | This is the same as @code{(car (car @var{cons-cell}))}. | |
347 | @end defun | |
348 | ||
349 | @defun cadr cons-cell | |
350 | This is the same as @code{(car (cdr @var{cons-cell}))} | |
351 | or @code{(nth 1 @var{cons-cell})}. | |
352 | @end defun | |
353 | ||
354 | @defun cdar cons-cell | |
355 | This is the same as @code{(cdr (car @var{cons-cell}))}. | |
356 | @end defun | |
357 | ||
358 | @defun cddr cons-cell | |
359 | This is the same as @code{(cdr (cdr @var{cons-cell}))} | |
360 | or @code{(nthcdr 2 @var{cons-cell})}. | |
361 | @end defun | |
362 | ||
363 | @defun butlast x &optional n | |
364 | This function returns the list @var{x} with the last element, | |
365 | or the last @var{n} elements, removed. If @var{n} is greater | |
366 | than zero it makes a copy of the list so as not to damage the | |
367 | original list. In general, @code{(append (butlast @var{x} @var{n}) | |
368 | (last @var{x} @var{n}))} will return a list equal to @var{x}. | |
369 | @end defun | |
370 | ||
371 | @defun nbutlast x &optional n | |
372 | This is a version of @code{butlast} that works by destructively | |
373 | modifying the @code{cdr} of the appropriate element, rather than | |
374 | making a copy of the list. | |
375 | @end defun | |
376 | ||
377 | @node Building Lists | |
378 | @comment node-name, next, previous, up | |
379 | @section Building Cons Cells and Lists | |
380 | @cindex cons cells | |
381 | @cindex building lists | |
382 | ||
383 | Many functions build lists, as lists reside at the very heart of Lisp. | |
384 | @code{cons} is the fundamental list-building function; however, it is | |
385 | interesting to note that @code{list} is used more times in the source | |
386 | code for Emacs than @code{cons}. | |
387 | ||
388 | @defun cons object1 object2 | |
389 | This function is the most basic function for building new list | |
390 | structure. It creates a new cons cell, making @var{object1} the | |
391 | @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new | |
392 | cons cell. The arguments @var{object1} and @var{object2} may be any | |
393 | Lisp objects, but most often @var{object2} is a list. | |
394 | ||
395 | @example | |
396 | @group | |
397 | (cons 1 '(2)) | |
398 | @result{} (1 2) | |
399 | @end group | |
400 | @group | |
401 | (cons 1 '()) | |
402 | @result{} (1) | |
403 | @end group | |
404 | @group | |
405 | (cons 1 2) | |
406 | @result{} (1 . 2) | |
407 | @end group | |
408 | @end example | |
409 | ||
410 | @cindex consing | |
411 | @code{cons} is often used to add a single element to the front of a | |
412 | list. This is called @dfn{consing the element onto the list}. | |
413 | @footnote{There is no strictly equivalent way to add an element to | |
414 | the end of a list. You can use @code{(append @var{listname} (list | |
415 | @var{newelt}))}, which creates a whole new list by copying @var{listname} | |
416 | and adding @var{newelt} to its end. Or you can use @code{(nconc | |
417 | @var{listname} (list @var{newelt}))}, which modifies @var{listname} | |
418 | by following all the @sc{cdr}s and then replacing the terminating | |
419 | @code{nil}. Compare this to adding an element to the beginning of a | |
420 | list with @code{cons}, which neither copies nor modifies the list.} | |
421 | For example: | |
422 | ||
423 | @example | |
424 | (setq list (cons newelt list)) | |
425 | @end example | |
426 | ||
427 | Note that there is no conflict between the variable named @code{list} | |
428 | used in this example and the function named @code{list} described below; | |
429 | any symbol can serve both purposes. | |
430 | @end defun | |
431 | ||
432 | @defun list &rest objects | |
433 | This function creates a list with @var{objects} as its elements. The | |
434 | resulting list is always @code{nil}-terminated. If no @var{objects} | |
435 | are given, the empty list is returned. | |
436 | ||
437 | @example | |
438 | @group | |
439 | (list 1 2 3 4 5) | |
440 | @result{} (1 2 3 4 5) | |
441 | @end group | |
442 | @group | |
443 | (list 1 2 '(3 4 5) 'foo) | |
444 | @result{} (1 2 (3 4 5) foo) | |
445 | @end group | |
446 | @group | |
447 | (list) | |
448 | @result{} nil | |
449 | @end group | |
450 | @end example | |
451 | @end defun | |
452 | ||
453 | @defun make-list length object | |
454 | This function creates a list of @var{length} elements, in which each | |
455 | element is @var{object}. Compare @code{make-list} with | |
456 | @code{make-string} (@pxref{Creating Strings}). | |
457 | ||
458 | @example | |
459 | @group | |
460 | (make-list 3 'pigs) | |
461 | @result{} (pigs pigs pigs) | |
462 | @end group | |
463 | @group | |
464 | (make-list 0 'pigs) | |
465 | @result{} nil | |
466 | @end group | |
467 | @group | |
468 | (setq l (make-list 3 '(a b)) | |
469 | @result{} ((a b) (a b) (a b)) | |
470 | (eq (car l) (cadr l)) | |
471 | @result{} t | |
472 | @end group | |
473 | @end example | |
474 | @end defun | |
475 | ||
476 | @defun append &rest sequences | |
477 | @cindex copying lists | |
478 | This function returns a list containing all the elements of | |
479 | @var{sequences}. The @var{sequences} may be lists, vectors, | |
480 | bool-vectors, or strings, but the last one should usually be a list. | |
481 | All arguments except the last one are copied, so none of the arguments | |
482 | is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join | |
483 | lists with no copying.) | |
484 | ||
485 | More generally, the final argument to @code{append} may be any Lisp | |
486 | object. The final argument is not copied or converted; it becomes the | |
487 | @sc{cdr} of the last cons cell in the new list. If the final argument | |
488 | is itself a list, then its elements become in effect elements of the | |
489 | result list. If the final element is not a list, the result is a | |
490 | dotted list since its final @sc{cdr} is not @code{nil} as required | |
491 | in a true list. | |
b8d4c8d0 GM |
492 | @end defun |
493 | ||
494 | Here is an example of using @code{append}: | |
495 | ||
496 | @example | |
497 | @group | |
498 | (setq trees '(pine oak)) | |
499 | @result{} (pine oak) | |
500 | (setq more-trees (append '(maple birch) trees)) | |
501 | @result{} (maple birch pine oak) | |
502 | @end group | |
503 | ||
504 | @group | |
505 | trees | |
506 | @result{} (pine oak) | |
507 | more-trees | |
508 | @result{} (maple birch pine oak) | |
509 | @end group | |
510 | @group | |
511 | (eq trees (cdr (cdr more-trees))) | |
512 | @result{} t | |
513 | @end group | |
514 | @end example | |
515 | ||
516 | You can see how @code{append} works by looking at a box diagram. The | |
517 | variable @code{trees} is set to the list @code{(pine oak)} and then the | |
518 | variable @code{more-trees} is set to the list @code{(maple birch pine | |
519 | oak)}. However, the variable @code{trees} continues to refer to the | |
520 | original list: | |
521 | ||
522 | @smallexample | |
523 | @group | |
524 | more-trees trees | |
525 | | | | |
526 | | --- --- --- --- -> --- --- --- --- | |
527 | --> | | |--> | | |--> | | |--> | | |--> nil | |
528 | --- --- --- --- --- --- --- --- | |
529 | | | | | | |
530 | | | | | | |
531 | --> maple -->birch --> pine --> oak | |
532 | @end group | |
533 | @end smallexample | |
534 | ||
535 | An empty sequence contributes nothing to the value returned by | |
536 | @code{append}. As a consequence of this, a final @code{nil} argument | |
537 | forces a copy of the previous argument: | |
538 | ||
539 | @example | |
540 | @group | |
541 | trees | |
542 | @result{} (pine oak) | |
543 | @end group | |
544 | @group | |
545 | (setq wood (append trees nil)) | |
546 | @result{} (pine oak) | |
547 | @end group | |
548 | @group | |
549 | wood | |
550 | @result{} (pine oak) | |
551 | @end group | |
552 | @group | |
553 | (eq wood trees) | |
554 | @result{} nil | |
555 | @end group | |
556 | @end example | |
557 | ||
558 | @noindent | |
559 | This once was the usual way to copy a list, before the function | |
560 | @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}. | |
561 | ||
562 | Here we show the use of vectors and strings as arguments to @code{append}: | |
563 | ||
564 | @example | |
565 | @group | |
566 | (append [a b] "cd" nil) | |
567 | @result{} (a b 99 100) | |
568 | @end group | |
569 | @end example | |
570 | ||
571 | With the help of @code{apply} (@pxref{Calling Functions}), we can append | |
572 | all the lists in a list of lists: | |
573 | ||
574 | @example | |
575 | @group | |
576 | (apply 'append '((a b c) nil (x y z) nil)) | |
577 | @result{} (a b c x y z) | |
578 | @end group | |
579 | @end example | |
580 | ||
581 | If no @var{sequences} are given, @code{nil} is returned: | |
582 | ||
583 | @example | |
584 | @group | |
585 | (append) | |
586 | @result{} nil | |
587 | @end group | |
588 | @end example | |
589 | ||
590 | Here are some examples where the final argument is not a list: | |
591 | ||
592 | @example | |
593 | (append '(x y) 'z) | |
594 | @result{} (x y . z) | |
595 | (append '(x y) [z]) | |
596 | @result{} (x y . [z]) | |
597 | @end example | |
598 | ||
599 | @noindent | |
600 | The second example shows that when the final argument is a sequence but | |
601 | not a list, the sequence's elements do not become elements of the | |
602 | resulting list. Instead, the sequence becomes the final @sc{cdr}, like | |
603 | any other non-list final argument. | |
604 | ||
605 | @defun reverse list | |
606 | This function creates a new list whose elements are the elements of | |
607 | @var{list}, but in reverse order. The original argument @var{list} is | |
608 | @emph{not} altered. | |
609 | ||
610 | @example | |
611 | @group | |
612 | (setq x '(1 2 3 4)) | |
613 | @result{} (1 2 3 4) | |
614 | @end group | |
615 | @group | |
616 | (reverse x) | |
617 | @result{} (4 3 2 1) | |
618 | x | |
619 | @result{} (1 2 3 4) | |
620 | @end group | |
621 | @end example | |
622 | @end defun | |
623 | ||
624 | @defun copy-tree tree &optional vecp | |
625 | This function returns a copy of the tree @code{tree}. If @var{tree} is a | |
626 | cons cell, this makes a new cons cell with the same @sc{car} and | |
627 | @sc{cdr}, then recursively copies the @sc{car} and @sc{cdr} in the | |
628 | same way. | |
629 | ||
630 | Normally, when @var{tree} is anything other than a cons cell, | |
631 | @code{copy-tree} simply returns @var{tree}. However, if @var{vecp} is | |
632 | non-@code{nil}, it copies vectors too (and operates recursively on | |
633 | their elements). | |
634 | @end defun | |
635 | ||
636 | @defun number-sequence from &optional to separation | |
637 | This returns a list of numbers starting with @var{from} and | |
638 | incrementing by @var{separation}, and ending at or just before | |
639 | @var{to}. @var{separation} can be positive or negative and defaults | |
640 | to 1. If @var{to} is @code{nil} or numerically equal to @var{from}, | |
641 | the value is the one-element list @code{(@var{from})}. If @var{to} is | |
642 | less than @var{from} with a positive @var{separation}, or greater than | |
643 | @var{from} with a negative @var{separation}, the value is @code{nil} | |
644 | because those arguments specify an empty sequence. | |
645 | ||
646 | If @var{separation} is 0 and @var{to} is neither @code{nil} nor | |
647 | numerically equal to @var{from}, @code{number-sequence} signals an | |
648 | error, since those arguments specify an infinite sequence. | |
649 | ||
650 | All arguments can be integers or floating point numbers. However, | |
651 | floating point arguments can be tricky, because floating point | |
652 | arithmetic is inexact. For instance, depending on the machine, it may | |
653 | quite well happen that @code{(number-sequence 0.4 0.6 0.2)} returns | |
654 | the one element list @code{(0.4)}, whereas | |
655 | @code{(number-sequence 0.4 0.8 0.2)} returns a list with three | |
656 | elements. The @var{n}th element of the list is computed by the exact | |
657 | formula @code{(+ @var{from} (* @var{n} @var{separation}))}. Thus, if | |
658 | one wants to make sure that @var{to} is included in the list, one can | |
659 | pass an expression of this exact type for @var{to}. Alternatively, | |
660 | one can replace @var{to} with a slightly larger value (or a slightly | |
661 | more negative value if @var{separation} is negative). | |
662 | ||
663 | Some examples: | |
664 | ||
665 | @example | |
666 | (number-sequence 4 9) | |
667 | @result{} (4 5 6 7 8 9) | |
668 | (number-sequence 9 4 -1) | |
669 | @result{} (9 8 7 6 5 4) | |
670 | (number-sequence 9 4 -2) | |
671 | @result{} (9 7 5) | |
672 | (number-sequence 8) | |
673 | @result{} (8) | |
674 | (number-sequence 8 5) | |
675 | @result{} nil | |
676 | (number-sequence 5 8 -1) | |
677 | @result{} nil | |
678 | (number-sequence 1.5 6 2) | |
679 | @result{} (1.5 3.5 5.5) | |
680 | @end example | |
681 | @end defun | |
682 | ||
683 | @node List Variables | |
684 | @section Modifying List Variables | |
685 | ||
686 | These functions, and one macro, provide convenient ways | |
687 | to modify a list which is stored in a variable. | |
688 | ||
689 | @defmac push newelt listname | |
690 | This macro provides an alternative way to write | |
691 | @code{(setq @var{listname} (cons @var{newelt} @var{listname}))}. | |
692 | ||
693 | @example | |
694 | (setq l '(a b)) | |
695 | @result{} (a b) | |
696 | (push 'c l) | |
697 | @result{} (c a b) | |
698 | l | |
699 | @result{} (c a b) | |
700 | @end example | |
701 | @end defmac | |
702 | ||
703 | Two functions modify lists that are the values of variables. | |
704 | ||
705 | @defun add-to-list symbol element &optional append compare-fn | |
706 | This function sets the variable @var{symbol} by consing @var{element} | |
707 | onto the old value, if @var{element} is not already a member of that | |
708 | value. It returns the resulting list, whether updated or not. The | |
709 | value of @var{symbol} had better be a list already before the call. | |
710 | @code{add-to-list} uses @var{compare-fn} to compare @var{element} | |
711 | against existing list members; if @var{compare-fn} is @code{nil}, it | |
712 | uses @code{equal}. | |
713 | ||
714 | Normally, if @var{element} is added, it is added to the front of | |
715 | @var{symbol}, but if the optional argument @var{append} is | |
716 | non-@code{nil}, it is added at the end. | |
717 | ||
718 | The argument @var{symbol} is not implicitly quoted; @code{add-to-list} | |
719 | is an ordinary function, like @code{set} and unlike @code{setq}. Quote | |
720 | the argument yourself if that is what you want. | |
721 | @end defun | |
722 | ||
723 | Here's a scenario showing how to use @code{add-to-list}: | |
724 | ||
725 | @example | |
726 | (setq foo '(a b)) | |
727 | @result{} (a b) | |
728 | ||
729 | (add-to-list 'foo 'c) ;; @r{Add @code{c}.} | |
730 | @result{} (c a b) | |
731 | ||
732 | (add-to-list 'foo 'b) ;; @r{No effect.} | |
733 | @result{} (c a b) | |
734 | ||
735 | foo ;; @r{@code{foo} was changed.} | |
736 | @result{} (c a b) | |
737 | @end example | |
738 | ||
739 | An equivalent expression for @code{(add-to-list '@var{var} | |
740 | @var{value})} is this: | |
741 | ||
742 | @example | |
743 | (or (member @var{value} @var{var}) | |
744 | (setq @var{var} (cons @var{value} @var{var}))) | |
745 | @end example | |
746 | ||
747 | @defun add-to-ordered-list symbol element &optional order | |
748 | This function sets the variable @var{symbol} by inserting | |
749 | @var{element} into the old value, which must be a list, at the | |
750 | position specified by @var{order}. If @var{element} is already a | |
751 | member of the list, its position in the list is adjusted according | |
752 | to @var{order}. Membership is tested using @code{eq}. | |
753 | This function returns the resulting list, whether updated or not. | |
754 | ||
755 | The @var{order} is typically a number (integer or float), and the | |
756 | elements of the list are sorted in non-decreasing numerical order. | |
757 | ||
758 | @var{order} may also be omitted or @code{nil}. Then the numeric order | |
759 | of @var{element} stays unchanged if it already has one; otherwise, | |
760 | @var{element} has no numeric order. Elements without a numeric list | |
761 | order are placed at the end of the list, in no particular order. | |
762 | ||
763 | Any other value for @var{order} removes the numeric order of @var{element} | |
764 | if it already has one; otherwise, it is equivalent to @code{nil}. | |
765 | ||
766 | The argument @var{symbol} is not implicitly quoted; | |
767 | @code{add-to-ordered-list} is an ordinary function, like @code{set} | |
768 | and unlike @code{setq}. Quote the argument yourself if that is what | |
769 | you want. | |
770 | ||
771 | The ordering information is stored in a hash table on @var{symbol}'s | |
772 | @code{list-order} property. | |
773 | @end defun | |
774 | ||
775 | Here's a scenario showing how to use @code{add-to-ordered-list}: | |
776 | ||
777 | @example | |
778 | (setq foo '()) | |
779 | @result{} nil | |
780 | ||
781 | (add-to-ordered-list 'foo 'a 1) ;; @r{Add @code{a}.} | |
782 | @result{} (a) | |
783 | ||
784 | (add-to-ordered-list 'foo 'c 3) ;; @r{Add @code{c}.} | |
785 | @result{} (a c) | |
786 | ||
787 | (add-to-ordered-list 'foo 'b 2) ;; @r{Add @code{b}.} | |
788 | @result{} (a b c) | |
789 | ||
790 | (add-to-ordered-list 'foo 'b 4) ;; @r{Move @code{b}.} | |
791 | @result{} (a c b) | |
792 | ||
793 | (add-to-ordered-list 'foo 'd) ;; @r{Append @code{d}.} | |
794 | @result{} (a c b d) | |
795 | ||
796 | (add-to-ordered-list 'foo 'e) ;; @r{Add @code{e}}. | |
797 | @result{} (a c b e d) | |
798 | ||
799 | foo ;; @r{@code{foo} was changed.} | |
800 | @result{} (a c b e d) | |
801 | @end example | |
802 | ||
803 | @node Modifying Lists | |
804 | @section Modifying Existing List Structure | |
805 | @cindex destructive list operations | |
806 | ||
807 | You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the | |
808 | primitives @code{setcar} and @code{setcdr}. We call these ``destructive'' | |
809 | operations because they change existing list structure. | |
810 | ||
811 | @cindex CL note---@code{rplaca} vs @code{setcar} | |
812 | @quotation | |
813 | @findex rplaca | |
814 | @findex rplacd | |
815 | @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and | |
816 | @code{rplacd} to alter list structure; they change structure the same | |
817 | way as @code{setcar} and @code{setcdr}, but the Common Lisp functions | |
818 | return the cons cell while @code{setcar} and @code{setcdr} return the | |
819 | new @sc{car} or @sc{cdr}. | |
820 | @end quotation | |
821 | ||
822 | @menu | |
823 | * Setcar:: Replacing an element in a list. | |
824 | * Setcdr:: Replacing part of the list backbone. | |
825 | This can be used to remove or add elements. | |
826 | * Rearrangement:: Reordering the elements in a list; combining lists. | |
827 | @end menu | |
828 | ||
829 | @node Setcar | |
830 | @subsection Altering List Elements with @code{setcar} | |
831 | ||
832 | Changing the @sc{car} of a cons cell is done with @code{setcar}. When | |
833 | used on a list, @code{setcar} replaces one element of a list with a | |
834 | different element. | |
835 | ||
836 | @defun setcar cons object | |
837 | This function stores @var{object} as the new @sc{car} of @var{cons}, | |
838 | replacing its previous @sc{car}. In other words, it changes the | |
839 | @sc{car} slot of @var{cons} to refer to @var{object}. It returns the | |
840 | value @var{object}. For example: | |
841 | ||
842 | @example | |
843 | @group | |
844 | (setq x '(1 2)) | |
845 | @result{} (1 2) | |
846 | @end group | |
847 | @group | |
848 | (setcar x 4) | |
849 | @result{} 4 | |
850 | @end group | |
851 | @group | |
852 | x | |
853 | @result{} (4 2) | |
854 | @end group | |
855 | @end example | |
856 | @end defun | |
857 | ||
858 | When a cons cell is part of the shared structure of several lists, | |
859 | storing a new @sc{car} into the cons changes one element of each of | |
860 | these lists. Here is an example: | |
861 | ||
862 | @example | |
863 | @group | |
864 | ;; @r{Create two lists that are partly shared.} | |
865 | (setq x1 '(a b c)) | |
866 | @result{} (a b c) | |
867 | (setq x2 (cons 'z (cdr x1))) | |
868 | @result{} (z b c) | |
869 | @end group | |
870 | ||
871 | @group | |
872 | ;; @r{Replace the @sc{car} of a shared link.} | |
873 | (setcar (cdr x1) 'foo) | |
874 | @result{} foo | |
875 | x1 ; @r{Both lists are changed.} | |
876 | @result{} (a foo c) | |
877 | x2 | |
878 | @result{} (z foo c) | |
879 | @end group | |
880 | ||
881 | @group | |
882 | ;; @r{Replace the @sc{car} of a link that is not shared.} | |
883 | (setcar x1 'baz) | |
884 | @result{} baz | |
885 | x1 ; @r{Only one list is changed.} | |
886 | @result{} (baz foo c) | |
887 | x2 | |
888 | @result{} (z foo c) | |
889 | @end group | |
890 | @end example | |
891 | ||
892 | Here is a graphical depiction of the shared structure of the two lists | |
893 | in the variables @code{x1} and @code{x2}, showing why replacing @code{b} | |
894 | changes them both: | |
895 | ||
896 | @example | |
897 | @group | |
898 | --- --- --- --- --- --- | |
899 | x1---> | | |----> | | |--> | | |--> nil | |
900 | --- --- --- --- --- --- | |
901 | | --> | | | |
902 | | | | | | |
903 | --> a | --> b --> c | |
904 | | | |
905 | --- --- | | |
906 | x2--> | | |-- | |
907 | --- --- | |
908 | | | |
909 | | | |
910 | --> z | |
911 | @end group | |
912 | @end example | |
913 | ||
914 | Here is an alternative form of box diagram, showing the same relationship: | |
915 | ||
916 | @example | |
917 | @group | |
918 | x1: | |
919 | -------------- -------------- -------------- | |
920 | | car | cdr | | car | cdr | | car | cdr | | |
921 | | a | o------->| b | o------->| c | nil | | |
922 | | | | -->| | | | | | | |
923 | -------------- | -------------- -------------- | |
924 | | | |
925 | x2: | | |
926 | -------------- | | |
927 | | car | cdr | | | |
928 | | z | o---- | |
929 | | | | | |
930 | -------------- | |
931 | @end group | |
932 | @end example | |
933 | ||
934 | @node Setcdr | |
935 | @subsection Altering the CDR of a List | |
936 | ||
937 | The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}: | |
938 | ||
939 | @defun setcdr cons object | |
940 | This function stores @var{object} as the new @sc{cdr} of @var{cons}, | |
941 | replacing its previous @sc{cdr}. In other words, it changes the | |
942 | @sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the | |
943 | value @var{object}. | |
944 | @end defun | |
945 | ||
946 | Here is an example of replacing the @sc{cdr} of a list with a | |
947 | different list. All but the first element of the list are removed in | |
948 | favor of a different sequence of elements. The first element is | |
949 | unchanged, because it resides in the @sc{car} of the list, and is not | |
950 | reached via the @sc{cdr}. | |
951 | ||
952 | @example | |
953 | @group | |
954 | (setq x '(1 2 3)) | |
955 | @result{} (1 2 3) | |
956 | @end group | |
957 | @group | |
958 | (setcdr x '(4)) | |
959 | @result{} (4) | |
960 | @end group | |
961 | @group | |
962 | x | |
963 | @result{} (1 4) | |
964 | @end group | |
965 | @end example | |
966 | ||
967 | You can delete elements from the middle of a list by altering the | |
968 | @sc{cdr}s of the cons cells in the list. For example, here we delete | |
969 | the second element, @code{b}, from the list @code{(a b c)}, by changing | |
970 | the @sc{cdr} of the first cons cell: | |
971 | ||
972 | @example | |
973 | @group | |
974 | (setq x1 '(a b c)) | |
975 | @result{} (a b c) | |
976 | (setcdr x1 (cdr (cdr x1))) | |
977 | @result{} (c) | |
978 | x1 | |
979 | @result{} (a c) | |
980 | @end group | |
981 | @end example | |
982 | ||
983 | Here is the result in box notation: | |
984 | ||
985 | @smallexample | |
986 | @group | |
987 | -------------------- | |
988 | | | | |
989 | -------------- | -------------- | -------------- | |
990 | | car | cdr | | | car | cdr | -->| car | cdr | | |
991 | | a | o----- | b | o-------->| c | nil | | |
992 | | | | | | | | | | | |
993 | -------------- -------------- -------------- | |
994 | @end group | |
995 | @end smallexample | |
996 | ||
997 | @noindent | |
998 | The second cons cell, which previously held the element @code{b}, still | |
999 | exists and its @sc{car} is still @code{b}, but it no longer forms part | |
1000 | of this list. | |
1001 | ||
1002 | It is equally easy to insert a new element by changing @sc{cdr}s: | |
1003 | ||
1004 | @example | |
1005 | @group | |
1006 | (setq x1 '(a b c)) | |
1007 | @result{} (a b c) | |
1008 | (setcdr x1 (cons 'd (cdr x1))) | |
1009 | @result{} (d b c) | |
1010 | x1 | |
1011 | @result{} (a d b c) | |
1012 | @end group | |
1013 | @end example | |
1014 | ||
1015 | Here is this result in box notation: | |
1016 | ||
1017 | @smallexample | |
1018 | @group | |
1019 | -------------- ------------- ------------- | |
1020 | | car | cdr | | car | cdr | | car | cdr | | |
1021 | | a | o | -->| b | o------->| c | nil | | |
1022 | | | | | | | | | | | | | |
1023 | --------- | -- | ------------- ------------- | |
1024 | | | | |
1025 | ----- -------- | |
1026 | | | | |
1027 | | --------------- | | |
1028 | | | car | cdr | | | |
1029 | -->| d | o------ | |
1030 | | | | | |
1031 | --------------- | |
1032 | @end group | |
1033 | @end smallexample | |
1034 | ||
1035 | @node Rearrangement | |
1036 | @subsection Functions that Rearrange Lists | |
1037 | @cindex rearrangement of lists | |
1038 | @cindex modification of lists | |
1039 | ||
1040 | Here are some functions that rearrange lists ``destructively'' by | |
1041 | modifying the @sc{cdr}s of their component cons cells. We call these | |
1042 | functions ``destructive'' because they chew up the original lists passed | |
1043 | to them as arguments, relinking their cons cells to form a new list that | |
1044 | is the returned value. | |
1045 | ||
1046 | @ifnottex | |
1047 | See @code{delq}, in @ref{Sets And Lists}, for another function | |
1048 | that modifies cons cells. | |
1049 | @end ifnottex | |
1050 | @iftex | |
1051 | The function @code{delq} in the following section is another example | |
1052 | of destructive list manipulation. | |
1053 | @end iftex | |
1054 | ||
1055 | @defun nconc &rest lists | |
1056 | @cindex concatenating lists | |
1057 | @cindex joining lists | |
1058 | This function returns a list containing all the elements of @var{lists}. | |
1059 | Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are | |
1060 | @emph{not} copied. Instead, the last @sc{cdr} of each of the | |
1061 | @var{lists} is changed to refer to the following list. The last of the | |
1062 | @var{lists} is not altered. For example: | |
1063 | ||
1064 | @example | |
1065 | @group | |
1066 | (setq x '(1 2 3)) | |
1067 | @result{} (1 2 3) | |
1068 | @end group | |
1069 | @group | |
1070 | (nconc x '(4 5)) | |
1071 | @result{} (1 2 3 4 5) | |
1072 | @end group | |
1073 | @group | |
1074 | x | |
1075 | @result{} (1 2 3 4 5) | |
1076 | @end group | |
1077 | @end example | |
1078 | ||
1079 | Since the last argument of @code{nconc} is not itself modified, it is | |
1080 | reasonable to use a constant list, such as @code{'(4 5)}, as in the | |
1081 | above example. For the same reason, the last argument need not be a | |
1082 | list: | |
1083 | ||
1084 | @example | |
1085 | @group | |
1086 | (setq x '(1 2 3)) | |
1087 | @result{} (1 2 3) | |
1088 | @end group | |
1089 | @group | |
1090 | (nconc x 'z) | |
1091 | @result{} (1 2 3 . z) | |
1092 | @end group | |
1093 | @group | |
1094 | x | |
1095 | @result{} (1 2 3 . z) | |
1096 | @end group | |
1097 | @end example | |
1098 | ||
1099 | However, the other arguments (all but the last) must be lists. | |
1100 | ||
1101 | A common pitfall is to use a quoted constant list as a non-last | |
1102 | argument to @code{nconc}. If you do this, your program will change | |
1103 | each time you run it! Here is what happens: | |
1104 | ||
1105 | @smallexample | |
1106 | @group | |
1107 | (defun add-foo (x) ; @r{We want this function to add} | |
1108 | (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.} | |
1109 | @end group | |
1110 | ||
1111 | @group | |
1112 | (symbol-function 'add-foo) | |
1113 | @result{} (lambda (x) (nconc (quote (foo)) x)) | |
1114 | @end group | |
1115 | ||
1116 | @group | |
1117 | (setq xx (add-foo '(1 2))) ; @r{It seems to work.} | |
1118 | @result{} (foo 1 2) | |
1119 | @end group | |
1120 | @group | |
1121 | (setq xy (add-foo '(3 4))) ; @r{What happened?} | |
1122 | @result{} (foo 1 2 3 4) | |
1123 | @end group | |
1124 | @group | |
1125 | (eq xx xy) | |
1126 | @result{} t | |
1127 | @end group | |
1128 | ||
1129 | @group | |
1130 | (symbol-function 'add-foo) | |
1131 | @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x))) | |
1132 | @end group | |
1133 | @end smallexample | |
1134 | @end defun | |
1135 | ||
1136 | @defun nreverse list | |
1137 | @cindex reversing a list | |
1138 | This function reverses the order of the elements of @var{list}. | |
1139 | Unlike @code{reverse}, @code{nreverse} alters its argument by reversing | |
1140 | the @sc{cdr}s in the cons cells forming the list. The cons cell that | |
1141 | used to be the last one in @var{list} becomes the first cons cell of the | |
1142 | value. | |
1143 | ||
1144 | For example: | |
1145 | ||
1146 | @example | |
1147 | @group | |
1148 | (setq x '(a b c)) | |
1149 | @result{} (a b c) | |
1150 | @end group | |
1151 | @group | |
1152 | x | |
1153 | @result{} (a b c) | |
1154 | (nreverse x) | |
1155 | @result{} (c b a) | |
1156 | @end group | |
1157 | @group | |
1158 | ;; @r{The cons cell that was first is now last.} | |
1159 | x | |
1160 | @result{} (a) | |
1161 | @end group | |
1162 | @end example | |
1163 | ||
1164 | To avoid confusion, we usually store the result of @code{nreverse} | |
1165 | back in the same variable which held the original list: | |
1166 | ||
1167 | @example | |
1168 | (setq x (nreverse x)) | |
1169 | @end example | |
1170 | ||
1171 | Here is the @code{nreverse} of our favorite example, @code{(a b c)}, | |
1172 | presented graphically: | |
1173 | ||
1174 | @smallexample | |
1175 | @group | |
1176 | @r{Original list head:} @r{Reversed list:} | |
1177 | ------------- ------------- ------------ | |
1178 | | car | cdr | | car | cdr | | car | cdr | | |
1179 | | a | nil |<-- | b | o |<-- | c | o | | |
1180 | | | | | | | | | | | | | | | |
1181 | ------------- | --------- | - | -------- | - | |
1182 | | | | | | |
1183 | ------------- ------------ | |
1184 | @end group | |
1185 | @end smallexample | |
1186 | @end defun | |
1187 | ||
1188 | @defun sort list predicate | |
1189 | @cindex stable sort | |
1190 | @cindex sorting lists | |
1191 | This function sorts @var{list} stably, though destructively, and | |
1192 | returns the sorted list. It compares elements using @var{predicate}. A | |
1193 | stable sort is one in which elements with equal sort keys maintain their | |
1194 | relative order before and after the sort. Stability is important when | |
1195 | successive sorts are used to order elements according to different | |
1196 | criteria. | |
1197 | ||
1198 | The argument @var{predicate} must be a function that accepts two | |
1199 | arguments. It is called with two elements of @var{list}. To get an | |
1200 | increasing order sort, the @var{predicate} should return non-@code{nil} if the | |
1201 | first element is ``less than'' the second, or @code{nil} if not. | |
1202 | ||
1203 | The comparison function @var{predicate} must give reliable results for | |
1204 | any given pair of arguments, at least within a single call to | |
1205 | @code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is | |
1206 | less than @var{b}, @var{b} must not be less than @var{a}. It must be | |
1207 | @dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b} | |
1208 | is less than @var{c}, then @var{a} must be less than @var{c}. If you | |
1209 | use a comparison function which does not meet these requirements, the | |
1210 | result of @code{sort} is unpredictable. | |
1211 | ||
1212 | The destructive aspect of @code{sort} is that it rearranges the cons | |
1213 | cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort | |
1214 | function would create new cons cells to store the elements in their | |
1215 | sorted order. If you wish to make a sorted copy without destroying the | |
1216 | original, copy it first with @code{copy-sequence} and then sort. | |
1217 | ||
1218 | Sorting does not change the @sc{car}s of the cons cells in @var{list}; | |
1219 | the cons cell that originally contained the element @code{a} in | |
1220 | @var{list} still has @code{a} in its @sc{car} after sorting, but it now | |
1221 | appears in a different position in the list due to the change of | |
1222 | @sc{cdr}s. For example: | |
1223 | ||
1224 | @example | |
1225 | @group | |
1226 | (setq nums '(1 3 2 6 5 4 0)) | |
1227 | @result{} (1 3 2 6 5 4 0) | |
1228 | @end group | |
1229 | @group | |
1230 | (sort nums '<) | |
1231 | @result{} (0 1 2 3 4 5 6) | |
1232 | @end group | |
1233 | @group | |
1234 | nums | |
1235 | @result{} (1 2 3 4 5 6) | |
1236 | @end group | |
1237 | @end example | |
1238 | ||
1239 | @noindent | |
1240 | @strong{Warning}: Note that the list in @code{nums} no longer contains | |
1241 | 0; this is the same cons cell that it was before, but it is no longer | |
1242 | the first one in the list. Don't assume a variable that formerly held | |
1243 | the argument now holds the entire sorted list! Instead, save the result | |
1244 | of @code{sort} and use that. Most often we store the result back into | |
1245 | the variable that held the original list: | |
1246 | ||
1247 | @example | |
1248 | (setq nums (sort nums '<)) | |
1249 | @end example | |
1250 | ||
1251 | @xref{Sorting}, for more functions that perform sorting. | |
1252 | See @code{documentation} in @ref{Accessing Documentation}, for a | |
1253 | useful example of @code{sort}. | |
1254 | @end defun | |
1255 | ||
1256 | @node Sets And Lists | |
1257 | @section Using Lists as Sets | |
1258 | @cindex lists as sets | |
1259 | @cindex sets | |
1260 | ||
1261 | A list can represent an unordered mathematical set---simply consider a | |
1262 | value an element of a set if it appears in the list, and ignore the | |
1263 | order of the list. To form the union of two sets, use @code{append} (as | |
1264 | long as you don't mind having duplicate elements). You can remove | |
1265 | @code{equal} duplicates using @code{delete-dups}. Other useful | |
1266 | functions for sets include @code{memq} and @code{delq}, and their | |
1267 | @code{equal} versions, @code{member} and @code{delete}. | |
1268 | ||
1269 | @cindex CL note---lack @code{union}, @code{intersection} | |
1270 | @quotation | |
1271 | @b{Common Lisp note:} Common Lisp has functions @code{union} (which | |
1272 | avoids duplicate elements) and @code{intersection} for set operations, | |
1273 | but GNU Emacs Lisp does not have them. You can write them in Lisp if | |
1274 | you wish. | |
1275 | @end quotation | |
1276 | ||
1277 | @defun memq object list | |
1278 | @cindex membership in a list | |
1279 | This function tests to see whether @var{object} is a member of | |
1280 | @var{list}. If it is, @code{memq} returns a list starting with the | |
1281 | first occurrence of @var{object}. Otherwise, it returns @code{nil}. | |
1282 | The letter @samp{q} in @code{memq} says that it uses @code{eq} to | |
1283 | compare @var{object} against the elements of the list. For example: | |
1284 | ||
1285 | @example | |
1286 | @group | |
1287 | (memq 'b '(a b c b a)) | |
1288 | @result{} (b c b a) | |
1289 | @end group | |
1290 | @group | |
1291 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1292 | @result{} nil | |
1293 | @end group | |
1294 | @end example | |
1295 | @end defun | |
1296 | ||
1297 | @defun delq object list | |
1298 | @cindex deleting list elements | |
1299 | This function destructively removes all elements @code{eq} to | |
1300 | @var{object} from @var{list}. The letter @samp{q} in @code{delq} says | |
1301 | that it uses @code{eq} to compare @var{object} against the elements of | |
1302 | the list, like @code{memq} and @code{remq}. | |
1303 | @end defun | |
1304 | ||
1305 | When @code{delq} deletes elements from the front of the list, it does so | |
1306 | simply by advancing down the list and returning a sublist that starts | |
1307 | after those elements: | |
1308 | ||
1309 | @example | |
1310 | @group | |
1311 | (delq 'a '(a b c)) @equiv{} (cdr '(a b c)) | |
1312 | @end group | |
1313 | @end example | |
1314 | ||
1315 | When an element to be deleted appears in the middle of the list, | |
1316 | removing it involves changing the @sc{cdr}s (@pxref{Setcdr}). | |
1317 | ||
1318 | @example | |
1319 | @group | |
1320 | (setq sample-list '(a b c (4))) | |
1321 | @result{} (a b c (4)) | |
1322 | @end group | |
1323 | @group | |
1324 | (delq 'a sample-list) | |
1325 | @result{} (b c (4)) | |
1326 | @end group | |
1327 | @group | |
1328 | sample-list | |
1329 | @result{} (a b c (4)) | |
1330 | @end group | |
1331 | @group | |
1332 | (delq 'c sample-list) | |
1333 | @result{} (a b (4)) | |
1334 | @end group | |
1335 | @group | |
1336 | sample-list | |
1337 | @result{} (a b (4)) | |
1338 | @end group | |
1339 | @end example | |
1340 | ||
1341 | Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to | |
1342 | splice out the third element, but @code{(delq 'a sample-list)} does not | |
1343 | splice anything---it just returns a shorter list. Don't assume that a | |
1344 | variable which formerly held the argument @var{list} now has fewer | |
1345 | elements, or that it still holds the original list! Instead, save the | |
1346 | result of @code{delq} and use that. Most often we store the result back | |
1347 | into the variable that held the original list: | |
1348 | ||
1349 | @example | |
1350 | (setq flowers (delq 'rose flowers)) | |
1351 | @end example | |
1352 | ||
1353 | In the following example, the @code{(4)} that @code{delq} attempts to match | |
1354 | and the @code{(4)} in the @code{sample-list} are not @code{eq}: | |
1355 | ||
1356 | @example | |
1357 | @group | |
1358 | (delq '(4) sample-list) | |
1359 | @result{} (a c (4)) | |
1360 | @end group | |
1361 | ||
1362 | If you want to delete elements that are @code{equal} to a given value, | |
1363 | use @code{delete} (see below). | |
1364 | @end example | |
1365 | ||
1366 | @defun remq object list | |
1367 | This function returns a copy of @var{list}, with all elements removed | |
1368 | which are @code{eq} to @var{object}. The letter @samp{q} in @code{remq} | |
1369 | says that it uses @code{eq} to compare @var{object} against the elements | |
1370 | of @code{list}. | |
1371 | ||
1372 | @example | |
1373 | @group | |
1374 | (setq sample-list '(a b c a b c)) | |
1375 | @result{} (a b c a b c) | |
1376 | @end group | |
1377 | @group | |
1378 | (remq 'a sample-list) | |
1379 | @result{} (b c b c) | |
1380 | @end group | |
1381 | @group | |
1382 | sample-list | |
1383 | @result{} (a b c a b c) | |
1384 | @end group | |
1385 | @end example | |
1386 | @end defun | |
1387 | ||
1388 | @defun memql object list | |
1389 | The function @code{memql} tests to see whether @var{object} is a member | |
1390 | of @var{list}, comparing members with @var{object} using @code{eql}, | |
1391 | so floating point elements are compared by value. | |
1392 | If @var{object} is a member, @code{memql} returns a list starting with | |
1393 | its first occurrence in @var{list}. Otherwise, it returns @code{nil}. | |
1394 | ||
1395 | Compare this with @code{memq}: | |
1396 | ||
1397 | @example | |
1398 | @group | |
1399 | (memql 1.2 '(1.1 1.2 1.3)) ; @r{@code{1.2} and @code{1.2} are @code{eql}.} | |
1400 | @result{} (1.2 1.3) | |
1401 | @end group | |
1402 | @group | |
1403 | (memq 1.2 '(1.1 1.2 1.3)) ; @r{@code{1.2} and @code{1.2} are not @code{eq}.} | |
1404 | @result{} nil | |
1405 | @end group | |
1406 | @end example | |
1407 | @end defun | |
1408 | ||
1409 | The following three functions are like @code{memq}, @code{delq} and | |
1410 | @code{remq}, but use @code{equal} rather than @code{eq} to compare | |
1411 | elements. @xref{Equality Predicates}. | |
1412 | ||
1413 | @defun member object list | |
1414 | The function @code{member} tests to see whether @var{object} is a member | |
1415 | of @var{list}, comparing members with @var{object} using @code{equal}. | |
1416 | If @var{object} is a member, @code{member} returns a list starting with | |
1417 | its first occurrence in @var{list}. Otherwise, it returns @code{nil}. | |
1418 | ||
1419 | Compare this with @code{memq}: | |
1420 | ||
1421 | @example | |
1422 | @group | |
1423 | (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.} | |
1424 | @result{} ((2)) | |
1425 | @end group | |
1426 | @group | |
1427 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1428 | @result{} nil | |
1429 | @end group | |
1430 | @group | |
1431 | ;; @r{Two strings with the same contents are @code{equal}.} | |
1432 | (member "foo" '("foo" "bar")) | |
1433 | @result{} ("foo" "bar") | |
1434 | @end group | |
1435 | @end example | |
1436 | @end defun | |
1437 | ||
1438 | @defun delete object sequence | |
1439 | If @code{sequence} is a list, this function destructively removes all | |
1440 | elements @code{equal} to @var{object} from @var{sequence}. For lists, | |
1441 | @code{delete} is to @code{delq} as @code{member} is to @code{memq}: it | |
1442 | uses @code{equal} to compare elements with @var{object}, like | |
1443 | @code{member}; when it finds an element that matches, it cuts the | |
1444 | element out just as @code{delq} would. | |
1445 | ||
1446 | If @code{sequence} is a vector or string, @code{delete} returns a copy | |
1447 | of @code{sequence} with all elements @code{equal} to @code{object} | |
1448 | removed. | |
1449 | ||
1450 | For example: | |
1451 | ||
1452 | @example | |
1453 | @group | |
1454 | (setq l '((2) (1) (2))) | |
1455 | (delete '(2) l) | |
1456 | @result{} ((1)) | |
1457 | l | |
1458 | @result{} ((2) (1)) | |
1459 | ;; @r{If you want to change @code{l} reliably,} | |
1460 | ;; @r{write @code{(setq l (delete elt l))}.} | |
1461 | @end group | |
1462 | @group | |
1463 | (setq l '((2) (1) (2))) | |
1464 | (delete '(1) l) | |
1465 | @result{} ((2) (2)) | |
1466 | l | |
1467 | @result{} ((2) (2)) | |
1468 | ;; @r{In this case, it makes no difference whether you set @code{l},} | |
1469 | ;; @r{but you should do so for the sake of the other case.} | |
1470 | @end group | |
1471 | @group | |
1472 | (delete '(2) [(2) (1) (2)]) | |
1473 | @result{} [(1)] | |
1474 | @end group | |
1475 | @end example | |
1476 | @end defun | |
1477 | ||
1478 | @defun remove object sequence | |
1479 | This function is the non-destructive counterpart of @code{delete}. It | |
1480 | returns a copy of @code{sequence}, a list, vector, or string, with | |
1481 | elements @code{equal} to @code{object} removed. For example: | |
1482 | ||
1483 | @example | |
1484 | @group | |
1485 | (remove '(2) '((2) (1) (2))) | |
1486 | @result{} ((1)) | |
1487 | @end group | |
1488 | @group | |
1489 | (remove '(2) [(2) (1) (2)]) | |
1490 | @result{} [(1)] | |
1491 | @end group | |
1492 | @end example | |
1493 | @end defun | |
1494 | ||
1495 | @quotation | |
1496 | @b{Common Lisp note:} The functions @code{member}, @code{delete} and | |
1497 | @code{remove} in GNU Emacs Lisp are derived from Maclisp, not Common | |
1498 | Lisp. The Common Lisp versions do not use @code{equal} to compare | |
1499 | elements. | |
1500 | @end quotation | |
1501 | ||
1502 | @defun member-ignore-case object list | |
1503 | This function is like @code{member}, except that @var{object} should | |
1504 | be a string and that it ignores differences in letter-case and text | |
1505 | representation: upper-case and lower-case letters are treated as | |
1506 | equal, and unibyte strings are converted to multibyte prior to | |
1507 | comparison. | |
1508 | @end defun | |
1509 | ||
1510 | @defun delete-dups list | |
1511 | This function destructively removes all @code{equal} duplicates from | |
1512 | @var{list}, stores the result in @var{list} and returns it. Of | |
1513 | several @code{equal} occurrences of an element in @var{list}, | |
1514 | @code{delete-dups} keeps the first one. | |
1515 | @end defun | |
1516 | ||
1517 | See also the function @code{add-to-list}, in @ref{List Variables}, | |
1518 | for a way to add an element to a list stored in a variable and used as a | |
1519 | set. | |
1520 | ||
1521 | @node Association Lists | |
1522 | @section Association Lists | |
1523 | @cindex association list | |
1524 | @cindex alist | |
1525 | ||
1526 | An @dfn{association list}, or @dfn{alist} for short, records a mapping | |
1527 | from keys to values. It is a list of cons cells called | |
1528 | @dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the | |
1529 | @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key'' | |
1530 | is not related to the term ``key sequence''; it means a value used to | |
1531 | look up an item in a table. In this case, the table is the alist, and | |
1532 | the alist associations are the items.} | |
1533 | ||
1534 | Here is an example of an alist. The key @code{pine} is associated with | |
1535 | the value @code{cones}; the key @code{oak} is associated with | |
1536 | @code{acorns}; and the key @code{maple} is associated with @code{seeds}. | |
1537 | ||
1538 | @example | |
1539 | @group | |
1540 | ((pine . cones) | |
1541 | (oak . acorns) | |
1542 | (maple . seeds)) | |
1543 | @end group | |
1544 | @end example | |
1545 | ||
1546 | Both the values and the keys in an alist may be any Lisp objects. | |
1547 | For example, in the following alist, the symbol @code{a} is | |
1548 | associated with the number @code{1}, and the string @code{"b"} is | |
1549 | associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of | |
1550 | the alist element: | |
1551 | ||
1552 | @example | |
1553 | ((a . 1) ("b" 2 3)) | |
1554 | @end example | |
1555 | ||
1556 | Sometimes it is better to design an alist to store the associated | |
1557 | value in the @sc{car} of the @sc{cdr} of the element. Here is an | |
1558 | example of such an alist: | |
1559 | ||
1560 | @example | |
1561 | ((rose red) (lily white) (buttercup yellow)) | |
1562 | @end example | |
1563 | ||
1564 | @noindent | |
1565 | Here we regard @code{red} as the value associated with @code{rose}. One | |
1566 | advantage of this kind of alist is that you can store other related | |
1567 | information---even a list of other items---in the @sc{cdr} of the | |
1568 | @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see | |
1569 | below) to find the element containing a given value. When neither of | |
1570 | these considerations is important, the choice is a matter of taste, as | |
1571 | long as you are consistent about it for any given alist. | |
1572 | ||
1573 | The same alist shown above could be regarded as having the | |
1574 | associated value in the @sc{cdr} of the element; the value associated | |
1575 | with @code{rose} would be the list @code{(red)}. | |
1576 | ||
1577 | Association lists are often used to record information that you might | |
1578 | otherwise keep on a stack, since new associations may be added easily to | |
1579 | the front of the list. When searching an association list for an | |
1580 | association with a given key, the first one found is returned, if there | |
1581 | is more than one. | |
1582 | ||
1583 | In Emacs Lisp, it is @emph{not} an error if an element of an | |
1584 | association list is not a cons cell. The alist search functions simply | |
1585 | ignore such elements. Many other versions of Lisp signal errors in such | |
1586 | cases. | |
1587 | ||
1588 | Note that property lists are similar to association lists in several | |
1589 | respects. A property list behaves like an association list in which | |
1590 | each key can occur only once. @xref{Property Lists}, for a comparison | |
1591 | of property lists and association lists. | |
1592 | ||
1593 | @defun assoc key alist | |
1594 | This function returns the first association for @var{key} in | |
1595 | @var{alist}, comparing @var{key} against the alist elements using | |
1596 | @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no | |
1597 | association in @var{alist} has a @sc{car} @code{equal} to @var{key}. | |
1598 | For example: | |
1599 | ||
1600 | @smallexample | |
1601 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1602 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1603 | (assoc 'oak trees) | |
1604 | @result{} (oak . acorns) | |
1605 | (cdr (assoc 'oak trees)) | |
1606 | @result{} acorns | |
1607 | (assoc 'birch trees) | |
1608 | @result{} nil | |
1609 | @end smallexample | |
1610 | ||
1611 | Here is another example, in which the keys and values are not symbols: | |
1612 | ||
1613 | @smallexample | |
1614 | (setq needles-per-cluster | |
1615 | '((2 "Austrian Pine" "Red Pine") | |
1616 | (3 "Pitch Pine") | |
1617 | (5 "White Pine"))) | |
1618 | ||
1619 | (cdr (assoc 3 needles-per-cluster)) | |
1620 | @result{} ("Pitch Pine") | |
1621 | (cdr (assoc 2 needles-per-cluster)) | |
1622 | @result{} ("Austrian Pine" "Red Pine") | |
1623 | @end smallexample | |
1624 | @end defun | |
1625 | ||
1626 | The function @code{assoc-string} is much like @code{assoc} except | |
1627 | that it ignores certain differences between strings. @xref{Text | |
1628 | Comparison}. | |
1629 | ||
1630 | @defun rassoc value alist | |
1631 | This function returns the first association with value @var{value} in | |
1632 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1633 | a @sc{cdr} @code{equal} to @var{value}. | |
1634 | ||
1635 | @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of | |
1636 | each @var{alist} association instead of the @sc{car}. You can think of | |
1637 | this as ``reverse @code{assoc},'' finding the key for a given value. | |
1638 | @end defun | |
1639 | ||
1640 | @defun assq key alist | |
1641 | This function is like @code{assoc} in that it returns the first | |
1642 | association for @var{key} in @var{alist}, but it makes the comparison | |
1643 | using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil} | |
1644 | if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}. | |
1645 | This function is used more often than @code{assoc}, since @code{eq} is | |
1646 | faster than @code{equal} and most alists use symbols as keys. | |
1647 | @xref{Equality Predicates}. | |
1648 | ||
1649 | @smallexample | |
1650 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1651 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1652 | (assq 'pine trees) | |
1653 | @result{} (pine . cones) | |
1654 | @end smallexample | |
1655 | ||
1656 | On the other hand, @code{assq} is not usually useful in alists where the | |
1657 | keys may not be symbols: | |
1658 | ||
1659 | @smallexample | |
1660 | (setq leaves | |
1661 | '(("simple leaves" . oak) | |
1662 | ("compound leaves" . horsechestnut))) | |
1663 | ||
1664 | (assq "simple leaves" leaves) | |
1665 | @result{} nil | |
1666 | (assoc "simple leaves" leaves) | |
1667 | @result{} ("simple leaves" . oak) | |
1668 | @end smallexample | |
1669 | @end defun | |
1670 | ||
1671 | @defun rassq value alist | |
1672 | This function returns the first association with value @var{value} in | |
1673 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1674 | a @sc{cdr} @code{eq} to @var{value}. | |
1675 | ||
1676 | @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of | |
1677 | each @var{alist} association instead of the @sc{car}. You can think of | |
1678 | this as ``reverse @code{assq},'' finding the key for a given value. | |
1679 | ||
1680 | For example: | |
1681 | ||
1682 | @smallexample | |
1683 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1684 | ||
1685 | (rassq 'acorns trees) | |
1686 | @result{} (oak . acorns) | |
1687 | (rassq 'spores trees) | |
1688 | @result{} nil | |
1689 | @end smallexample | |
1690 | ||
1691 | @code{rassq} cannot search for a value stored in the @sc{car} | |
1692 | of the @sc{cdr} of an element: | |
1693 | ||
1694 | @smallexample | |
1695 | (setq colors '((rose red) (lily white) (buttercup yellow))) | |
1696 | ||
1697 | (rassq 'white colors) | |
1698 | @result{} nil | |
1699 | @end smallexample | |
1700 | ||
1701 | In this case, the @sc{cdr} of the association @code{(lily white)} is not | |
1702 | the symbol @code{white}, but rather the list @code{(white)}. This | |
1703 | becomes clearer if the association is written in dotted pair notation: | |
1704 | ||
1705 | @smallexample | |
1706 | (lily white) @equiv{} (lily . (white)) | |
1707 | @end smallexample | |
1708 | @end defun | |
1709 | ||
1710 | @defun assoc-default key alist &optional test default | |
1711 | This function searches @var{alist} for a match for @var{key}. For each | |
1712 | element of @var{alist}, it compares the element (if it is an atom) or | |
1713 | the element's @sc{car} (if it is a cons) against @var{key}, by calling | |
1714 | @var{test} with two arguments: the element or its @sc{car}, and | |
1715 | @var{key}. The arguments are passed in that order so that you can get | |
1716 | useful results using @code{string-match} with an alist that contains | |
1717 | regular expressions (@pxref{Regexp Search}). If @var{test} is omitted | |
1718 | or @code{nil}, @code{equal} is used for comparison. | |
1719 | ||
1720 | If an alist element matches @var{key} by this criterion, | |
1721 | then @code{assoc-default} returns a value based on this element. | |
1722 | If the element is a cons, then the value is the element's @sc{cdr}. | |
1723 | Otherwise, the return value is @var{default}. | |
1724 | ||
1725 | If no alist element matches @var{key}, @code{assoc-default} returns | |
1726 | @code{nil}. | |
1727 | @end defun | |
1728 | ||
1729 | @defun copy-alist alist | |
1730 | @cindex copying alists | |
1731 | This function returns a two-level deep copy of @var{alist}: it creates a | |
1732 | new copy of each association, so that you can alter the associations of | |
1733 | the new alist without changing the old one. | |
1734 | ||
1735 | @smallexample | |
1736 | @group | |
1737 | (setq needles-per-cluster | |
1738 | '((2 . ("Austrian Pine" "Red Pine")) | |
1739 | (3 . ("Pitch Pine")) | |
1740 | @end group | |
1741 | (5 . ("White Pine")))) | |
1742 | @result{} | |
1743 | ((2 "Austrian Pine" "Red Pine") | |
1744 | (3 "Pitch Pine") | |
1745 | (5 "White Pine")) | |
1746 | ||
1747 | (setq copy (copy-alist needles-per-cluster)) | |
1748 | @result{} | |
1749 | ((2 "Austrian Pine" "Red Pine") | |
1750 | (3 "Pitch Pine") | |
1751 | (5 "White Pine")) | |
1752 | ||
1753 | (eq needles-per-cluster copy) | |
1754 | @result{} nil | |
1755 | (equal needles-per-cluster copy) | |
1756 | @result{} t | |
1757 | (eq (car needles-per-cluster) (car copy)) | |
1758 | @result{} nil | |
1759 | (cdr (car (cdr needles-per-cluster))) | |
1760 | @result{} ("Pitch Pine") | |
1761 | @group | |
1762 | (eq (cdr (car (cdr needles-per-cluster))) | |
1763 | (cdr (car (cdr copy)))) | |
1764 | @result{} t | |
1765 | @end group | |
1766 | @end smallexample | |
1767 | ||
1768 | This example shows how @code{copy-alist} makes it possible to change | |
1769 | the associations of one copy without affecting the other: | |
1770 | ||
1771 | @smallexample | |
1772 | @group | |
1773 | (setcdr (assq 3 copy) '("Martian Vacuum Pine")) | |
1774 | (cdr (assq 3 needles-per-cluster)) | |
1775 | @result{} ("Pitch Pine") | |
1776 | @end group | |
1777 | @end smallexample | |
1778 | @end defun | |
1779 | ||
1780 | @defun assq-delete-all key alist | |
1781 | This function deletes from @var{alist} all the elements whose @sc{car} | |
1782 | is @code{eq} to @var{key}, much as if you used @code{delq} to delete | |
1783 | each such element one by one. It returns the shortened alist, and | |
1784 | often modifies the original list structure of @var{alist}. For | |
1785 | correct results, use the return value of @code{assq-delete-all} rather | |
1786 | than looking at the saved value of @var{alist}. | |
1787 | ||
1788 | @example | |
1789 | (setq alist '((foo 1) (bar 2) (foo 3) (lose 4))) | |
1790 | @result{} ((foo 1) (bar 2) (foo 3) (lose 4)) | |
1791 | (assq-delete-all 'foo alist) | |
1792 | @result{} ((bar 2) (lose 4)) | |
1793 | alist | |
1794 | @result{} ((foo 1) (bar 2) (lose 4)) | |
1795 | @end example | |
1796 | @end defun | |
1797 | ||
1798 | @defun rassq-delete-all value alist | |
1799 | This function deletes from @var{alist} all the elements whose @sc{cdr} | |
1800 | is @code{eq} to @var{value}. It returns the shortened alist, and | |
1801 | often modifies the original list structure of @var{alist}. | |
1802 | @code{rassq-delete-all} is like @code{assq-delete-all} except that it | |
1803 | compares the @sc{cdr} of each @var{alist} association instead of the | |
1804 | @sc{car}. | |
1805 | @end defun | |
1806 | ||
1807 | @node Rings | |
1808 | @section Managing a Fixed-Size Ring of Objects | |
1809 | ||
1810 | @cindex ring data structure | |
1811 | This section describes functions for operating on rings. A | |
1812 | @dfn{ring} is a fixed-size data structure that supports insertion, | |
1813 | deletion, rotation, and modulo-indexed reference and traversal. | |
1814 | ||
1815 | @defun make-ring size | |
1816 | This returns a new ring capable of holding @var{size} objects. | |
1817 | @var{size} should be an integer. | |
1818 | @end defun | |
1819 | ||
1820 | @defun ring-p object | |
1821 | This returns @code{t} if @var{object} is a ring, @code{nil} otherwise. | |
1822 | @end defun | |
1823 | ||
1824 | @defun ring-size ring | |
1825 | This returns the maximum capacity of the @var{ring}. | |
1826 | @end defun | |
1827 | ||
1828 | @defun ring-length ring | |
1829 | This returns the number of objects that @var{ring} currently contains. | |
1830 | The value will never exceed that returned by @code{ring-size}. | |
1831 | @end defun | |
1832 | ||
1833 | @defun ring-elements ring | |
1834 | This returns a list of the objects in @var{ring}, in order, newest first. | |
1835 | @end defun | |
1836 | ||
1837 | @defun ring-copy ring | |
1838 | This returns a new ring which is a copy of @var{ring}. | |
1839 | The new ring contains the same (@code{eq}) objects as @var{ring}. | |
1840 | @end defun | |
1841 | ||
1842 | @defun ring-empty-p ring | |
1843 | This returns @code{t} if @var{ring} is empty, @code{nil} otherwise. | |
1844 | @end defun | |
1845 | ||
1846 | The newest element in the ring always has index 0. Higher indices | |
1847 | correspond to older elements. Indices are computed modulo the ring | |
1848 | length. Index @minus{}1 corresponds to the oldest element, @minus{}2 | |
1849 | to the next-oldest, and so forth. | |
1850 | ||
1851 | @defun ring-ref ring index | |
1852 | This returns the object in @var{ring} found at index @var{index}. | |
1853 | @var{index} may be negative or greater than the ring length. If | |
1854 | @var{ring} is empty, @code{ring-ref} signals an error. | |
1855 | @end defun | |
1856 | ||
1857 | @defun ring-insert ring object | |
1858 | This inserts @var{object} into @var{ring}, making it the newest | |
1859 | element, and returns @var{object}. | |
1860 | ||
1861 | If the ring is full, insertion removes the oldest element to | |
1862 | make room for the new element. | |
1863 | @end defun | |
1864 | ||
1865 | @defun ring-remove ring &optional index | |
1866 | Remove an object from @var{ring}, and return that object. The | |
1867 | argument @var{index} specifies which item to remove; if it is | |
1868 | @code{nil}, that means to remove the oldest item. If @var{ring} is | |
1869 | empty, @code{ring-remove} signals an error. | |
1870 | @end defun | |
1871 | ||
1872 | @defun ring-insert-at-beginning ring object | |
1873 | This inserts @var{object} into @var{ring}, treating it as the oldest | |
1874 | element. The return value is not significant. | |
1875 | ||
1876 | If the ring is full, this function removes the newest element to make | |
1877 | room for the inserted element. | |
1878 | @end defun | |
1879 | ||
1880 | @cindex fifo data structure | |
1881 | If you are careful not to exceed the ring size, you can | |
1882 | use the ring as a first-in-first-out queue. For example: | |
1883 | ||
1884 | @lisp | |
1885 | (let ((fifo (make-ring 5))) | |
1886 | (mapc (lambda (obj) (ring-insert fifo obj)) | |
1887 | '(0 one "two")) | |
1888 | (list (ring-remove fifo) t | |
1889 | (ring-remove fifo) t | |
1890 | (ring-remove fifo))) | |
1891 | @result{} (0 t one t "two") | |
1892 | @end lisp | |
1893 | ||
1894 | @ignore | |
1895 | arch-tag: 31fb8a4e-4aa8-4a74-a206-aa00451394d4 | |
1896 | @end ignore |