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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 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/lists | |
6 | @node Lists, Sequences Arrays Vectors, Strings and Characters, Top | |
7 | @chapter Lists | |
8 | @cindex list | |
9 | @cindex element (of list) | |
10 | ||
11 | A @dfn{list} represents a sequence of zero or more elements (which may | |
12 | be any Lisp objects). The important difference between lists and | |
13 | vectors is that two or more lists can share part of their structure; in | |
14 | addition, you can insert or delete elements in a list without copying | |
15 | the whole list. | |
16 | ||
17 | @menu | |
18 | * Cons Cells:: How lists are made out of cons cells. | |
19 | * Lists as Boxes:: Graphical notation to explain lists. | |
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 | * Modifying Lists:: Storing new pieces into an existing list. | |
24 | * Sets And Lists:: A list can represent a finite mathematical set. | |
25 | * Association Lists:: A list can represent a finite relation or mapping. | |
26 | @end menu | |
27 | ||
28 | @node Cons Cells | |
29 | @section Lists and Cons Cells | |
30 | @cindex lists and cons cells | |
31 | @cindex @code{nil} and lists | |
32 | ||
33 | Lists in Lisp are not a primitive data type; they are built up from | |
2b3fc6c3 RS |
34 | @dfn{cons cells}. A cons cell is a data object that represents an |
35 | ordered pair. It records two Lisp objects, one labeled as the @sc{car}, | |
36 | and the other labeled as the @sc{cdr}. These names are traditional; see | |
37 | @ref{Cons Cell Type}. @sc{cdr} is pronounced ``could-er.'' | |
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38 | |
39 | A list is a series of cons cells chained together, one cons cell per | |
40 | element of the list. By convention, the @sc{car}s of the cons cells are | |
41 | the elements of the list, and the @sc{cdr}s are used to chain the list: | |
42 | the @sc{cdr} of each cons cell is the following cons cell. The @sc{cdr} | |
43 | of the last cons cell is @code{nil}. This asymmetry between the | |
44 | @sc{car} and the @sc{cdr} is entirely a matter of convention; at the | |
45 | level of cons cells, the @sc{car} and @sc{cdr} slots have the same | |
46 | characteristics. | |
47 | ||
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48 | @cindex list structure |
49 | Because most cons cells are used as part of lists, the phrase | |
50 | @dfn{list structure} has come to mean any structure made out of cons | |
51 | cells. | |
52 | ||
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53 | The symbol @code{nil} is considered a list as well as a symbol; it is |
54 | the list with no elements. For convenience, the symbol @code{nil} is | |
55 | considered to have @code{nil} as its @sc{cdr} (and also as its | |
56 | @sc{car}). | |
57 | ||
58 | The @sc{cdr} of any nonempty list @var{l} is a list containing all the | |
59 | elements of @var{l} except the first. | |
60 | ||
61 | @node Lists as Boxes | |
62 | @comment node-name, next, previous, up | |
63 | @section Lists as Linked Pairs of Boxes | |
64 | @cindex box representation for lists | |
65 | @cindex lists represented as boxes | |
66 | @cindex cons cell as box | |
67 | ||
68 | A cons cell can be illustrated as a pair of boxes. The first box | |
69 | represents the @sc{car} and the second box represents the @sc{cdr}. | |
70 | Here is an illustration of the two-element list, @code{(tulip lily)}, | |
71 | made from two cons cells: | |
72 | ||
73 | @example | |
74 | @group | |
75 | --------------- --------------- | |
76 | | car | cdr | | car | cdr | | |
77 | | tulip | o---------->| lily | nil | | |
78 | | | | | | | | |
79 | --------------- --------------- | |
80 | @end group | |
81 | @end example | |
82 | ||
83 | Each pair of boxes represents a cons cell. Each box ``refers to'', | |
84 | ``points to'' or ``contains'' a Lisp object. (These terms are | |
85 | synonymous.) The first box, which is the @sc{car} of the first cons | |
86 | cell, contains the symbol @code{tulip}. The arrow from the @sc{cdr} of | |
87 | the first cons cell to the second cons cell indicates that the @sc{cdr} | |
88 | of the first cons cell points to the second cons cell. | |
89 | ||
90 | The same list can be illustrated in a different sort of box notation | |
91 | like this: | |
92 | ||
93 | @example | |
94 | @group | |
95 | ___ ___ ___ ___ | |
96 | |___|___|--> |___|___|--> nil | |
97 | | | | |
98 | | | | |
99 | --> tulip --> lily | |
100 | @end group | |
101 | @end example | |
102 | ||
103 | Here is a more complex illustration, showing the three-element list, | |
104 | @code{((pine needles) oak maple)}, the first element of which is a | |
105 | two-element list: | |
106 | ||
107 | @example | |
108 | @group | |
109 | ___ ___ ___ ___ ___ ___ | |
110 | |___|___|--> |___|___|--> |___|___|--> nil | |
111 | | | | | |
112 | | | | | |
113 | | --> oak --> maple | |
114 | | | |
115 | | ___ ___ ___ ___ | |
116 | --> |___|___|--> |___|___|--> nil | |
117 | | | | |
118 | | | | |
119 | --> pine --> needles | |
120 | @end group | |
121 | @end example | |
122 | ||
123 | The same list represented in the first box notation looks like this: | |
124 | ||
125 | @example | |
126 | @group | |
127 | -------------- -------------- -------------- | |
128 | | car | cdr | | car | cdr | | car | cdr | | |
129 | | o | o------->| oak | o------->| maple | nil | | |
130 | | | | | | | | | | | | |
131 | -- | --------- -------------- -------------- | |
132 | | | |
133 | | | |
134 | | -------------- ---------------- | |
135 | | | car | cdr | | car | cdr | | |
136 | ------>| pine | o------->| needles | nil | | |
137 | | | | | | | | |
138 | -------------- ---------------- | |
139 | @end group | |
140 | @end example | |
141 | ||
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142 | @xref{Cons Cell Type}, for the read and print syntax of cons cells and |
143 | lists, and for more ``box and arrow'' illustrations of lists. | |
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144 | |
145 | @node List-related Predicates | |
146 | @section Predicates on Lists | |
147 | ||
148 | The following predicates test whether a Lisp object is an atom, is a | |
149 | cons cell or is a list, or whether it is the distinguished object | |
150 | @code{nil}. (Many of these predicates can be defined in terms of the | |
151 | others, but they are used so often that it is worth having all of them.) | |
152 | ||
153 | @defun consp object | |
154 | This function returns @code{t} if @var{object} is a cons cell, @code{nil} | |
155 | otherwise. @code{nil} is not a cons cell, although it @emph{is} a list. | |
156 | @end defun | |
157 | ||
158 | @defun atom object | |
159 | @cindex atoms | |
160 | This function returns @code{t} if @var{object} is an atom, @code{nil} | |
161 | otherwise. All objects except cons cells are atoms. The symbol | |
162 | @code{nil} is an atom and is also a list; it is the only Lisp object | |
2b3fc6c3 | 163 | that is both. |
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164 | |
165 | @example | |
166 | (atom @var{object}) @equiv{} (not (consp @var{object})) | |
167 | @end example | |
168 | @end defun | |
169 | ||
170 | @defun listp object | |
171 | This function returns @code{t} if @var{object} is a cons cell or | |
172 | @code{nil}. Otherwise, it returns @code{nil}. | |
173 | ||
174 | @example | |
175 | @group | |
176 | (listp '(1)) | |
177 | @result{} t | |
178 | @end group | |
179 | @group | |
180 | (listp '()) | |
181 | @result{} t | |
182 | @end group | |
183 | @end example | |
184 | @end defun | |
185 | ||
186 | @defun nlistp object | |
187 | This function is the opposite of @code{listp}: it returns @code{t} if | |
188 | @var{object} is not a list. Otherwise, it returns @code{nil}. | |
189 | ||
190 | @example | |
191 | (listp @var{object}) @equiv{} (not (nlistp @var{object})) | |
192 | @end example | |
193 | @end defun | |
194 | ||
195 | @defun null object | |
196 | This function returns @code{t} if @var{object} is @code{nil}, and | |
197 | returns @code{nil} otherwise. This function is identical to @code{not}, | |
198 | but as a matter of clarity we use @code{null} when @var{object} is | |
199 | considered a list and @code{not} when it is considered a truth value | |
200 | (see @code{not} in @ref{Combining Conditions}). | |
201 | ||
202 | @example | |
203 | @group | |
204 | (null '(1)) | |
205 | @result{} nil | |
206 | @end group | |
207 | @group | |
208 | (null '()) | |
209 | @result{} t | |
210 | @end group | |
211 | @end example | |
212 | @end defun | |
213 | ||
ec221d13 | 214 | @need 2000 |
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215 | |
216 | @node List Elements | |
217 | @section Accessing Elements of Lists | |
218 | @cindex list elements | |
219 | ||
220 | @defun car cons-cell | |
221 | This function returns the value pointed to by the first pointer of the | |
222 | cons cell @var{cons-cell}. Expressed another way, this function | |
223 | returns the @sc{car} of @var{cons-cell}. | |
224 | ||
225 | As a special case, if @var{cons-cell} is @code{nil}, then @code{car} | |
226 | is defined to return @code{nil}; therefore, any list is a valid argument | |
227 | for @code{car}. An error is signaled if the argument is not a cons cell | |
228 | or @code{nil}. | |
229 | ||
230 | @example | |
231 | @group | |
232 | (car '(a b c)) | |
233 | @result{} a | |
234 | @end group | |
235 | @group | |
236 | (car '()) | |
237 | @result{} nil | |
238 | @end group | |
239 | @end example | |
240 | @end defun | |
241 | ||
242 | @defun cdr cons-cell | |
243 | This function returns the value pointed to by the second pointer of | |
244 | the cons cell @var{cons-cell}. Expressed another way, this function | |
245 | returns the @sc{cdr} of @var{cons-cell}. | |
246 | ||
247 | As a special case, if @var{cons-cell} is @code{nil}, then @code{cdr} | |
248 | is defined to return @code{nil}; therefore, any list is a valid argument | |
249 | for @code{cdr}. An error is signaled if the argument is not a cons cell | |
250 | or @code{nil}. | |
251 | ||
252 | @example | |
253 | @group | |
254 | (cdr '(a b c)) | |
255 | @result{} (b c) | |
256 | @end group | |
257 | @group | |
258 | (cdr '()) | |
259 | @result{} nil | |
260 | @end group | |
261 | @end example | |
262 | @end defun | |
263 | ||
264 | @defun car-safe object | |
265 | This function lets you take the @sc{car} of a cons cell while avoiding | |
266 | errors for other data types. It returns the @sc{car} of @var{object} if | |
267 | @var{object} is a cons cell, @code{nil} otherwise. This is in contrast | |
268 | to @code{car}, which signals an error if @var{object} is not a list. | |
269 | ||
270 | @example | |
271 | @group | |
272 | (car-safe @var{object}) | |
273 | @equiv{} | |
274 | (let ((x @var{object})) | |
275 | (if (consp x) | |
276 | (car x) | |
277 | nil)) | |
278 | @end group | |
279 | @end example | |
280 | @end defun | |
281 | ||
282 | @defun cdr-safe object | |
283 | This function lets you take the @sc{cdr} of a cons cell while | |
284 | avoiding errors for other data types. It returns the @sc{cdr} of | |
285 | @var{object} if @var{object} is a cons cell, @code{nil} otherwise. | |
286 | This is in contrast to @code{cdr}, which signals an error if | |
287 | @var{object} is not a list. | |
288 | ||
289 | @example | |
290 | @group | |
291 | (cdr-safe @var{object}) | |
292 | @equiv{} | |
293 | (let ((x @var{object})) | |
294 | (if (consp x) | |
295 | (cdr x) | |
296 | nil)) | |
297 | @end group | |
298 | @end example | |
299 | @end defun | |
300 | ||
301 | @defun nth n list | |
302 | This function returns the @var{n}th element of @var{list}. Elements | |
303 | are numbered starting with zero, so the @sc{car} of @var{list} is | |
304 | element number zero. If the length of @var{list} is @var{n} or less, | |
305 | the value is @code{nil}. | |
306 | ||
307 | If @var{n} is negative, @code{nth} returns the first element of | |
308 | @var{list}. | |
309 | ||
310 | @example | |
311 | @group | |
312 | (nth 2 '(1 2 3 4)) | |
313 | @result{} 3 | |
314 | @end group | |
315 | @group | |
316 | (nth 10 '(1 2 3 4)) | |
317 | @result{} nil | |
318 | @end group | |
319 | @group | |
320 | (nth -3 '(1 2 3 4)) | |
321 | @result{} 1 | |
322 | ||
323 | (nth n x) @equiv{} (car (nthcdr n x)) | |
324 | @end group | |
325 | @end example | |
326 | @end defun | |
327 | ||
328 | @defun nthcdr n list | |
329 | This function returns the @var{n}th @sc{cdr} of @var{list}. In other | |
330 | words, it removes the first @var{n} links of @var{list} and returns | |
331 | what follows. | |
332 | ||
333 | If @var{n} is zero or negative, @code{nthcdr} returns all of | |
334 | @var{list}. If the length of @var{list} is @var{n} or less, | |
335 | @code{nthcdr} returns @code{nil}. | |
336 | ||
337 | @example | |
338 | @group | |
339 | (nthcdr 1 '(1 2 3 4)) | |
340 | @result{} (2 3 4) | |
341 | @end group | |
342 | @group | |
343 | (nthcdr 10 '(1 2 3 4)) | |
344 | @result{} nil | |
345 | @end group | |
346 | @group | |
347 | (nthcdr -3 '(1 2 3 4)) | |
348 | @result{} (1 2 3 4) | |
349 | @end group | |
350 | @end example | |
351 | @end defun | |
352 | ||
353 | @node Building Lists | |
354 | @comment node-name, next, previous, up | |
355 | @section Building Cons Cells and Lists | |
356 | @cindex cons cells | |
357 | @cindex building lists | |
358 | ||
359 | Many functions build lists, as lists reside at the very heart of Lisp. | |
360 | @code{cons} is the fundamental list-building function; however, it is | |
361 | interesting to note that @code{list} is used more times in the source | |
362 | code for Emacs than @code{cons}. | |
363 | ||
364 | @defun cons object1 object2 | |
365 | This function is the fundamental function used to build new list | |
366 | structure. It creates a new cons cell, making @var{object1} the | |
367 | @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new cons | |
368 | cell. The arguments @var{object1} and @var{object2} may be any Lisp | |
369 | objects, but most often @var{object2} is a list. | |
370 | ||
371 | @example | |
372 | @group | |
373 | (cons 1 '(2)) | |
374 | @result{} (1 2) | |
375 | @end group | |
376 | @group | |
377 | (cons 1 '()) | |
378 | @result{} (1) | |
379 | @end group | |
380 | @group | |
381 | (cons 1 2) | |
382 | @result{} (1 . 2) | |
383 | @end group | |
384 | @end example | |
385 | ||
386 | @cindex consing | |
387 | @code{cons} is often used to add a single element to the front of a | |
388 | list. This is called @dfn{consing the element onto the list}. For | |
389 | example: | |
390 | ||
391 | @example | |
392 | (setq list (cons newelt list)) | |
393 | @end example | |
394 | ||
395 | Note that there is no conflict between the variable named @code{list} | |
396 | used in this example and the function named @code{list} described below; | |
397 | any symbol can serve both purposes. | |
398 | @end defun | |
399 | ||
400 | @defun list &rest objects | |
401 | This function creates a list with @var{objects} as its elements. The | |
402 | resulting list is always @code{nil}-terminated. If no @var{objects} | |
403 | are given, the empty list is returned. | |
404 | ||
405 | @example | |
406 | @group | |
407 | (list 1 2 3 4 5) | |
408 | @result{} (1 2 3 4 5) | |
409 | @end group | |
410 | @group | |
411 | (list 1 2 '(3 4 5) 'foo) | |
412 | @result{} (1 2 (3 4 5) foo) | |
413 | @end group | |
414 | @group | |
415 | (list) | |
416 | @result{} nil | |
417 | @end group | |
418 | @end example | |
419 | @end defun | |
420 | ||
421 | @defun make-list length object | |
422 | This function creates a list of length @var{length}, in which all the | |
423 | elements have the identical value @var{object}. Compare | |
424 | @code{make-list} with @code{make-string} (@pxref{Creating Strings}). | |
425 | ||
426 | @example | |
427 | @group | |
428 | (make-list 3 'pigs) | |
429 | @result{} (pigs pigs pigs) | |
430 | @end group | |
431 | @group | |
432 | (make-list 0 'pigs) | |
433 | @result{} nil | |
434 | @end group | |
435 | @end example | |
436 | @end defun | |
437 | ||
438 | @defun append &rest sequences | |
439 | @cindex copying lists | |
440 | This function returns a list containing all the elements of | |
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441 | @var{sequences}. The @var{sequences} may be lists, vectors, or strings, |
442 | but the last one should be a list. All arguments except the last one | |
443 | are copied, so none of them are altered. | |
444 | ||
445 | More generally, the final argument to @code{append} may be any Lisp | |
446 | object. The final argument is not copied or converted; it becomes the | |
447 | @sc{cdr} of the last cons cell in the new list. If the final argument | |
448 | is itself a list, then its elements become in effect elements of the | |
449 | result list. If the final element is not a list, the result is a | |
450 | ``dotted list'' since its final @sc{cdr} is not @code{nil} as required | |
451 | in a true list. | |
73804d4b | 452 | |
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453 | See @code{nconc} in @ref{Rearrangement}, for a way to join lists with no |
454 | copying. | |
73804d4b | 455 | |
2b3fc6c3 | 456 | Here is an example of using @code{append}: |
73804d4b RS |
457 | |
458 | @example | |
459 | @group | |
460 | (setq trees '(pine oak)) | |
461 | @result{} (pine oak) | |
462 | (setq more-trees (append '(maple birch) trees)) | |
463 | @result{} (maple birch pine oak) | |
464 | @end group | |
465 | ||
466 | @group | |
467 | trees | |
468 | @result{} (pine oak) | |
469 | more-trees | |
470 | @result{} (maple birch pine oak) | |
471 | @end group | |
472 | @group | |
473 | (eq trees (cdr (cdr more-trees))) | |
474 | @result{} t | |
475 | @end group | |
476 | @end example | |
477 | ||
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478 | You can see how @code{append} works by looking at a box diagram. The |
479 | variable @code{trees} is set to the list @code{(pine oak)} and then the | |
480 | variable @code{more-trees} is set to the list @code{(maple birch pine | |
481 | oak)}. However, the variable @code{trees} continues to refer to the | |
482 | original list: | |
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483 | |
484 | @smallexample | |
485 | @group | |
486 | more-trees trees | |
487 | | | | |
488 | | ___ ___ ___ ___ -> ___ ___ ___ ___ | |
489 | --> |___|___|--> |___|___|--> |___|___|--> |___|___|--> nil | |
490 | | | | | | |
491 | | | | | | |
492 | --> maple -->birch --> pine --> oak | |
493 | @end group | |
494 | @end smallexample | |
495 | ||
496 | An empty sequence contributes nothing to the value returned by | |
497 | @code{append}. As a consequence of this, a final @code{nil} argument | |
498 | forces a copy of the previous argument. | |
499 | ||
500 | @example | |
501 | @group | |
502 | trees | |
503 | @result{} (pine oak) | |
504 | @end group | |
505 | @group | |
506 | (setq wood (append trees ())) | |
507 | @result{} (pine oak) | |
508 | @end group | |
509 | @group | |
510 | wood | |
511 | @result{} (pine oak) | |
512 | @end group | |
513 | @group | |
514 | (eq wood trees) | |
515 | @result{} nil | |
516 | @end group | |
517 | @end example | |
518 | ||
519 | @noindent | |
520 | This once was the usual way to copy a list, before the function | |
521 | @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}. | |
522 | ||
523 | With the help of @code{apply}, we can append all the lists in a list of | |
524 | lists: | |
525 | ||
526 | @example | |
527 | @group | |
528 | (apply 'append '((a b c) nil (x y z) nil)) | |
529 | @result{} (a b c x y z) | |
530 | @end group | |
531 | @end example | |
532 | ||
533 | If no @var{sequences} are given, @code{nil} is returned: | |
534 | ||
535 | @example | |
536 | @group | |
537 | (append) | |
538 | @result{} nil | |
539 | @end group | |
540 | @end example | |
541 | ||
2b3fc6c3 RS |
542 | Here are some examples where the final argument is not a list: |
543 | ||
544 | @example | |
545 | (append '(x y) 'z) | |
bfe721d1 | 546 | @result{} (x y . z) |
2b3fc6c3 | 547 | (append '(x y) [z]) |
bfe721d1 | 548 | @result{} (x y . [z]) |
2b3fc6c3 RS |
549 | @end example |
550 | ||
551 | @noindent | |
552 | The second example shows that when the final argument is a sequence but | |
553 | not a list, the sequence's elements do not become elements of the | |
554 | resulting list. Instead, the sequence becomes the final @sc{cdr}, like | |
555 | any other non-list final argument. | |
73804d4b | 556 | |
22697dac KH |
557 | The @code{append} function also allows integers as arguments. It |
558 | converts them to strings of digits, making up the decimal print | |
559 | representation of the integer, and then uses the strings instead of the | |
560 | original integers. @strong{Don't use this feature; we plan to eliminate | |
561 | it. If you already use this feature, change your programs now!} The | |
562 | proper way to convert an integer to a decimal number in this way is with | |
563 | @code{format} (@pxref{Formatting Strings}) or @code{number-to-string} | |
564 | (@pxref{String Conversion}). | |
73804d4b RS |
565 | @end defun |
566 | ||
567 | @defun reverse list | |
568 | This function creates a new list whose elements are the elements of | |
569 | @var{list}, but in reverse order. The original argument @var{list} is | |
570 | @emph{not} altered. | |
571 | ||
572 | @example | |
573 | @group | |
574 | (setq x '(1 2 3 4)) | |
575 | @result{} (1 2 3 4) | |
576 | @end group | |
577 | @group | |
578 | (reverse x) | |
579 | @result{} (4 3 2 1) | |
580 | x | |
581 | @result{} (1 2 3 4) | |
582 | @end group | |
583 | @end example | |
584 | @end defun | |
585 | ||
586 | @node Modifying Lists | |
587 | @section Modifying Existing List Structure | |
588 | ||
589 | You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the | |
590 | primitives @code{setcar} and @code{setcdr}. | |
591 | ||
592 | @cindex CL note---@code{rplaca} vrs @code{setcar} | |
593 | @quotation | |
594 | @findex rplaca | |
595 | @findex rplacd | |
596 | @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and | |
597 | @code{rplacd} to alter list structure; they change structure the same | |
598 | way as @code{setcar} and @code{setcdr}, but the Common Lisp functions | |
599 | return the cons cell while @code{setcar} and @code{setcdr} return the | |
600 | new @sc{car} or @sc{cdr}. | |
601 | @end quotation | |
602 | ||
603 | @menu | |
604 | * Setcar:: Replacing an element in a list. | |
605 | * Setcdr:: Replacing part of the list backbone. | |
606 | This can be used to remove or add elements. | |
607 | * Rearrangement:: Reordering the elements in a list; combining lists. | |
608 | @end menu | |
609 | ||
610 | @node Setcar | |
611 | @subsection Altering List Elements with @code{setcar} | |
612 | ||
2b3fc6c3 RS |
613 | Changing the @sc{car} of a cons cell is done with @code{setcar}. When |
614 | used on a list, @code{setcar} replaces one element of a list with a | |
615 | different element. | |
73804d4b RS |
616 | |
617 | @defun setcar cons object | |
618 | This function stores @var{object} as the new @sc{car} of @var{cons}, | |
619 | replacing its previous @sc{car}. It returns the value @var{object}. | |
620 | For example: | |
621 | ||
622 | @example | |
623 | @group | |
624 | (setq x '(1 2)) | |
625 | @result{} (1 2) | |
626 | @end group | |
627 | @group | |
628 | (setcar x 4) | |
629 | @result{} 4 | |
630 | @end group | |
631 | @group | |
632 | x | |
633 | @result{} (4 2) | |
634 | @end group | |
635 | @end example | |
636 | @end defun | |
637 | ||
638 | When a cons cell is part of the shared structure of several lists, | |
639 | storing a new @sc{car} into the cons changes one element of each of | |
640 | these lists. Here is an example: | |
641 | ||
642 | @example | |
643 | @group | |
644 | ;; @r{Create two lists that are partly shared.} | |
645 | (setq x1 '(a b c)) | |
646 | @result{} (a b c) | |
647 | (setq x2 (cons 'z (cdr x1))) | |
648 | @result{} (z b c) | |
649 | @end group | |
650 | ||
651 | @group | |
652 | ;; @r{Replace the @sc{car} of a shared link.} | |
653 | (setcar (cdr x1) 'foo) | |
654 | @result{} foo | |
655 | x1 ; @r{Both lists are changed.} | |
656 | @result{} (a foo c) | |
657 | x2 | |
658 | @result{} (z foo c) | |
659 | @end group | |
660 | ||
661 | @group | |
662 | ;; @r{Replace the @sc{car} of a link that is not shared.} | |
663 | (setcar x1 'baz) | |
664 | @result{} baz | |
665 | x1 ; @r{Only one list is changed.} | |
666 | @result{} (baz foo c) | |
667 | x2 | |
668 | @result{} (z foo c) | |
669 | @end group | |
670 | @end example | |
671 | ||
672 | Here is a graphical depiction of the shared structure of the two lists | |
673 | in the variables @code{x1} and @code{x2}, showing why replacing @code{b} | |
674 | changes them both: | |
675 | ||
676 | @example | |
677 | @group | |
678 | ___ ___ ___ ___ ___ ___ | |
679 | x1---> |___|___|----> |___|___|--> |___|___|--> nil | |
680 | | --> | | | |
681 | | | | | | |
682 | --> a | --> b --> c | |
683 | | | |
684 | ___ ___ | | |
685 | x2--> |___|___|-- | |
686 | | | |
687 | | | |
688 | --> z | |
689 | @end group | |
690 | @end example | |
691 | ||
692 | Here is an alternative form of box diagram, showing the same relationship: | |
693 | ||
694 | @example | |
695 | @group | |
696 | x1: | |
697 | -------------- -------------- -------------- | |
698 | | car | cdr | | car | cdr | | car | cdr | | |
699 | | a | o------->| b | o------->| c | nil | | |
700 | | | | -->| | | | | | | |
701 | -------------- | -------------- -------------- | |
702 | | | |
703 | x2: | | |
704 | -------------- | | |
705 | | car | cdr | | | |
706 | | z | o---- | |
707 | | | | | |
708 | -------------- | |
709 | @end group | |
710 | @end example | |
711 | ||
712 | @node Setcdr | |
713 | @subsection Altering the CDR of a List | |
714 | ||
715 | The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}: | |
716 | ||
717 | @defun setcdr cons object | |
2b3fc6c3 RS |
718 | This function stores @var{object} as the new @sc{cdr} of @var{cons}, |
719 | replacing its previous @sc{cdr}. It returns the value @var{object}. | |
73804d4b RS |
720 | @end defun |
721 | ||
722 | Here is an example of replacing the @sc{cdr} of a list with a | |
723 | different list. All but the first element of the list are removed in | |
724 | favor of a different sequence of elements. The first element is | |
725 | unchanged, because it resides in the @sc{car} of the list, and is not | |
726 | reached via the @sc{cdr}. | |
727 | ||
728 | @example | |
729 | @group | |
730 | (setq x '(1 2 3)) | |
731 | @result{} (1 2 3) | |
732 | @end group | |
733 | @group | |
734 | (setcdr x '(4)) | |
735 | @result{} (4) | |
736 | @end group | |
737 | @group | |
738 | x | |
739 | @result{} (1 4) | |
740 | @end group | |
741 | @end example | |
742 | ||
743 | You can delete elements from the middle of a list by altering the | |
744 | @sc{cdr}s of the cons cells in the list. For example, here we delete | |
745 | the second element, @code{b}, from the list @code{(a b c)}, by changing | |
746 | the @sc{cdr} of the first cell: | |
747 | ||
748 | @example | |
749 | @group | |
750 | (setq x1 '(a b c)) | |
751 | @result{} (a b c) | |
752 | (setcdr x1 (cdr (cdr x1))) | |
753 | @result{} (c) | |
754 | x1 | |
755 | @result{} (a c) | |
756 | @end group | |
757 | @end example | |
758 | ||
bda144f4 | 759 | @need 4000 |
73804d4b RS |
760 | Here is the result in box notation: |
761 | ||
762 | @example | |
763 | @group | |
764 | -------------------- | |
765 | | | | |
766 | -------------- | -------------- | -------------- | |
767 | | car | cdr | | | car | cdr | -->| car | cdr | | |
768 | | a | o----- | b | o-------->| c | nil | | |
769 | | | | | | | | | | | |
770 | -------------- -------------- -------------- | |
771 | @end group | |
772 | @end example | |
773 | ||
774 | @noindent | |
775 | The second cons cell, which previously held the element @code{b}, still | |
776 | exists and its @sc{car} is still @code{b}, but it no longer forms part | |
777 | of this list. | |
778 | ||
779 | It is equally easy to insert a new element by changing @sc{cdr}s: | |
780 | ||
781 | @example | |
782 | @group | |
783 | (setq x1 '(a b c)) | |
784 | @result{} (a b c) | |
785 | (setcdr x1 (cons 'd (cdr x1))) | |
786 | @result{} (d b c) | |
787 | x1 | |
788 | @result{} (a d b c) | |
789 | @end group | |
790 | @end example | |
791 | ||
792 | Here is this result in box notation: | |
793 | ||
794 | @smallexample | |
795 | @group | |
796 | -------------- ------------- ------------- | |
797 | | car | cdr | | car | cdr | | car | cdr | | |
798 | | a | o | -->| b | o------->| c | nil | | |
799 | | | | | | | | | | | | | |
800 | --------- | -- | ------------- ------------- | |
801 | | | | |
802 | ----- -------- | |
803 | | | | |
804 | | --------------- | | |
805 | | | car | cdr | | | |
806 | -->| d | o------ | |
807 | | | | | |
808 | --------------- | |
809 | @end group | |
810 | @end smallexample | |
811 | ||
812 | @node Rearrangement | |
813 | @subsection Functions that Rearrange Lists | |
814 | @cindex rearrangement of lists | |
815 | @cindex modification of lists | |
816 | ||
817 | Here are some functions that rearrange lists ``destructively'' by | |
818 | modifying the @sc{cdr}s of their component cons cells. We call these | |
819 | functions ``destructive'' because they chew up the original lists passed | |
820 | to them as arguments, to produce a new list that is the returned value. | |
821 | ||
2b3fc6c3 RS |
822 | @ifinfo |
823 | See @code{delq}, in @ref{Sets And Lists}, for another function | |
824 | that modifies cons cells. | |
825 | @end ifinfo | |
826 | @iftex | |
827 | The function @code{delq} in the following section is another example | |
828 | of destructive list manipulation. | |
829 | @end iftex | |
830 | ||
73804d4b RS |
831 | @defun nconc &rest lists |
832 | @cindex concatenating lists | |
833 | @cindex joining lists | |
834 | This function returns a list containing all the elements of @var{lists}. | |
835 | Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are | |
836 | @emph{not} copied. Instead, the last @sc{cdr} of each of the | |
837 | @var{lists} is changed to refer to the following list. The last of the | |
838 | @var{lists} is not altered. For example: | |
839 | ||
840 | @example | |
841 | @group | |
842 | (setq x '(1 2 3)) | |
843 | @result{} (1 2 3) | |
844 | @end group | |
845 | @group | |
846 | (nconc x '(4 5)) | |
847 | @result{} (1 2 3 4 5) | |
848 | @end group | |
849 | @group | |
850 | x | |
851 | @result{} (1 2 3 4 5) | |
852 | @end group | |
853 | @end example | |
854 | ||
855 | Since the last argument of @code{nconc} is not itself modified, it is | |
856 | reasonable to use a constant list, such as @code{'(4 5)}, as in the | |
857 | above example. For the same reason, the last argument need not be a | |
858 | list: | |
859 | ||
860 | @example | |
861 | @group | |
862 | (setq x '(1 2 3)) | |
863 | @result{} (1 2 3) | |
864 | @end group | |
865 | @group | |
866 | (nconc x 'z) | |
867 | @result{} (1 2 3 . z) | |
868 | @end group | |
869 | @group | |
870 | x | |
871 | @result{} (1 2 3 . z) | |
872 | @end group | |
873 | @end example | |
874 | ||
875 | A common pitfall is to use a quoted constant list as a non-last | |
876 | argument to @code{nconc}. If you do this, your program will change | |
877 | each time you run it! Here is what happens: | |
878 | ||
879 | @smallexample | |
880 | @group | |
881 | (defun add-foo (x) ; @r{We want this function to add} | |
882 | (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.} | |
883 | @end group | |
884 | ||
885 | @group | |
886 | (symbol-function 'add-foo) | |
887 | @result{} (lambda (x) (nconc (quote (foo)) x)) | |
888 | @end group | |
889 | ||
890 | @group | |
891 | (setq xx (add-foo '(1 2))) ; @r{It seems to work.} | |
892 | @result{} (foo 1 2) | |
893 | @end group | |
894 | @group | |
895 | (setq xy (add-foo '(3 4))) ; @r{What happened?} | |
896 | @result{} (foo 1 2 3 4) | |
897 | @end group | |
898 | @group | |
899 | (eq xx xy) | |
900 | @result{} t | |
901 | @end group | |
902 | ||
903 | @group | |
904 | (symbol-function 'add-foo) | |
905 | @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x))) | |
906 | @end group | |
907 | @end smallexample | |
908 | @end defun | |
909 | ||
910 | @defun nreverse list | |
911 | @cindex reversing a list | |
912 | This function reverses the order of the elements of @var{list}. | |
2b3fc6c3 RS |
913 | Unlike @code{reverse}, @code{nreverse} alters its argument by reversing |
914 | the @sc{cdr}s in the cons cells forming the list. The cons cell that | |
915 | used to be the last one in @var{list} becomes the first cell of the | |
916 | value. | |
73804d4b RS |
917 | |
918 | For example: | |
919 | ||
920 | @example | |
921 | @group | |
922 | (setq x '(1 2 3 4)) | |
923 | @result{} (1 2 3 4) | |
924 | @end group | |
925 | @group | |
926 | x | |
927 | @result{} (1 2 3 4) | |
928 | (nreverse x) | |
929 | @result{} (4 3 2 1) | |
930 | @end group | |
931 | @group | |
932 | ;; @r{The cell that was first is now last.} | |
933 | x | |
934 | @result{} (1) | |
935 | @end group | |
936 | @end example | |
937 | ||
938 | To avoid confusion, we usually store the result of @code{nreverse} | |
939 | back in the same variable which held the original list: | |
940 | ||
941 | @example | |
942 | (setq x (nreverse x)) | |
943 | @end example | |
944 | ||
945 | Here is the @code{nreverse} of our favorite example, @code{(a b c)}, | |
946 | presented graphically: | |
947 | ||
948 | @smallexample | |
949 | @group | |
950 | @r{Original list head:} @r{Reversed list:} | |
951 | ------------- ------------- ------------ | |
952 | | car | cdr | | car | cdr | | car | cdr | | |
953 | | a | nil |<-- | b | o |<-- | c | o | | |
954 | | | | | | | | | | | | | | | |
955 | ------------- | --------- | - | -------- | - | |
956 | | | | | | |
957 | ------------- ------------ | |
958 | @end group | |
959 | @end smallexample | |
960 | @end defun | |
961 | ||
962 | @defun sort list predicate | |
963 | @cindex stable sort | |
964 | @cindex sorting lists | |
965 | This function sorts @var{list} stably, though destructively, and | |
966 | returns the sorted list. It compares elements using @var{predicate}. A | |
967 | stable sort is one in which elements with equal sort keys maintain their | |
968 | relative order before and after the sort. Stability is important when | |
969 | successive sorts are used to order elements according to different | |
970 | criteria. | |
971 | ||
972 | The argument @var{predicate} must be a function that accepts two | |
973 | arguments. It is called with two elements of @var{list}. To get an | |
974 | increasing order sort, the @var{predicate} should return @code{t} if the | |
975 | first element is ``less than'' the second, or @code{nil} if not. | |
976 | ||
977 | The destructive aspect of @code{sort} is that it rearranges the cons | |
978 | cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort | |
979 | function would create new cons cells to store the elements in their | |
980 | sorted order. If you wish to make a sorted copy without destroying the | |
981 | original, copy it first with @code{copy-sequence} and then sort. | |
982 | ||
983 | Sorting does not change the @sc{car}s of the cons cells in @var{list}; | |
984 | the cons cell that originally contained the element @code{a} in | |
985 | @var{list} still has @code{a} in its @sc{car} after sorting, but it now | |
986 | appears in a different position in the list due to the change of | |
987 | @sc{cdr}s. For example: | |
988 | ||
989 | @example | |
990 | @group | |
991 | (setq nums '(1 3 2 6 5 4 0)) | |
992 | @result{} (1 3 2 6 5 4 0) | |
993 | @end group | |
994 | @group | |
995 | (sort nums '<) | |
996 | @result{} (0 1 2 3 4 5 6) | |
997 | @end group | |
998 | @group | |
999 | nums | |
1000 | @result{} (1 2 3 4 5 6) | |
1001 | @end group | |
1002 | @end example | |
1003 | ||
1004 | @noindent | |
1005 | Note that the list in @code{nums} no longer contains 0; this is the same | |
1006 | cons cell that it was before, but it is no longer the first one in the | |
1007 | list. Don't assume a variable that formerly held the argument now holds | |
1008 | the entire sorted list! Instead, save the result of @code{sort} and use | |
1009 | that. Most often we store the result back into the variable that held | |
1010 | the original list: | |
1011 | ||
1012 | @example | |
1013 | (setq nums (sort nums '<)) | |
1014 | @end example | |
1015 | ||
1016 | @xref{Sorting}, for more functions that perform sorting. | |
1017 | See @code{documentation} in @ref{Accessing Documentation}, for a | |
1018 | useful example of @code{sort}. | |
1019 | @end defun | |
1020 | ||
73804d4b RS |
1021 | @node Sets And Lists |
1022 | @section Using Lists as Sets | |
1023 | @cindex lists as sets | |
1024 | @cindex sets | |
1025 | ||
1026 | A list can represent an unordered mathematical set---simply consider a | |
1027 | value an element of a set if it appears in the list, and ignore the | |
1028 | order of the list. To form the union of two sets, use @code{append} (as | |
1029 | long as you don't mind having duplicate elements). Other useful | |
1030 | functions for sets include @code{memq} and @code{delq}, and their | |
1031 | @code{equal} versions, @code{member} and @code{delete}. | |
1032 | ||
b5ef0e92 | 1033 | @cindex CL note---lack @code{union}, @code{intersection} |
73804d4b RS |
1034 | @quotation |
1035 | @b{Common Lisp note:} Common Lisp has functions @code{union} (which | |
1036 | avoids duplicate elements) and @code{intersection} for set operations, | |
1037 | but GNU Emacs Lisp does not have them. You can write them in Lisp if | |
1038 | you wish. | |
1039 | @end quotation | |
1040 | ||
1041 | @defun memq object list | |
1042 | @cindex membership in a list | |
1043 | This function tests to see whether @var{object} is a member of | |
1044 | @var{list}. If it is, @code{memq} returns a list starting with the | |
1045 | first occurrence of @var{object}. Otherwise, it returns @code{nil}. | |
1046 | The letter @samp{q} in @code{memq} says that it uses @code{eq} to | |
1047 | compare @var{object} against the elements of the list. For example: | |
1048 | ||
1049 | @example | |
1050 | @group | |
2b3fc6c3 RS |
1051 | (memq 'b '(a b c b a)) |
1052 | @result{} (b c b a) | |
73804d4b RS |
1053 | @end group |
1054 | @group | |
1055 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1056 | @result{} nil | |
1057 | @end group | |
1058 | @end example | |
1059 | @end defun | |
1060 | ||
1061 | @defun delq object list | |
1062 | @cindex deletion of elements | |
1063 | This function destructively removes all elements @code{eq} to | |
1064 | @var{object} from @var{list}. The letter @samp{q} in @code{delq} says | |
1065 | that it uses @code{eq} to compare @var{object} against the elements of | |
1066 | the list, like @code{memq}. | |
1067 | @end defun | |
1068 | ||
1069 | When @code{delq} deletes elements from the front of the list, it does so | |
1070 | simply by advancing down the list and returning a sublist that starts | |
1071 | after those elements: | |
1072 | ||
1073 | @example | |
1074 | @group | |
1075 | (delq 'a '(a b c)) @equiv{} (cdr '(a b c)) | |
1076 | @end group | |
1077 | @end example | |
1078 | ||
1079 | When an element to be deleted appears in the middle of the list, | |
1080 | removing it involves changing the @sc{cdr}s (@pxref{Setcdr}). | |
1081 | ||
1082 | @example | |
1083 | @group | |
2b3fc6c3 RS |
1084 | (setq sample-list '(a b c (4))) |
1085 | @result{} (a b c (4)) | |
73804d4b RS |
1086 | @end group |
1087 | @group | |
2b3fc6c3 RS |
1088 | (delq 'a sample-list) |
1089 | @result{} (b c (4)) | |
73804d4b RS |
1090 | @end group |
1091 | @group | |
1092 | sample-list | |
2b3fc6c3 | 1093 | @result{} (a b c (4)) |
73804d4b RS |
1094 | @end group |
1095 | @group | |
2b3fc6c3 | 1096 | (delq 'c sample-list) |
34e1af81 | 1097 | @result{} (a b (4)) |
73804d4b RS |
1098 | @end group |
1099 | @group | |
1100 | sample-list | |
34e1af81 | 1101 | @result{} (a b (4)) |
73804d4b RS |
1102 | @end group |
1103 | @end example | |
1104 | ||
bfe721d1 KH |
1105 | Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to |
1106 | splice out the third element, but @code{(delq 'a sample-list)} does not | |
73804d4b RS |
1107 | splice anything---it just returns a shorter list. Don't assume that a |
1108 | variable which formerly held the argument @var{list} now has fewer | |
1109 | elements, or that it still holds the original list! Instead, save the | |
1110 | result of @code{delq} and use that. Most often we store the result back | |
1111 | into the variable that held the original list: | |
1112 | ||
1113 | @example | |
1114 | (setq flowers (delq 'rose flowers)) | |
1115 | @end example | |
1116 | ||
1117 | In the following example, the @code{(4)} that @code{delq} attempts to match | |
1118 | and the @code{(4)} in the @code{sample-list} are not @code{eq}: | |
1119 | ||
1120 | @example | |
1121 | @group | |
1122 | (delq '(4) sample-list) | |
2b3fc6c3 | 1123 | @result{} (a c (4)) |
73804d4b RS |
1124 | @end group |
1125 | @end example | |
1126 | ||
1127 | The following two functions are like @code{memq} and @code{delq} but use | |
1128 | @code{equal} rather than @code{eq} to compare elements. They are new in | |
1129 | Emacs 19. | |
1130 | ||
1131 | @defun member object list | |
1132 | The function @code{member} tests to see whether @var{object} is a member | |
1133 | of @var{list}, comparing members with @var{object} using @code{equal}. | |
1134 | If @var{object} is a member, @code{member} returns a list starting with | |
1135 | its first occurrence in @var{list}. Otherwise, it returns @code{nil}. | |
1136 | ||
1137 | Compare this with @code{memq}: | |
1138 | ||
1139 | @example | |
1140 | @group | |
1141 | (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.} | |
1142 | @result{} ((2)) | |
1143 | @end group | |
1144 | @group | |
1145 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1146 | @result{} nil | |
1147 | @end group | |
1148 | @group | |
1149 | ;; @r{Two strings with the same contents are @code{equal}.} | |
1150 | (member "foo" '("foo" "bar")) | |
1151 | @result{} ("foo" "bar") | |
1152 | @end group | |
1153 | @end example | |
1154 | @end defun | |
1155 | ||
1156 | @defun delete object list | |
1157 | This function destructively removes all elements @code{equal} to | |
1158 | @var{object} from @var{list}. It is to @code{delq} as @code{member} is | |
1159 | to @code{memq}: it uses @code{equal} to compare elements with | |
1160 | @var{object}, like @code{member}; when it finds an element that matches, | |
1161 | it removes the element just as @code{delq} would. For example: | |
1162 | ||
1163 | @example | |
1164 | @group | |
1165 | (delete '(2) '((2) (1) (2))) | |
b5ef0e92 | 1166 | @result{} ((1)) |
73804d4b RS |
1167 | @end group |
1168 | @end example | |
1169 | @end defun | |
1170 | ||
1171 | @quotation | |
1172 | @b{Common Lisp note:} The functions @code{member} and @code{delete} in | |
1173 | GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common | |
1174 | Lisp versions do not use @code{equal} to compare elements. | |
1175 | @end quotation | |
1176 | ||
bfe721d1 KH |
1177 | See also the function @code{add-to-list}, in @ref{Setting Variables}, |
1178 | for another way to add an element to a list stored in a variable. | |
1179 | ||
73804d4b RS |
1180 | @node Association Lists |
1181 | @section Association Lists | |
1182 | @cindex association list | |
1183 | @cindex alist | |
1184 | ||
1185 | An @dfn{association list}, or @dfn{alist} for short, records a mapping | |
1186 | from keys to values. It is a list of cons cells called | |
1187 | @dfn{associations}: the @sc{car} of each cell is the @dfn{key}, and the | |
1188 | @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key'' | |
1189 | is not related to the term ``key sequence''; it means a value used to | |
1190 | look up an item in a table. In this case, the table is the alist, and | |
1191 | the alist associations are the items.} | |
1192 | ||
1193 | Here is an example of an alist. The key @code{pine} is associated with | |
1194 | the value @code{cones}; the key @code{oak} is associated with | |
1195 | @code{acorns}; and the key @code{maple} is associated with @code{seeds}. | |
1196 | ||
1197 | @example | |
1198 | @group | |
1199 | '((pine . cones) | |
1200 | (oak . acorns) | |
1201 | (maple . seeds)) | |
1202 | @end group | |
1203 | @end example | |
1204 | ||
1205 | The associated values in an alist may be any Lisp objects; so may the | |
1206 | keys. For example, in the following alist, the symbol @code{a} is | |
1207 | associated with the number @code{1}, and the string @code{"b"} is | |
1208 | associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of | |
1209 | the alist element: | |
1210 | ||
1211 | @example | |
1212 | ((a . 1) ("b" 2 3)) | |
1213 | @end example | |
1214 | ||
1215 | Sometimes it is better to design an alist to store the associated | |
1216 | value in the @sc{car} of the @sc{cdr} of the element. Here is an | |
1217 | example: | |
1218 | ||
1219 | @example | |
1220 | '((rose red) (lily white) (buttercup yellow)) | |
1221 | @end example | |
1222 | ||
1223 | @noindent | |
1224 | Here we regard @code{red} as the value associated with @code{rose}. One | |
1225 | advantage of this method is that you can store other related | |
1226 | information---even a list of other items---in the @sc{cdr} of the | |
1227 | @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see | |
1228 | below) to find the element containing a given value. When neither of | |
1229 | these considerations is important, the choice is a matter of taste, as | |
1230 | long as you are consistent about it for any given alist. | |
1231 | ||
1232 | Note that the same alist shown above could be regarded as having the | |
1233 | associated value in the @sc{cdr} of the element; the value associated | |
1234 | with @code{rose} would be the list @code{(red)}. | |
1235 | ||
1236 | Association lists are often used to record information that you might | |
1237 | otherwise keep on a stack, since new associations may be added easily to | |
1238 | the front of the list. When searching an association list for an | |
1239 | association with a given key, the first one found is returned, if there | |
1240 | is more than one. | |
1241 | ||
1242 | In Emacs Lisp, it is @emph{not} an error if an element of an | |
1243 | association list is not a cons cell. The alist search functions simply | |
1244 | ignore such elements. Many other versions of Lisp signal errors in such | |
1245 | cases. | |
1246 | ||
1247 | Note that property lists are similar to association lists in several | |
1248 | respects. A property list behaves like an association list in which | |
1249 | each key can occur only once. @xref{Property Lists}, for a comparison | |
1250 | of property lists and association lists. | |
1251 | ||
1252 | @defun assoc key alist | |
1253 | This function returns the first association for @var{key} in | |
1254 | @var{alist}. It compares @var{key} against the alist elements using | |
1255 | @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no | |
1256 | association in @var{alist} has a @sc{car} @code{equal} to @var{key}. | |
1257 | For example: | |
1258 | ||
1259 | @smallexample | |
1260 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1261 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1262 | (assoc 'oak trees) | |
1263 | @result{} (oak . acorns) | |
1264 | (cdr (assoc 'oak trees)) | |
1265 | @result{} acorns | |
1266 | (assoc 'birch trees) | |
1267 | @result{} nil | |
1268 | @end smallexample | |
1269 | ||
2b3fc6c3 | 1270 | Here is another example, in which the keys and values are not symbols: |
73804d4b RS |
1271 | |
1272 | @smallexample | |
1273 | (setq needles-per-cluster | |
1274 | '((2 "Austrian Pine" "Red Pine") | |
1275 | (3 "Pitch Pine") | |
1276 | (5 "White Pine"))) | |
1277 | ||
1278 | (cdr (assoc 3 needles-per-cluster)) | |
1279 | @result{} ("Pitch Pine") | |
1280 | (cdr (assoc 2 needles-per-cluster)) | |
1281 | @result{} ("Austrian Pine" "Red Pine") | |
1282 | @end smallexample | |
1283 | @end defun | |
1284 | ||
22697dac KH |
1285 | @defun rassoc value alist |
1286 | This function returns the first association with value @var{value} in | |
1287 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1288 | a @sc{cdr} @code{equal} to @var{value}. | |
1289 | ||
1290 | @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of | |
1291 | each @var{alist} association instead of the @sc{car}. You can think of | |
1292 | this as ``reverse @code{assoc}'', finding the key for a given value. | |
1293 | @end defun | |
1294 | ||
73804d4b RS |
1295 | @defun assq key alist |
1296 | This function is like @code{assoc} in that it returns the first | |
1297 | association for @var{key} in @var{alist}, but it makes the comparison | |
1298 | using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil} | |
1299 | if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}. | |
1300 | This function is used more often than @code{assoc}, since @code{eq} is | |
1301 | faster than @code{equal} and most alists use symbols as keys. | |
1302 | @xref{Equality Predicates}. | |
1303 | ||
1304 | @smallexample | |
1305 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1306 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1307 | (assq 'pine trees) | |
1308 | @result{} (pine . cones) | |
1309 | @end smallexample | |
1310 | ||
1311 | On the other hand, @code{assq} is not usually useful in alists where the | |
1312 | keys may not be symbols: | |
1313 | ||
1314 | @smallexample | |
1315 | (setq leaves | |
1316 | '(("simple leaves" . oak) | |
1317 | ("compound leaves" . horsechestnut))) | |
1318 | ||
1319 | (assq "simple leaves" leaves) | |
1320 | @result{} nil | |
1321 | (assoc "simple leaves" leaves) | |
1322 | @result{} ("simple leaves" . oak) | |
1323 | @end smallexample | |
1324 | @end defun | |
1325 | ||
1326 | @defun rassq value alist | |
1327 | This function returns the first association with value @var{value} in | |
1328 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1329 | a @sc{cdr} @code{eq} to @var{value}. | |
1330 | ||
1331 | @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of | |
1332 | each @var{alist} association instead of the @sc{car}. You can think of | |
1333 | this as ``reverse @code{assq}'', finding the key for a given value. | |
1334 | ||
1335 | For example: | |
1336 | ||
1337 | @smallexample | |
1338 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1339 | ||
1340 | (rassq 'acorns trees) | |
1341 | @result{} (oak . acorns) | |
1342 | (rassq 'spores trees) | |
1343 | @result{} nil | |
1344 | @end smallexample | |
1345 | ||
1346 | Note that @code{rassq} cannot search for a value stored in the @sc{car} | |
1347 | of the @sc{cdr} of an element: | |
1348 | ||
1349 | @smallexample | |
1350 | (setq colors '((rose red) (lily white) (buttercup yellow))) | |
1351 | ||
1352 | (rassq 'white colors) | |
1353 | @result{} nil | |
1354 | @end smallexample | |
1355 | ||
1356 | In this case, the @sc{cdr} of the association @code{(lily white)} is not | |
1357 | the symbol @code{white}, but rather the list @code{(white)}. This | |
1358 | becomes clearer if the association is written in dotted pair notation: | |
1359 | ||
1360 | @smallexample | |
1361 | (lily white) @equiv{} (lily . (white)) | |
1362 | @end smallexample | |
1363 | @end defun | |
1364 | ||
1365 | @defun copy-alist alist | |
1366 | @cindex copying alists | |
1367 | This function returns a two-level deep copy of @var{alist}: it creates a | |
1368 | new copy of each association, so that you can alter the associations of | |
1369 | the new alist without changing the old one. | |
1370 | ||
1371 | @smallexample | |
1372 | @group | |
1373 | (setq needles-per-cluster | |
1374 | '((2 . ("Austrian Pine" "Red Pine")) | |
2b3fc6c3 | 1375 | (3 . ("Pitch Pine")) |
ec221d13 | 1376 | @end group |
2b3fc6c3 | 1377 | (5 . ("White Pine")))) |
73804d4b RS |
1378 | @result{} |
1379 | ((2 "Austrian Pine" "Red Pine") | |
2b3fc6c3 RS |
1380 | (3 "Pitch Pine") |
1381 | (5 "White Pine")) | |
73804d4b RS |
1382 | |
1383 | (setq copy (copy-alist needles-per-cluster)) | |
1384 | @result{} | |
1385 | ((2 "Austrian Pine" "Red Pine") | |
2b3fc6c3 RS |
1386 | (3 "Pitch Pine") |
1387 | (5 "White Pine")) | |
73804d4b RS |
1388 | |
1389 | (eq needles-per-cluster copy) | |
1390 | @result{} nil | |
1391 | (equal needles-per-cluster copy) | |
1392 | @result{} t | |
1393 | (eq (car needles-per-cluster) (car copy)) | |
1394 | @result{} nil | |
1395 | (cdr (car (cdr needles-per-cluster))) | |
2b3fc6c3 | 1396 | @result{} ("Pitch Pine") |
ec221d13 | 1397 | @group |
73804d4b RS |
1398 | (eq (cdr (car (cdr needles-per-cluster))) |
1399 | (cdr (car (cdr copy)))) | |
1400 | @result{} t | |
1401 | @end group | |
3e099569 | 1402 | @end smallexample |
2b3fc6c3 RS |
1403 | |
1404 | This example shows how @code{copy-alist} makes it possible to change | |
1405 | the associations of one copy without affecting the other: | |
1406 | ||
3e099569 | 1407 | @smallexample |
2b3fc6c3 | 1408 | @group |
c74c521d | 1409 | (setcdr (assq 3 copy) '("Martian Vacuum Pine")) |
2b3fc6c3 RS |
1410 | (cdr (assq 3 needles-per-cluster)) |
1411 | @result{} ("Pitch Pine") | |
1412 | @end group | |
73804d4b RS |
1413 | @end smallexample |
1414 | @end defun | |
1415 | ||
1416 |