Commit | Line | Data |
---|---|---|
73804d4b RS |
1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
f9f59935 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. |
73804d4b RS |
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 | 34 | @dfn{cons cells}. A cons cell is a data object that represents an |
3998eed0 RS |
35 | ordered pair. That is, it has two slots, and each slot @dfn{holds}, or |
36 | @dfn{refers to}, some Lisp object. One slot is known as the @sc{car}, | |
37 | and the other is known as the @sc{cdr}. (These names are traditional; | |
38 | see @ref{Cons Cell Type}.) @sc{cdr} is pronounced ``could-er.'' | |
73804d4b | 39 | |
3998eed0 RS |
40 | We say that ``the @sc{car} of this cons cell is'' whatever object |
41 | its @sc{car} slot currently holds, and likewise for the @sc{cdr}. | |
42 | ||
43 | A list is a series of cons cells ``chained together,'' so that each | |
44 | cell refers to the next one. There one cons cell for each element of | |
45 | the list. By convention, the @sc{car}s of the cons cells hold the | |
46 | elements of the list, and the @sc{cdr}s are used to chain the list: the | |
47 | @sc{cdr} slot of each cons cell refers to the following cons cell. The | |
48 | @sc{cdr} of the last cons cell is @code{nil}. This asymmetry between | |
49 | the @sc{car} and the @sc{cdr} is entirely a matter of convention; at the | |
73804d4b RS |
50 | level of cons cells, the @sc{car} and @sc{cdr} slots have the same |
51 | characteristics. | |
52 | ||
2b3fc6c3 RS |
53 | @cindex list structure |
54 | Because most cons cells are used as part of lists, the phrase | |
55 | @dfn{list structure} has come to mean any structure made out of cons | |
56 | cells. | |
57 | ||
73804d4b RS |
58 | The symbol @code{nil} is considered a list as well as a symbol; it is |
59 | the list with no elements. For convenience, the symbol @code{nil} is | |
60 | considered to have @code{nil} as its @sc{cdr} (and also as its | |
61 | @sc{car}). | |
62 | ||
63 | The @sc{cdr} of any nonempty list @var{l} is a list containing all the | |
64 | elements of @var{l} except the first. | |
65 | ||
66 | @node Lists as Boxes | |
67 | @comment node-name, next, previous, up | |
68 | @section Lists as Linked Pairs of Boxes | |
69 | @cindex box representation for lists | |
70 | @cindex lists represented as boxes | |
71 | @cindex cons cell as box | |
72 | ||
73 | A cons cell can be illustrated as a pair of boxes. The first box | |
74 | represents the @sc{car} and the second box represents the @sc{cdr}. | |
75 | Here is an illustration of the two-element list, @code{(tulip lily)}, | |
76 | made from two cons cells: | |
77 | ||
78 | @example | |
79 | @group | |
80 | --------------- --------------- | |
81 | | car | cdr | | car | cdr | | |
82 | | tulip | o---------->| lily | nil | | |
83 | | | | | | | | |
84 | --------------- --------------- | |
85 | @end group | |
86 | @end example | |
87 | ||
88 | Each pair of boxes represents a cons cell. Each box ``refers to'', | |
b6954afd | 89 | ``points to'' or ``holds'' a Lisp object. (These terms are |
74490e55 RS |
90 | synonymous.) The first box, which describes the @sc{car} of the first |
91 | cons cell, contains the symbol @code{tulip}. The arrow from the | |
92 | @sc{cdr} box of the first cons cell to the second cons cell indicates | |
93 | that the @sc{cdr} of the first cons cell is the second cons cell. | |
73804d4b RS |
94 | |
95 | The same list can be illustrated in a different sort of box notation | |
96 | like this: | |
97 | ||
98 | @example | |
99 | @group | |
969fe9b5 RS |
100 | --- --- --- --- |
101 | | | |--> | | |--> nil | |
102 | --- --- --- --- | |
73804d4b RS |
103 | | | |
104 | | | | |
105 | --> tulip --> lily | |
106 | @end group | |
107 | @end example | |
108 | ||
109 | Here is a more complex illustration, showing the three-element list, | |
110 | @code{((pine needles) oak maple)}, the first element of which is a | |
111 | two-element list: | |
112 | ||
113 | @example | |
114 | @group | |
969fe9b5 RS |
115 | --- --- --- --- --- --- |
116 | | | |--> | | |--> | | |--> nil | |
117 | --- --- --- --- --- --- | |
73804d4b RS |
118 | | | | |
119 | | | | | |
120 | | --> oak --> maple | |
121 | | | |
969fe9b5 RS |
122 | | --- --- --- --- |
123 | --> | | |--> | | |--> nil | |
124 | --- --- --- --- | |
73804d4b RS |
125 | | | |
126 | | | | |
127 | --> pine --> needles | |
128 | @end group | |
129 | @end example | |
130 | ||
131 | The same list represented in the first box notation looks like this: | |
132 | ||
133 | @example | |
134 | @group | |
135 | -------------- -------------- -------------- | |
136 | | car | cdr | | car | cdr | | car | cdr | | |
137 | | o | o------->| oak | o------->| maple | nil | | |
138 | | | | | | | | | | | | |
139 | -- | --------- -------------- -------------- | |
140 | | | |
141 | | | |
142 | | -------------- ---------------- | |
143 | | | car | cdr | | car | cdr | | |
144 | ------>| pine | o------->| needles | nil | | |
145 | | | | | | | | |
146 | -------------- ---------------- | |
147 | @end group | |
148 | @end example | |
149 | ||
2b3fc6c3 RS |
150 | @xref{Cons Cell Type}, for the read and print syntax of cons cells and |
151 | lists, and for more ``box and arrow'' illustrations of lists. | |
73804d4b RS |
152 | |
153 | @node List-related Predicates | |
154 | @section Predicates on Lists | |
155 | ||
156 | The following predicates test whether a Lisp object is an atom, is a | |
157 | cons cell or is a list, or whether it is the distinguished object | |
158 | @code{nil}. (Many of these predicates can be defined in terms of the | |
159 | others, but they are used so often that it is worth having all of them.) | |
160 | ||
161 | @defun consp object | |
162 | This function returns @code{t} if @var{object} is a cons cell, @code{nil} | |
163 | otherwise. @code{nil} is not a cons cell, although it @emph{is} a list. | |
164 | @end defun | |
165 | ||
166 | @defun atom object | |
167 | @cindex atoms | |
168 | This function returns @code{t} if @var{object} is an atom, @code{nil} | |
169 | otherwise. All objects except cons cells are atoms. The symbol | |
170 | @code{nil} is an atom and is also a list; it is the only Lisp object | |
2b3fc6c3 | 171 | that is both. |
73804d4b RS |
172 | |
173 | @example | |
174 | (atom @var{object}) @equiv{} (not (consp @var{object})) | |
175 | @end example | |
176 | @end defun | |
177 | ||
178 | @defun listp object | |
179 | This function returns @code{t} if @var{object} is a cons cell or | |
180 | @code{nil}. Otherwise, it returns @code{nil}. | |
181 | ||
182 | @example | |
183 | @group | |
184 | (listp '(1)) | |
185 | @result{} t | |
186 | @end group | |
187 | @group | |
188 | (listp '()) | |
189 | @result{} t | |
190 | @end group | |
191 | @end example | |
192 | @end defun | |
193 | ||
194 | @defun nlistp object | |
195 | This function is the opposite of @code{listp}: it returns @code{t} if | |
196 | @var{object} is not a list. Otherwise, it returns @code{nil}. | |
197 | ||
198 | @example | |
199 | (listp @var{object}) @equiv{} (not (nlistp @var{object})) | |
200 | @end example | |
201 | @end defun | |
202 | ||
203 | @defun null object | |
204 | This function returns @code{t} if @var{object} is @code{nil}, and | |
205 | returns @code{nil} otherwise. This function is identical to @code{not}, | |
206 | but as a matter of clarity we use @code{null} when @var{object} is | |
207 | considered a list and @code{not} when it is considered a truth value | |
208 | (see @code{not} in @ref{Combining Conditions}). | |
209 | ||
210 | @example | |
211 | @group | |
212 | (null '(1)) | |
213 | @result{} nil | |
214 | @end group | |
215 | @group | |
216 | (null '()) | |
217 | @result{} t | |
218 | @end group | |
219 | @end example | |
220 | @end defun | |
221 | ||
ec221d13 | 222 | @need 2000 |
73804d4b RS |
223 | |
224 | @node List Elements | |
225 | @section Accessing Elements of Lists | |
226 | @cindex list elements | |
227 | ||
228 | @defun car cons-cell | |
b6954afd | 229 | This function returns the value referred to by the first slot of the |
73804d4b RS |
230 | cons cell @var{cons-cell}. Expressed another way, this function |
231 | returns the @sc{car} of @var{cons-cell}. | |
232 | ||
233 | As a special case, if @var{cons-cell} is @code{nil}, then @code{car} | |
234 | is defined to return @code{nil}; therefore, any list is a valid argument | |
235 | for @code{car}. An error is signaled if the argument is not a cons cell | |
236 | or @code{nil}. | |
237 | ||
238 | @example | |
239 | @group | |
240 | (car '(a b c)) | |
241 | @result{} a | |
242 | @end group | |
243 | @group | |
244 | (car '()) | |
245 | @result{} nil | |
246 | @end group | |
247 | @end example | |
248 | @end defun | |
249 | ||
250 | @defun cdr cons-cell | |
b6954afd | 251 | This function returns the value referred to by the second slot of |
73804d4b RS |
252 | the cons cell @var{cons-cell}. Expressed another way, this function |
253 | returns the @sc{cdr} of @var{cons-cell}. | |
254 | ||
255 | As a special case, if @var{cons-cell} is @code{nil}, then @code{cdr} | |
256 | is defined to return @code{nil}; therefore, any list is a valid argument | |
257 | for @code{cdr}. An error is signaled if the argument is not a cons cell | |
258 | or @code{nil}. | |
259 | ||
260 | @example | |
261 | @group | |
262 | (cdr '(a b c)) | |
263 | @result{} (b c) | |
264 | @end group | |
265 | @group | |
266 | (cdr '()) | |
267 | @result{} nil | |
268 | @end group | |
269 | @end example | |
270 | @end defun | |
271 | ||
272 | @defun car-safe object | |
273 | This function lets you take the @sc{car} of a cons cell while avoiding | |
274 | errors for other data types. It returns the @sc{car} of @var{object} if | |
275 | @var{object} is a cons cell, @code{nil} otherwise. This is in contrast | |
276 | to @code{car}, which signals an error if @var{object} is not a list. | |
277 | ||
278 | @example | |
279 | @group | |
280 | (car-safe @var{object}) | |
281 | @equiv{} | |
282 | (let ((x @var{object})) | |
283 | (if (consp x) | |
284 | (car x) | |
285 | nil)) | |
286 | @end group | |
287 | @end example | |
288 | @end defun | |
289 | ||
290 | @defun cdr-safe object | |
291 | This function lets you take the @sc{cdr} of a cons cell while | |
292 | avoiding errors for other data types. It returns the @sc{cdr} of | |
293 | @var{object} if @var{object} is a cons cell, @code{nil} otherwise. | |
294 | This is in contrast to @code{cdr}, which signals an error if | |
295 | @var{object} is not a list. | |
296 | ||
297 | @example | |
298 | @group | |
299 | (cdr-safe @var{object}) | |
300 | @equiv{} | |
301 | (let ((x @var{object})) | |
302 | (if (consp x) | |
303 | (cdr x) | |
304 | nil)) | |
305 | @end group | |
306 | @end example | |
307 | @end defun | |
308 | ||
8241495d RS |
309 | @tindex pop |
310 | @defmac pop listname | |
311 | This macro is a way of examining the @sc{car} of a list, | |
312 | and taking it off the list, all at once. It is new in Emacs 21. | |
313 | ||
314 | It operates on the list which is stored in the symbol @var{listname}. | |
315 | It removes this element from the list by setting @var{listname} | |
316 | to the @sc{cdr} of its old value---but it also returns the @sc{car} | |
317 | of that list, which is the element being removed. | |
318 | ||
319 | @example | |
320 | x | |
321 | @result{} (a b c) | |
322 | (pop x) | |
323 | @result{} a | |
324 | x | |
325 | @result{} (b c) | |
326 | @end example | |
327 | @end defmac | |
328 | ||
73804d4b RS |
329 | @defun nth n list |
330 | This function returns the @var{n}th element of @var{list}. Elements | |
331 | are numbered starting with zero, so the @sc{car} of @var{list} is | |
332 | element number zero. If the length of @var{list} is @var{n} or less, | |
333 | the value is @code{nil}. | |
334 | ||
335 | If @var{n} is negative, @code{nth} returns the first element of | |
336 | @var{list}. | |
337 | ||
338 | @example | |
339 | @group | |
340 | (nth 2 '(1 2 3 4)) | |
341 | @result{} 3 | |
342 | @end group | |
343 | @group | |
344 | (nth 10 '(1 2 3 4)) | |
345 | @result{} nil | |
346 | @end group | |
347 | @group | |
348 | (nth -3 '(1 2 3 4)) | |
349 | @result{} 1 | |
350 | ||
351 | (nth n x) @equiv{} (car (nthcdr n x)) | |
352 | @end group | |
353 | @end example | |
969fe9b5 RS |
354 | |
355 | The function @code{elt} is similar, but applies to any kind of sequence. | |
356 | For historical reasons, it takes its arguments in the opposite order. | |
357 | @xref{Sequence Functions}. | |
73804d4b RS |
358 | @end defun |
359 | ||
360 | @defun nthcdr n list | |
361 | This function returns the @var{n}th @sc{cdr} of @var{list}. In other | |
f9f59935 | 362 | words, it skips past the first @var{n} links of @var{list} and returns |
73804d4b RS |
363 | what follows. |
364 | ||
365 | If @var{n} is zero or negative, @code{nthcdr} returns all of | |
366 | @var{list}. If the length of @var{list} is @var{n} or less, | |
367 | @code{nthcdr} returns @code{nil}. | |
368 | ||
369 | @example | |
370 | @group | |
371 | (nthcdr 1 '(1 2 3 4)) | |
372 | @result{} (2 3 4) | |
373 | @end group | |
374 | @group | |
375 | (nthcdr 10 '(1 2 3 4)) | |
376 | @result{} nil | |
377 | @end group | |
378 | @group | |
379 | (nthcdr -3 '(1 2 3 4)) | |
380 | @result{} (1 2 3 4) | |
381 | @end group | |
382 | @end example | |
383 | @end defun | |
384 | ||
969fe9b5 | 385 | @defun safe-length list |
f9f59935 RS |
386 | This function returns the length of @var{list}, with no risk |
387 | of either an error or an infinite loop. | |
388 | ||
389 | If @var{list} is not really a list, @code{safe-length} returns 0. If | |
390 | @var{list} is circular, it returns a finite value which is at least the | |
391 | number of distinct elements. | |
392 | @end defun | |
393 | ||
969fe9b5 RS |
394 | The most common way to compute the length of a list, when you are not |
395 | worried that it may be circular, is with @code{length}. @xref{Sequence | |
396 | Functions}. | |
397 | ||
969fe9b5 RS |
398 | @defun caar cons-cell |
399 | This is the same as @code{(car (car @var{cons-cell}))}. | |
f9f59935 RS |
400 | @end defun |
401 | ||
969fe9b5 RS |
402 | @defun cadr cons-cell |
403 | This is the same as @code{(car (cdr @var{cons-cell}))} | |
404 | or @code{(nth 1 @var{cons-cell})}. | |
f9f59935 RS |
405 | @end defun |
406 | ||
969fe9b5 RS |
407 | @defun cdar cons-cell |
408 | This is the same as @code{(cdr (car @var{cons-cell}))}. | |
f9f59935 RS |
409 | @end defun |
410 | ||
969fe9b5 RS |
411 | @defun cddr cons-cell |
412 | This is the same as @code{(cdr (cdr @var{cons-cell}))} | |
413 | or @code{(nthcdr 2 @var{cons-cell})}. | |
f9f59935 RS |
414 | @end defun |
415 | ||
73804d4b RS |
416 | @node Building Lists |
417 | @comment node-name, next, previous, up | |
418 | @section Building Cons Cells and Lists | |
419 | @cindex cons cells | |
420 | @cindex building lists | |
421 | ||
422 | Many functions build lists, as lists reside at the very heart of Lisp. | |
423 | @code{cons} is the fundamental list-building function; however, it is | |
424 | interesting to note that @code{list} is used more times in the source | |
425 | code for Emacs than @code{cons}. | |
426 | ||
427 | @defun cons object1 object2 | |
428 | This function is the fundamental function used to build new list | |
429 | structure. It creates a new cons cell, making @var{object1} the | |
430 | @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new cons | |
431 | cell. The arguments @var{object1} and @var{object2} may be any Lisp | |
432 | objects, but most often @var{object2} is a list. | |
433 | ||
434 | @example | |
435 | @group | |
436 | (cons 1 '(2)) | |
437 | @result{} (1 2) | |
438 | @end group | |
439 | @group | |
440 | (cons 1 '()) | |
441 | @result{} (1) | |
442 | @end group | |
443 | @group | |
444 | (cons 1 2) | |
445 | @result{} (1 . 2) | |
446 | @end group | |
447 | @end example | |
448 | ||
449 | @cindex consing | |
450 | @code{cons} is often used to add a single element to the front of a | |
451 | list. This is called @dfn{consing the element onto the list}. For | |
452 | example: | |
453 | ||
454 | @example | |
455 | (setq list (cons newelt list)) | |
456 | @end example | |
457 | ||
458 | Note that there is no conflict between the variable named @code{list} | |
459 | used in this example and the function named @code{list} described below; | |
460 | any symbol can serve both purposes. | |
461 | @end defun | |
462 | ||
8241495d RS |
463 | @tindex push |
464 | @defmac push newelt listname | |
465 | This macro provides an alternative way to write | |
466 | @code{(setq @var{listname} (cons @var{newelt} @var{listname}))}. | |
467 | It is new in Emacs 21. | |
468 | @end defmac | |
469 | ||
73804d4b RS |
470 | @defun list &rest objects |
471 | This function creates a list with @var{objects} as its elements. The | |
472 | resulting list is always @code{nil}-terminated. If no @var{objects} | |
473 | are given, the empty list is returned. | |
474 | ||
475 | @example | |
476 | @group | |
477 | (list 1 2 3 4 5) | |
478 | @result{} (1 2 3 4 5) | |
479 | @end group | |
480 | @group | |
481 | (list 1 2 '(3 4 5) 'foo) | |
482 | @result{} (1 2 (3 4 5) foo) | |
483 | @end group | |
484 | @group | |
485 | (list) | |
486 | @result{} nil | |
487 | @end group | |
488 | @end example | |
489 | @end defun | |
490 | ||
491 | @defun make-list length object | |
492 | This function creates a list of length @var{length}, in which all the | |
493 | elements have the identical value @var{object}. Compare | |
494 | @code{make-list} with @code{make-string} (@pxref{Creating Strings}). | |
495 | ||
496 | @example | |
497 | @group | |
498 | (make-list 3 'pigs) | |
499 | @result{} (pigs pigs pigs) | |
500 | @end group | |
501 | @group | |
502 | (make-list 0 'pigs) | |
503 | @result{} nil | |
504 | @end group | |
505 | @end example | |
506 | @end defun | |
507 | ||
508 | @defun append &rest sequences | |
509 | @cindex copying lists | |
510 | This function returns a list containing all the elements of | |
969fe9b5 RS |
511 | @var{sequences}. The @var{sequences} may be lists, vectors, |
512 | bool-vectors, or strings, but the last one should usually be a list. | |
513 | All arguments except the last one are copied, so none of the arguments | |
514 | is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join | |
515 | lists with no copying.) | |
2b3fc6c3 RS |
516 | |
517 | More generally, the final argument to @code{append} may be any Lisp | |
518 | object. The final argument is not copied or converted; it becomes the | |
519 | @sc{cdr} of the last cons cell in the new list. If the final argument | |
520 | is itself a list, then its elements become in effect elements of the | |
521 | result list. If the final element is not a list, the result is a | |
522 | ``dotted list'' since its final @sc{cdr} is not @code{nil} as required | |
523 | in a true list. | |
73804d4b | 524 | |
7dd3d99f RS |
525 | The @code{append} function also allows integers as arguments. It |
526 | converts them to strings of digits, making up the decimal print | |
527 | representation of the integer, and then uses the strings instead of the | |
528 | original integers. @strong{Don't use this feature; we plan to eliminate | |
529 | it. If you already use this feature, change your programs now!} The | |
530 | proper way to convert an integer to a decimal number in this way is with | |
531 | @code{format} (@pxref{Formatting Strings}) or @code{number-to-string} | |
532 | (@pxref{String Conversion}). | |
533 | @end defun | |
534 | ||
535 | Here is an example of using @code{append}: | |
73804d4b RS |
536 | |
537 | @example | |
538 | @group | |
539 | (setq trees '(pine oak)) | |
540 | @result{} (pine oak) | |
541 | (setq more-trees (append '(maple birch) trees)) | |
542 | @result{} (maple birch pine oak) | |
543 | @end group | |
544 | ||
545 | @group | |
546 | trees | |
547 | @result{} (pine oak) | |
548 | more-trees | |
549 | @result{} (maple birch pine oak) | |
550 | @end group | |
551 | @group | |
552 | (eq trees (cdr (cdr more-trees))) | |
553 | @result{} t | |
554 | @end group | |
555 | @end example | |
556 | ||
7dd3d99f | 557 | You can see how @code{append} works by looking at a box diagram. The |
2b3fc6c3 RS |
558 | variable @code{trees} is set to the list @code{(pine oak)} and then the |
559 | variable @code{more-trees} is set to the list @code{(maple birch pine | |
560 | oak)}. However, the variable @code{trees} continues to refer to the | |
561 | original list: | |
73804d4b RS |
562 | |
563 | @smallexample | |
564 | @group | |
565 | more-trees trees | |
566 | | | | |
969fe9b5 RS |
567 | | --- --- --- --- -> --- --- --- --- |
568 | --> | | |--> | | |--> | | |--> | | |--> nil | |
569 | --- --- --- --- --- --- --- --- | |
73804d4b RS |
570 | | | | | |
571 | | | | | | |
572 | --> maple -->birch --> pine --> oak | |
573 | @end group | |
574 | @end smallexample | |
575 | ||
7dd3d99f | 576 | An empty sequence contributes nothing to the value returned by |
73804d4b | 577 | @code{append}. As a consequence of this, a final @code{nil} argument |
7dd3d99f | 578 | forces a copy of the previous argument: |
73804d4b RS |
579 | |
580 | @example | |
581 | @group | |
582 | trees | |
583 | @result{} (pine oak) | |
584 | @end group | |
585 | @group | |
969fe9b5 | 586 | (setq wood (append trees nil)) |
73804d4b RS |
587 | @result{} (pine oak) |
588 | @end group | |
589 | @group | |
590 | wood | |
591 | @result{} (pine oak) | |
592 | @end group | |
593 | @group | |
594 | (eq wood trees) | |
595 | @result{} nil | |
596 | @end group | |
597 | @end example | |
598 | ||
599 | @noindent | |
600 | This once was the usual way to copy a list, before the function | |
601 | @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}. | |
602 | ||
7dd3d99f | 603 | Here we show the use of vectors and strings as arguments to @code{append}: |
969fe9b5 RS |
604 | |
605 | @example | |
606 | @group | |
607 | (append [a b] "cd" nil) | |
608 | @result{} (a b 99 100) | |
609 | @end group | |
610 | @end example | |
611 | ||
7dd3d99f | 612 | With the help of @code{apply} (@pxref{Calling Functions}), we can append |
a9f0a989 | 613 | all the lists in a list of lists: |
73804d4b RS |
614 | |
615 | @example | |
616 | @group | |
617 | (apply 'append '((a b c) nil (x y z) nil)) | |
618 | @result{} (a b c x y z) | |
619 | @end group | |
620 | @end example | |
621 | ||
7dd3d99f | 622 | If no @var{sequences} are given, @code{nil} is returned: |
73804d4b RS |
623 | |
624 | @example | |
625 | @group | |
626 | (append) | |
627 | @result{} nil | |
628 | @end group | |
629 | @end example | |
630 | ||
7dd3d99f | 631 | Here are some examples where the final argument is not a list: |
2b3fc6c3 RS |
632 | |
633 | @example | |
634 | (append '(x y) 'z) | |
bfe721d1 | 635 | @result{} (x y . z) |
2b3fc6c3 | 636 | (append '(x y) [z]) |
bfe721d1 | 637 | @result{} (x y . [z]) |
2b3fc6c3 RS |
638 | @end example |
639 | ||
640 | @noindent | |
641 | The second example shows that when the final argument is a sequence but | |
642 | not a list, the sequence's elements do not become elements of the | |
643 | resulting list. Instead, the sequence becomes the final @sc{cdr}, like | |
644 | any other non-list final argument. | |
73804d4b | 645 | |
73804d4b RS |
646 | @defun reverse list |
647 | This function creates a new list whose elements are the elements of | |
648 | @var{list}, but in reverse order. The original argument @var{list} is | |
649 | @emph{not} altered. | |
650 | ||
651 | @example | |
652 | @group | |
653 | (setq x '(1 2 3 4)) | |
654 | @result{} (1 2 3 4) | |
655 | @end group | |
656 | @group | |
657 | (reverse x) | |
658 | @result{} (4 3 2 1) | |
659 | x | |
660 | @result{} (1 2 3 4) | |
661 | @end group | |
662 | @end example | |
663 | @end defun | |
664 | ||
665 | @node Modifying Lists | |
666 | @section Modifying Existing List Structure | |
f1e2c45e | 667 | @cindex destructive list operations |
73804d4b RS |
668 | |
669 | You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the | |
f1e2c45e RS |
670 | primitives @code{setcar} and @code{setcdr}. We call these ``destructive'' |
671 | operations because they change existing list structure. | |
73804d4b RS |
672 | |
673 | @cindex CL note---@code{rplaca} vrs @code{setcar} | |
674 | @quotation | |
675 | @findex rplaca | |
676 | @findex rplacd | |
677 | @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and | |
678 | @code{rplacd} to alter list structure; they change structure the same | |
679 | way as @code{setcar} and @code{setcdr}, but the Common Lisp functions | |
680 | return the cons cell while @code{setcar} and @code{setcdr} return the | |
681 | new @sc{car} or @sc{cdr}. | |
682 | @end quotation | |
683 | ||
684 | @menu | |
685 | * Setcar:: Replacing an element in a list. | |
686 | * Setcdr:: Replacing part of the list backbone. | |
687 | This can be used to remove or add elements. | |
688 | * Rearrangement:: Reordering the elements in a list; combining lists. | |
689 | @end menu | |
690 | ||
691 | @node Setcar | |
692 | @subsection Altering List Elements with @code{setcar} | |
693 | ||
2b3fc6c3 RS |
694 | Changing the @sc{car} of a cons cell is done with @code{setcar}. When |
695 | used on a list, @code{setcar} replaces one element of a list with a | |
696 | different element. | |
73804d4b RS |
697 | |
698 | @defun setcar cons object | |
699 | This function stores @var{object} as the new @sc{car} of @var{cons}, | |
74490e55 | 700 | replacing its previous @sc{car}. In other words, it changes the |
b6954afd | 701 | @sc{car} slot of @var{cons} to refer to @var{object}. It returns the |
74490e55 | 702 | value @var{object}. For example: |
73804d4b RS |
703 | |
704 | @example | |
705 | @group | |
706 | (setq x '(1 2)) | |
707 | @result{} (1 2) | |
708 | @end group | |
709 | @group | |
710 | (setcar x 4) | |
711 | @result{} 4 | |
712 | @end group | |
713 | @group | |
714 | x | |
715 | @result{} (4 2) | |
716 | @end group | |
717 | @end example | |
718 | @end defun | |
719 | ||
720 | When a cons cell is part of the shared structure of several lists, | |
721 | storing a new @sc{car} into the cons changes one element of each of | |
722 | these lists. Here is an example: | |
723 | ||
724 | @example | |
725 | @group | |
726 | ;; @r{Create two lists that are partly shared.} | |
727 | (setq x1 '(a b c)) | |
728 | @result{} (a b c) | |
729 | (setq x2 (cons 'z (cdr x1))) | |
730 | @result{} (z b c) | |
731 | @end group | |
732 | ||
733 | @group | |
734 | ;; @r{Replace the @sc{car} of a shared link.} | |
735 | (setcar (cdr x1) 'foo) | |
736 | @result{} foo | |
737 | x1 ; @r{Both lists are changed.} | |
738 | @result{} (a foo c) | |
739 | x2 | |
740 | @result{} (z foo c) | |
741 | @end group | |
742 | ||
743 | @group | |
744 | ;; @r{Replace the @sc{car} of a link that is not shared.} | |
745 | (setcar x1 'baz) | |
746 | @result{} baz | |
747 | x1 ; @r{Only one list is changed.} | |
748 | @result{} (baz foo c) | |
749 | x2 | |
750 | @result{} (z foo c) | |
751 | @end group | |
752 | @end example | |
753 | ||
754 | Here is a graphical depiction of the shared structure of the two lists | |
755 | in the variables @code{x1} and @code{x2}, showing why replacing @code{b} | |
756 | changes them both: | |
757 | ||
758 | @example | |
759 | @group | |
969fe9b5 RS |
760 | --- --- --- --- --- --- |
761 | x1---> | | |----> | | |--> | | |--> nil | |
762 | --- --- --- --- --- --- | |
73804d4b RS |
763 | | --> | | |
764 | | | | | | |
765 | --> a | --> b --> c | |
766 | | | |
969fe9b5 RS |
767 | --- --- | |
768 | x2--> | | |-- | |
769 | --- --- | |
73804d4b RS |
770 | | |
771 | | | |
772 | --> z | |
773 | @end group | |
774 | @end example | |
775 | ||
776 | Here is an alternative form of box diagram, showing the same relationship: | |
777 | ||
778 | @example | |
779 | @group | |
780 | x1: | |
781 | -------------- -------------- -------------- | |
782 | | car | cdr | | car | cdr | | car | cdr | | |
783 | | a | o------->| b | o------->| c | nil | | |
784 | | | | -->| | | | | | | |
785 | -------------- | -------------- -------------- | |
786 | | | |
787 | x2: | | |
788 | -------------- | | |
789 | | car | cdr | | | |
790 | | z | o---- | |
791 | | | | | |
792 | -------------- | |
793 | @end group | |
794 | @end example | |
795 | ||
796 | @node Setcdr | |
797 | @subsection Altering the CDR of a List | |
798 | ||
799 | The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}: | |
800 | ||
801 | @defun setcdr cons object | |
2b3fc6c3 | 802 | This function stores @var{object} as the new @sc{cdr} of @var{cons}, |
74490e55 | 803 | replacing its previous @sc{cdr}. In other words, it changes the |
b6954afd | 804 | @sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the |
74490e55 | 805 | value @var{object}. |
73804d4b RS |
806 | @end defun |
807 | ||
808 | Here is an example of replacing the @sc{cdr} of a list with a | |
809 | different list. All but the first element of the list are removed in | |
810 | favor of a different sequence of elements. The first element is | |
811 | unchanged, because it resides in the @sc{car} of the list, and is not | |
812 | reached via the @sc{cdr}. | |
813 | ||
814 | @example | |
815 | @group | |
816 | (setq x '(1 2 3)) | |
817 | @result{} (1 2 3) | |
818 | @end group | |
819 | @group | |
820 | (setcdr x '(4)) | |
821 | @result{} (4) | |
822 | @end group | |
823 | @group | |
824 | x | |
825 | @result{} (1 4) | |
826 | @end group | |
827 | @end example | |
828 | ||
829 | You can delete elements from the middle of a list by altering the | |
830 | @sc{cdr}s of the cons cells in the list. For example, here we delete | |
831 | the second element, @code{b}, from the list @code{(a b c)}, by changing | |
74490e55 | 832 | the @sc{cdr} of the first cons cell: |
73804d4b RS |
833 | |
834 | @example | |
835 | @group | |
836 | (setq x1 '(a b c)) | |
837 | @result{} (a b c) | |
838 | (setcdr x1 (cdr (cdr x1))) | |
839 | @result{} (c) | |
840 | x1 | |
841 | @result{} (a c) | |
842 | @end group | |
843 | @end example | |
844 | ||
bda144f4 | 845 | @need 4000 |
73804d4b RS |
846 | Here is the result in box notation: |
847 | ||
848 | @example | |
849 | @group | |
850 | -------------------- | |
851 | | | | |
852 | -------------- | -------------- | -------------- | |
853 | | car | cdr | | | car | cdr | -->| car | cdr | | |
854 | | a | o----- | b | o-------->| c | nil | | |
855 | | | | | | | | | | | |
856 | -------------- -------------- -------------- | |
857 | @end group | |
858 | @end example | |
859 | ||
860 | @noindent | |
861 | The second cons cell, which previously held the element @code{b}, still | |
862 | exists and its @sc{car} is still @code{b}, but it no longer forms part | |
863 | of this list. | |
864 | ||
865 | It is equally easy to insert a new element by changing @sc{cdr}s: | |
866 | ||
867 | @example | |
868 | @group | |
869 | (setq x1 '(a b c)) | |
870 | @result{} (a b c) | |
871 | (setcdr x1 (cons 'd (cdr x1))) | |
872 | @result{} (d b c) | |
873 | x1 | |
874 | @result{} (a d b c) | |
875 | @end group | |
876 | @end example | |
877 | ||
878 | Here is this result in box notation: | |
879 | ||
880 | @smallexample | |
881 | @group | |
882 | -------------- ------------- ------------- | |
883 | | car | cdr | | car | cdr | | car | cdr | | |
884 | | a | o | -->| b | o------->| c | nil | | |
885 | | | | | | | | | | | | | |
886 | --------- | -- | ------------- ------------- | |
887 | | | | |
888 | ----- -------- | |
889 | | | | |
890 | | --------------- | | |
891 | | | car | cdr | | | |
892 | -->| d | o------ | |
893 | | | | | |
894 | --------------- | |
895 | @end group | |
896 | @end smallexample | |
897 | ||
898 | @node Rearrangement | |
899 | @subsection Functions that Rearrange Lists | |
900 | @cindex rearrangement of lists | |
901 | @cindex modification of lists | |
902 | ||
903 | Here are some functions that rearrange lists ``destructively'' by | |
904 | modifying the @sc{cdr}s of their component cons cells. We call these | |
905 | functions ``destructive'' because they chew up the original lists passed | |
f1e2c45e RS |
906 | to them as arguments, relinking their cons cells to form a new list that |
907 | is the returned value. | |
73804d4b | 908 | |
2b3fc6c3 RS |
909 | @ifinfo |
910 | See @code{delq}, in @ref{Sets And Lists}, for another function | |
911 | that modifies cons cells. | |
912 | @end ifinfo | |
913 | @iftex | |
914 | The function @code{delq} in the following section is another example | |
915 | of destructive list manipulation. | |
916 | @end iftex | |
917 | ||
73804d4b RS |
918 | @defun nconc &rest lists |
919 | @cindex concatenating lists | |
920 | @cindex joining lists | |
921 | This function returns a list containing all the elements of @var{lists}. | |
922 | Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are | |
923 | @emph{not} copied. Instead, the last @sc{cdr} of each of the | |
924 | @var{lists} is changed to refer to the following list. The last of the | |
925 | @var{lists} is not altered. For example: | |
926 | ||
927 | @example | |
928 | @group | |
929 | (setq x '(1 2 3)) | |
930 | @result{} (1 2 3) | |
931 | @end group | |
932 | @group | |
933 | (nconc x '(4 5)) | |
934 | @result{} (1 2 3 4 5) | |
935 | @end group | |
936 | @group | |
937 | x | |
938 | @result{} (1 2 3 4 5) | |
939 | @end group | |
940 | @end example | |
941 | ||
942 | Since the last argument of @code{nconc} is not itself modified, it is | |
943 | reasonable to use a constant list, such as @code{'(4 5)}, as in the | |
944 | above example. For the same reason, the last argument need not be a | |
945 | list: | |
946 | ||
947 | @example | |
948 | @group | |
949 | (setq x '(1 2 3)) | |
950 | @result{} (1 2 3) | |
951 | @end group | |
952 | @group | |
953 | (nconc x 'z) | |
954 | @result{} (1 2 3 . z) | |
955 | @end group | |
956 | @group | |
957 | x | |
958 | @result{} (1 2 3 . z) | |
959 | @end group | |
960 | @end example | |
961 | ||
969fe9b5 RS |
962 | However, the other arguments (all but the last) must be lists. |
963 | ||
73804d4b RS |
964 | A common pitfall is to use a quoted constant list as a non-last |
965 | argument to @code{nconc}. If you do this, your program will change | |
966 | each time you run it! Here is what happens: | |
967 | ||
968 | @smallexample | |
969 | @group | |
970 | (defun add-foo (x) ; @r{We want this function to add} | |
971 | (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.} | |
972 | @end group | |
973 | ||
974 | @group | |
975 | (symbol-function 'add-foo) | |
976 | @result{} (lambda (x) (nconc (quote (foo)) x)) | |
977 | @end group | |
978 | ||
979 | @group | |
980 | (setq xx (add-foo '(1 2))) ; @r{It seems to work.} | |
981 | @result{} (foo 1 2) | |
982 | @end group | |
983 | @group | |
984 | (setq xy (add-foo '(3 4))) ; @r{What happened?} | |
985 | @result{} (foo 1 2 3 4) | |
986 | @end group | |
987 | @group | |
988 | (eq xx xy) | |
989 | @result{} t | |
990 | @end group | |
991 | ||
992 | @group | |
993 | (symbol-function 'add-foo) | |
994 | @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x))) | |
995 | @end group | |
996 | @end smallexample | |
997 | @end defun | |
998 | ||
999 | @defun nreverse list | |
1000 | @cindex reversing a list | |
1001 | This function reverses the order of the elements of @var{list}. | |
2b3fc6c3 RS |
1002 | Unlike @code{reverse}, @code{nreverse} alters its argument by reversing |
1003 | the @sc{cdr}s in the cons cells forming the list. The cons cell that | |
74490e55 | 1004 | used to be the last one in @var{list} becomes the first cons cell of the |
2b3fc6c3 | 1005 | value. |
73804d4b RS |
1006 | |
1007 | For example: | |
1008 | ||
1009 | @example | |
1010 | @group | |
1011 | (setq x '(1 2 3 4)) | |
1012 | @result{} (1 2 3 4) | |
1013 | @end group | |
1014 | @group | |
1015 | x | |
1016 | @result{} (1 2 3 4) | |
1017 | (nreverse x) | |
1018 | @result{} (4 3 2 1) | |
1019 | @end group | |
1020 | @group | |
74490e55 | 1021 | ;; @r{The cons cell that was first is now last.} |
73804d4b RS |
1022 | x |
1023 | @result{} (1) | |
1024 | @end group | |
1025 | @end example | |
1026 | ||
1027 | To avoid confusion, we usually store the result of @code{nreverse} | |
1028 | back in the same variable which held the original list: | |
1029 | ||
1030 | @example | |
1031 | (setq x (nreverse x)) | |
1032 | @end example | |
1033 | ||
1034 | Here is the @code{nreverse} of our favorite example, @code{(a b c)}, | |
1035 | presented graphically: | |
1036 | ||
1037 | @smallexample | |
1038 | @group | |
1039 | @r{Original list head:} @r{Reversed list:} | |
1040 | ------------- ------------- ------------ | |
1041 | | car | cdr | | car | cdr | | car | cdr | | |
1042 | | a | nil |<-- | b | o |<-- | c | o | | |
1043 | | | | | | | | | | | | | | | |
1044 | ------------- | --------- | - | -------- | - | |
1045 | | | | | | |
1046 | ------------- ------------ | |
1047 | @end group | |
1048 | @end smallexample | |
1049 | @end defun | |
1050 | ||
1051 | @defun sort list predicate | |
1052 | @cindex stable sort | |
1053 | @cindex sorting lists | |
1054 | This function sorts @var{list} stably, though destructively, and | |
1055 | returns the sorted list. It compares elements using @var{predicate}. A | |
1056 | stable sort is one in which elements with equal sort keys maintain their | |
1057 | relative order before and after the sort. Stability is important when | |
1058 | successive sorts are used to order elements according to different | |
1059 | criteria. | |
1060 | ||
1061 | The argument @var{predicate} must be a function that accepts two | |
1062 | arguments. It is called with two elements of @var{list}. To get an | |
1063 | increasing order sort, the @var{predicate} should return @code{t} if the | |
1064 | first element is ``less than'' the second, or @code{nil} if not. | |
1065 | ||
a9f0a989 RS |
1066 | The comparison function @var{predicate} must give reliable results for |
1067 | any given pair of arguments, at least within a single call to | |
1068 | @code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is | |
1069 | less than @var{b}, @var{b} must not be less than @var{a}. It must be | |
1070 | @dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b} | |
1071 | is less than @var{c}, then @var{a} must be less than @var{c}. If you | |
1072 | use a comparison function which does not meet these requirements, the | |
1073 | result of @code{sort} is unpredictable. | |
1074 | ||
73804d4b RS |
1075 | The destructive aspect of @code{sort} is that it rearranges the cons |
1076 | cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort | |
1077 | function would create new cons cells to store the elements in their | |
1078 | sorted order. If you wish to make a sorted copy without destroying the | |
1079 | original, copy it first with @code{copy-sequence} and then sort. | |
1080 | ||
1081 | Sorting does not change the @sc{car}s of the cons cells in @var{list}; | |
1082 | the cons cell that originally contained the element @code{a} in | |
1083 | @var{list} still has @code{a} in its @sc{car} after sorting, but it now | |
1084 | appears in a different position in the list due to the change of | |
1085 | @sc{cdr}s. For example: | |
1086 | ||
1087 | @example | |
1088 | @group | |
1089 | (setq nums '(1 3 2 6 5 4 0)) | |
1090 | @result{} (1 3 2 6 5 4 0) | |
1091 | @end group | |
1092 | @group | |
1093 | (sort nums '<) | |
1094 | @result{} (0 1 2 3 4 5 6) | |
1095 | @end group | |
1096 | @group | |
1097 | nums | |
1098 | @result{} (1 2 3 4 5 6) | |
1099 | @end group | |
1100 | @end example | |
1101 | ||
1102 | @noindent | |
f9f59935 RS |
1103 | @strong{Warning}: Note that the list in @code{nums} no longer contains |
1104 | 0; this is the same cons cell that it was before, but it is no longer | |
1105 | the first one in the list. Don't assume a variable that formerly held | |
1106 | the argument now holds the entire sorted list! Instead, save the result | |
1107 | of @code{sort} and use that. Most often we store the result back into | |
1108 | the variable that held the original list: | |
73804d4b RS |
1109 | |
1110 | @example | |
1111 | (setq nums (sort nums '<)) | |
1112 | @end example | |
1113 | ||
1114 | @xref{Sorting}, for more functions that perform sorting. | |
1115 | See @code{documentation} in @ref{Accessing Documentation}, for a | |
1116 | useful example of @code{sort}. | |
1117 | @end defun | |
1118 | ||
73804d4b RS |
1119 | @node Sets And Lists |
1120 | @section Using Lists as Sets | |
1121 | @cindex lists as sets | |
1122 | @cindex sets | |
1123 | ||
1124 | A list can represent an unordered mathematical set---simply consider a | |
1125 | value an element of a set if it appears in the list, and ignore the | |
1126 | order of the list. To form the union of two sets, use @code{append} (as | |
1127 | long as you don't mind having duplicate elements). Other useful | |
1128 | functions for sets include @code{memq} and @code{delq}, and their | |
1129 | @code{equal} versions, @code{member} and @code{delete}. | |
1130 | ||
b5ef0e92 | 1131 | @cindex CL note---lack @code{union}, @code{intersection} |
73804d4b RS |
1132 | @quotation |
1133 | @b{Common Lisp note:} Common Lisp has functions @code{union} (which | |
1134 | avoids duplicate elements) and @code{intersection} for set operations, | |
1135 | but GNU Emacs Lisp does not have them. You can write them in Lisp if | |
1136 | you wish. | |
1137 | @end quotation | |
1138 | ||
1139 | @defun memq object list | |
1140 | @cindex membership in a list | |
1141 | This function tests to see whether @var{object} is a member of | |
1142 | @var{list}. If it is, @code{memq} returns a list starting with the | |
1143 | first occurrence of @var{object}. Otherwise, it returns @code{nil}. | |
1144 | The letter @samp{q} in @code{memq} says that it uses @code{eq} to | |
1145 | compare @var{object} against the elements of the list. For example: | |
1146 | ||
1147 | @example | |
1148 | @group | |
2b3fc6c3 RS |
1149 | (memq 'b '(a b c b a)) |
1150 | @result{} (b c b a) | |
73804d4b RS |
1151 | @end group |
1152 | @group | |
1153 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1154 | @result{} nil | |
1155 | @end group | |
1156 | @end example | |
1157 | @end defun | |
1158 | ||
1159 | @defun delq object list | |
1160 | @cindex deletion of elements | |
1161 | This function destructively removes all elements @code{eq} to | |
1162 | @var{object} from @var{list}. The letter @samp{q} in @code{delq} says | |
1163 | that it uses @code{eq} to compare @var{object} against the elements of | |
1164 | the list, like @code{memq}. | |
1165 | @end defun | |
1166 | ||
1167 | When @code{delq} deletes elements from the front of the list, it does so | |
1168 | simply by advancing down the list and returning a sublist that starts | |
1169 | after those elements: | |
1170 | ||
1171 | @example | |
1172 | @group | |
1173 | (delq 'a '(a b c)) @equiv{} (cdr '(a b c)) | |
1174 | @end group | |
1175 | @end example | |
1176 | ||
1177 | When an element to be deleted appears in the middle of the list, | |
1178 | removing it involves changing the @sc{cdr}s (@pxref{Setcdr}). | |
1179 | ||
1180 | @example | |
1181 | @group | |
2b3fc6c3 RS |
1182 | (setq sample-list '(a b c (4))) |
1183 | @result{} (a b c (4)) | |
73804d4b RS |
1184 | @end group |
1185 | @group | |
2b3fc6c3 RS |
1186 | (delq 'a sample-list) |
1187 | @result{} (b c (4)) | |
73804d4b RS |
1188 | @end group |
1189 | @group | |
1190 | sample-list | |
2b3fc6c3 | 1191 | @result{} (a b c (4)) |
73804d4b RS |
1192 | @end group |
1193 | @group | |
2b3fc6c3 | 1194 | (delq 'c sample-list) |
34e1af81 | 1195 | @result{} (a b (4)) |
73804d4b RS |
1196 | @end group |
1197 | @group | |
1198 | sample-list | |
34e1af81 | 1199 | @result{} (a b (4)) |
73804d4b RS |
1200 | @end group |
1201 | @end example | |
1202 | ||
bfe721d1 KH |
1203 | Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to |
1204 | splice out the third element, but @code{(delq 'a sample-list)} does not | |
73804d4b RS |
1205 | splice anything---it just returns a shorter list. Don't assume that a |
1206 | variable which formerly held the argument @var{list} now has fewer | |
1207 | elements, or that it still holds the original list! Instead, save the | |
1208 | result of @code{delq} and use that. Most often we store the result back | |
1209 | into the variable that held the original list: | |
1210 | ||
1211 | @example | |
1212 | (setq flowers (delq 'rose flowers)) | |
1213 | @end example | |
1214 | ||
1215 | In the following example, the @code{(4)} that @code{delq} attempts to match | |
1216 | and the @code{(4)} in the @code{sample-list} are not @code{eq}: | |
1217 | ||
1218 | @example | |
1219 | @group | |
1220 | (delq '(4) sample-list) | |
2b3fc6c3 | 1221 | @result{} (a c (4)) |
73804d4b RS |
1222 | @end group |
1223 | @end example | |
1224 | ||
1225 | The following two functions are like @code{memq} and @code{delq} but use | |
969fe9b5 RS |
1226 | @code{equal} rather than @code{eq} to compare elements. @xref{Equality |
1227 | Predicates}. | |
73804d4b RS |
1228 | |
1229 | @defun member object list | |
1230 | The function @code{member} tests to see whether @var{object} is a member | |
1231 | of @var{list}, comparing members with @var{object} using @code{equal}. | |
1232 | If @var{object} is a member, @code{member} returns a list starting with | |
1233 | its first occurrence in @var{list}. Otherwise, it returns @code{nil}. | |
1234 | ||
1235 | Compare this with @code{memq}: | |
1236 | ||
1237 | @example | |
1238 | @group | |
1239 | (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.} | |
1240 | @result{} ((2)) | |
1241 | @end group | |
1242 | @group | |
1243 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} | |
1244 | @result{} nil | |
1245 | @end group | |
1246 | @group | |
1247 | ;; @r{Two strings with the same contents are @code{equal}.} | |
1248 | (member "foo" '("foo" "bar")) | |
1249 | @result{} ("foo" "bar") | |
1250 | @end group | |
1251 | @end example | |
1252 | @end defun | |
1253 | ||
1254 | @defun delete object list | |
1255 | This function destructively removes all elements @code{equal} to | |
1256 | @var{object} from @var{list}. It is to @code{delq} as @code{member} is | |
1257 | to @code{memq}: it uses @code{equal} to compare elements with | |
1258 | @var{object}, like @code{member}; when it finds an element that matches, | |
1259 | it removes the element just as @code{delq} would. For example: | |
1260 | ||
1261 | @example | |
1262 | @group | |
1263 | (delete '(2) '((2) (1) (2))) | |
b5ef0e92 | 1264 | @result{} ((1)) |
73804d4b RS |
1265 | @end group |
1266 | @end example | |
1267 | @end defun | |
1268 | ||
1269 | @quotation | |
1270 | @b{Common Lisp note:} The functions @code{member} and @code{delete} in | |
1271 | GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common | |
1272 | Lisp versions do not use @code{equal} to compare elements. | |
1273 | @end quotation | |
1274 | ||
bfe721d1 KH |
1275 | See also the function @code{add-to-list}, in @ref{Setting Variables}, |
1276 | for another way to add an element to a list stored in a variable. | |
1277 | ||
73804d4b RS |
1278 | @node Association Lists |
1279 | @section Association Lists | |
1280 | @cindex association list | |
1281 | @cindex alist | |
1282 | ||
1283 | An @dfn{association list}, or @dfn{alist} for short, records a mapping | |
1284 | from keys to values. It is a list of cons cells called | |
74490e55 | 1285 | @dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the |
73804d4b RS |
1286 | @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key'' |
1287 | is not related to the term ``key sequence''; it means a value used to | |
1288 | look up an item in a table. In this case, the table is the alist, and | |
1289 | the alist associations are the items.} | |
1290 | ||
1291 | Here is an example of an alist. The key @code{pine} is associated with | |
1292 | the value @code{cones}; the key @code{oak} is associated with | |
1293 | @code{acorns}; and the key @code{maple} is associated with @code{seeds}. | |
1294 | ||
1295 | @example | |
1296 | @group | |
1297 | '((pine . cones) | |
1298 | (oak . acorns) | |
1299 | (maple . seeds)) | |
1300 | @end group | |
1301 | @end example | |
1302 | ||
1303 | The associated values in an alist may be any Lisp objects; so may the | |
1304 | keys. For example, in the following alist, the symbol @code{a} is | |
1305 | associated with the number @code{1}, and the string @code{"b"} is | |
1306 | associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of | |
1307 | the alist element: | |
1308 | ||
1309 | @example | |
1310 | ((a . 1) ("b" 2 3)) | |
1311 | @end example | |
1312 | ||
1313 | Sometimes it is better to design an alist to store the associated | |
1314 | value in the @sc{car} of the @sc{cdr} of the element. Here is an | |
1315 | example: | |
1316 | ||
1317 | @example | |
1318 | '((rose red) (lily white) (buttercup yellow)) | |
1319 | @end example | |
1320 | ||
1321 | @noindent | |
1322 | Here we regard @code{red} as the value associated with @code{rose}. One | |
f9f59935 | 1323 | advantage of this kind of alist is that you can store other related |
73804d4b RS |
1324 | information---even a list of other items---in the @sc{cdr} of the |
1325 | @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see | |
1326 | below) to find the element containing a given value. When neither of | |
1327 | these considerations is important, the choice is a matter of taste, as | |
1328 | long as you are consistent about it for any given alist. | |
1329 | ||
1330 | Note that the same alist shown above could be regarded as having the | |
1331 | associated value in the @sc{cdr} of the element; the value associated | |
1332 | with @code{rose} would be the list @code{(red)}. | |
1333 | ||
1334 | Association lists are often used to record information that you might | |
1335 | otherwise keep on a stack, since new associations may be added easily to | |
1336 | the front of the list. When searching an association list for an | |
1337 | association with a given key, the first one found is returned, if there | |
1338 | is more than one. | |
1339 | ||
1340 | In Emacs Lisp, it is @emph{not} an error if an element of an | |
1341 | association list is not a cons cell. The alist search functions simply | |
1342 | ignore such elements. Many other versions of Lisp signal errors in such | |
1343 | cases. | |
1344 | ||
1345 | Note that property lists are similar to association lists in several | |
1346 | respects. A property list behaves like an association list in which | |
1347 | each key can occur only once. @xref{Property Lists}, for a comparison | |
1348 | of property lists and association lists. | |
1349 | ||
1350 | @defun assoc key alist | |
1351 | This function returns the first association for @var{key} in | |
1352 | @var{alist}. It compares @var{key} against the alist elements using | |
1353 | @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no | |
1354 | association in @var{alist} has a @sc{car} @code{equal} to @var{key}. | |
1355 | For example: | |
1356 | ||
1357 | @smallexample | |
1358 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1359 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1360 | (assoc 'oak trees) | |
1361 | @result{} (oak . acorns) | |
1362 | (cdr (assoc 'oak trees)) | |
1363 | @result{} acorns | |
1364 | (assoc 'birch trees) | |
1365 | @result{} nil | |
1366 | @end smallexample | |
1367 | ||
2b3fc6c3 | 1368 | Here is another example, in which the keys and values are not symbols: |
73804d4b RS |
1369 | |
1370 | @smallexample | |
1371 | (setq needles-per-cluster | |
1372 | '((2 "Austrian Pine" "Red Pine") | |
1373 | (3 "Pitch Pine") | |
1374 | (5 "White Pine"))) | |
1375 | ||
1376 | (cdr (assoc 3 needles-per-cluster)) | |
1377 | @result{} ("Pitch Pine") | |
1378 | (cdr (assoc 2 needles-per-cluster)) | |
1379 | @result{} ("Austrian Pine" "Red Pine") | |
1380 | @end smallexample | |
1381 | @end defun | |
1382 | ||
a9f0a989 RS |
1383 | The functions @code{assoc-ignore-representation} and |
1384 | @code{assoc-ignore-case} are much like @code{assoc} except using | |
1385 | @code{compare-strings} to do the comparison. @xref{Text Comparison}. | |
1386 | ||
22697dac KH |
1387 | @defun rassoc value alist |
1388 | This function returns the first association with value @var{value} in | |
1389 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1390 | a @sc{cdr} @code{equal} to @var{value}. | |
1391 | ||
1392 | @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of | |
1393 | each @var{alist} association instead of the @sc{car}. You can think of | |
1394 | this as ``reverse @code{assoc}'', finding the key for a given value. | |
1395 | @end defun | |
1396 | ||
73804d4b RS |
1397 | @defun assq key alist |
1398 | This function is like @code{assoc} in that it returns the first | |
1399 | association for @var{key} in @var{alist}, but it makes the comparison | |
1400 | using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil} | |
1401 | if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}. | |
1402 | This function is used more often than @code{assoc}, since @code{eq} is | |
1403 | faster than @code{equal} and most alists use symbols as keys. | |
1404 | @xref{Equality Predicates}. | |
1405 | ||
1406 | @smallexample | |
1407 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1408 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) | |
1409 | (assq 'pine trees) | |
1410 | @result{} (pine . cones) | |
1411 | @end smallexample | |
1412 | ||
1413 | On the other hand, @code{assq} is not usually useful in alists where the | |
1414 | keys may not be symbols: | |
1415 | ||
1416 | @smallexample | |
1417 | (setq leaves | |
1418 | '(("simple leaves" . oak) | |
1419 | ("compound leaves" . horsechestnut))) | |
1420 | ||
1421 | (assq "simple leaves" leaves) | |
1422 | @result{} nil | |
1423 | (assoc "simple leaves" leaves) | |
1424 | @result{} ("simple leaves" . oak) | |
1425 | @end smallexample | |
1426 | @end defun | |
1427 | ||
1428 | @defun rassq value alist | |
1429 | This function returns the first association with value @var{value} in | |
1430 | @var{alist}. It returns @code{nil} if no association in @var{alist} has | |
1431 | a @sc{cdr} @code{eq} to @var{value}. | |
1432 | ||
1433 | @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of | |
1434 | each @var{alist} association instead of the @sc{car}. You can think of | |
1435 | this as ``reverse @code{assq}'', finding the key for a given value. | |
1436 | ||
1437 | For example: | |
1438 | ||
1439 | @smallexample | |
1440 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) | |
1441 | ||
1442 | (rassq 'acorns trees) | |
1443 | @result{} (oak . acorns) | |
1444 | (rassq 'spores trees) | |
1445 | @result{} nil | |
1446 | @end smallexample | |
1447 | ||
1448 | Note that @code{rassq} cannot search for a value stored in the @sc{car} | |
1449 | of the @sc{cdr} of an element: | |
1450 | ||
1451 | @smallexample | |
1452 | (setq colors '((rose red) (lily white) (buttercup yellow))) | |
1453 | ||
1454 | (rassq 'white colors) | |
1455 | @result{} nil | |
1456 | @end smallexample | |
1457 | ||
1458 | In this case, the @sc{cdr} of the association @code{(lily white)} is not | |
1459 | the symbol @code{white}, but rather the list @code{(white)}. This | |
1460 | becomes clearer if the association is written in dotted pair notation: | |
1461 | ||
1462 | @smallexample | |
1463 | (lily white) @equiv{} (lily . (white)) | |
1464 | @end smallexample | |
1465 | @end defun | |
1466 | ||
a46dba07 RS |
1467 | @defun assoc-default key alist test default |
1468 | This function searches @var{alist} for a match for @var{key}. For each | |
1469 | element of @var{alist}, it compares the element (if it is an atom) or | |
1470 | the element's @sc{car} (if it is a cons) against @var{key}, by calling | |
1471 | @var{test} with two arguments: the element or its @sc{car}, and | |
1472 | @var{key}. The arguments are passed in that order so that you can get | |
1473 | useful results using @code{string-match} with an alist that contains | |
1474 | regular expressions (@pxref{Regexp Search}). If @var{test} is omitted | |
1475 | or @code{nil}, @code{equal} is used for comparison. | |
1476 | ||
1477 | If an alist element matches @var{key} by this criterion, | |
1478 | then @code{assoc-default} returns a value based on this element. | |
1479 | If the element is a cons, then the value is the element's @sc{cdr}. | |
1480 | Otherwise, the return value is @var{default}. | |
1481 | ||
1482 | If no alist element matches @var{key}, @code{assoc-default} returns | |
1483 | @code{nil}. | |
1484 | @end defun | |
1485 | ||
73804d4b RS |
1486 | @defun copy-alist alist |
1487 | @cindex copying alists | |
1488 | This function returns a two-level deep copy of @var{alist}: it creates a | |
1489 | new copy of each association, so that you can alter the associations of | |
1490 | the new alist without changing the old one. | |
1491 | ||
1492 | @smallexample | |
1493 | @group | |
1494 | (setq needles-per-cluster | |
1495 | '((2 . ("Austrian Pine" "Red Pine")) | |
2b3fc6c3 | 1496 | (3 . ("Pitch Pine")) |
ec221d13 | 1497 | @end group |
2b3fc6c3 | 1498 | (5 . ("White Pine")))) |
73804d4b RS |
1499 | @result{} |
1500 | ((2 "Austrian Pine" "Red Pine") | |
2b3fc6c3 RS |
1501 | (3 "Pitch Pine") |
1502 | (5 "White Pine")) | |
73804d4b RS |
1503 | |
1504 | (setq copy (copy-alist needles-per-cluster)) | |
1505 | @result{} | |
1506 | ((2 "Austrian Pine" "Red Pine") | |
2b3fc6c3 RS |
1507 | (3 "Pitch Pine") |
1508 | (5 "White Pine")) | |
73804d4b RS |
1509 | |
1510 | (eq needles-per-cluster copy) | |
1511 | @result{} nil | |
1512 | (equal needles-per-cluster copy) | |
1513 | @result{} t | |
1514 | (eq (car needles-per-cluster) (car copy)) | |
1515 | @result{} nil | |
1516 | (cdr (car (cdr needles-per-cluster))) | |
2b3fc6c3 | 1517 | @result{} ("Pitch Pine") |
ec221d13 | 1518 | @group |
73804d4b RS |
1519 | (eq (cdr (car (cdr needles-per-cluster))) |
1520 | (cdr (car (cdr copy)))) | |
1521 | @result{} t | |
1522 | @end group | |
3e099569 | 1523 | @end smallexample |
2b3fc6c3 RS |
1524 | |
1525 | This example shows how @code{copy-alist} makes it possible to change | |
1526 | the associations of one copy without affecting the other: | |
1527 | ||
3e099569 | 1528 | @smallexample |
2b3fc6c3 | 1529 | @group |
c74c521d | 1530 | (setcdr (assq 3 copy) '("Martian Vacuum Pine")) |
2b3fc6c3 RS |
1531 | (cdr (assq 3 needles-per-cluster)) |
1532 | @result{} ("Pitch Pine") | |
1533 | @end group | |
73804d4b RS |
1534 | @end smallexample |
1535 | @end defun | |
1536 | ||
8241495d RS |
1537 | @defun assoc-delete-all key alist |
1538 | @tindex assoc-delete-all | |
1539 | This function deletes from @var{alist} all the elements whose @sc{car} | |
1540 | is @var{key}. It returns the modified alist. | |
73804d4b | 1541 | |
8241495d RS |
1542 | @example |
1543 | (assoc-delete-all 'foo | |
1544 | '((foo 1) (bar 2) (foo 3) (lose 4))) | |
1545 | @result{} ((bar 2) (lose 4)) | |
1546 | @end example | |
1547 | @end defun |