2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 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
9 @cindex element (of list)
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
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.
29 @section Lists and Cons Cells
30 @cindex lists and cons cells
31 @cindex @code{nil} and lists
33 Lists in Lisp are not a primitive data type; they are built up from
34 @dfn{cons cells}. A cons cell is a data object that represents an
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.''
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}.
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
50 level of cons cells, the @sc{car} and @sc{cdr} slots have the same
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
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
63 The @sc{cdr} of any nonempty list @var{l} is a list containing all the
64 elements of @var{l} except the first.
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
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:
80 --------------- ---------------
81 | car | cdr | | car | cdr |
82 | tulip | o---------->| lily | nil |
84 --------------- ---------------
88 Each pair of boxes represents a cons cell. Each box ``refers to'',
89 ``points to'' or ``holds'' a Lisp object. (These terms are
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.
95 The same list can be illustrated in a different sort of box notation
101 | | |--> | | |--> nil
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
115 --- --- --- --- --- ---
116 | | |--> | | |--> | | |--> nil
117 --- --- --- --- --- ---
123 --> | | |--> | | |--> nil
131 The same list represented in the first box notation looks like this:
135 -------------- -------------- --------------
136 | car | cdr | | car | cdr | | car | cdr |
137 | o | o------->| oak | o------->| maple | nil |
139 -- | --------- -------------- --------------
142 | -------------- ----------------
143 | | car | cdr | | car | cdr |
144 ------>| pine | o------->| needles | nil |
146 -------------- ----------------
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.
153 @node List-related Predicates
154 @section Predicates on Lists
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.)
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.
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
174 (atom @var{object}) @equiv{} (not (consp @var{object}))
179 This function returns @code{t} if @var{object} is a cons cell or
180 @code{nil}. Otherwise, it returns @code{nil}.
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}.
199 (listp @var{object}) @equiv{} (not (nlistp @var{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}).
225 @section Accessing Elements of Lists
226 @cindex list elements
229 This function returns the value referred to by the first slot of the
230 cons cell @var{cons-cell}. Expressed another way, this function
231 returns the @sc{car} of @var{cons-cell}.
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
251 This function returns the value referred to by the second slot of
252 the cons cell @var{cons-cell}. Expressed another way, this function
253 returns the @sc{cdr} of @var{cons-cell}.
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
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.
280 (car-safe @var{object})
282 (let ((x @var{object}))
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.
299 (cdr-safe @var{object})
301 (let ((x @var{object}))
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.
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.
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}.
335 If @var{n} is negative, @code{nth} returns the first element of
351 (nth n x) @equiv{} (car (nthcdr n x))
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}.
361 This function returns the @var{n}th @sc{cdr} of @var{list}. In other
362 words, it skips past the first @var{n} links of @var{list} and returns
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}.
371 (nthcdr 1 '(1 2 3 4))
375 (nthcdr 10 '(1 2 3 4))
379 (nthcdr -3 '(1 2 3 4))
385 @defun safe-length list
387 This function returns the length of @var{list}, with no risk
388 of either an error or an infinite loop.
390 If @var{list} is not really a list, @code{safe-length} returns 0. If
391 @var{list} is circular, it returns a finite value which is at least the
392 number of distinct elements.
395 The most common way to compute the length of a list, when you are not
396 worried that it may be circular, is with @code{length}. @xref{Sequence
399 @defun caar cons-cell
401 This is the same as @code{(car (car @var{cons-cell}))}.
404 @defun cadr cons-cell
406 This is the same as @code{(car (cdr @var{cons-cell}))}
407 or @code{(nth 1 @var{cons-cell})}.
410 @defun cdar cons-cell
412 This is the same as @code{(cdr (car @var{cons-cell}))}.
415 @defun cddr cons-cell
417 This is the same as @code{(cdr (cdr @var{cons-cell}))}
418 or @code{(nthcdr 2 @var{cons-cell})}.
422 @comment node-name, next, previous, up
423 @section Building Cons Cells and Lists
425 @cindex building lists
427 Many functions build lists, as lists reside at the very heart of Lisp.
428 @code{cons} is the fundamental list-building function; however, it is
429 interesting to note that @code{list} is used more times in the source
430 code for Emacs than @code{cons}.
432 @defun cons object1 object2
433 This function is the fundamental function used to build new list
434 structure. It creates a new cons cell, making @var{object1} the
435 @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new cons
436 cell. The arguments @var{object1} and @var{object2} may be any Lisp
437 objects, but most often @var{object2} is a list.
455 @code{cons} is often used to add a single element to the front of a
456 list. This is called @dfn{consing the element onto the list}. For
460 (setq list (cons newelt list))
463 Note that there is no conflict between the variable named @code{list}
464 used in this example and the function named @code{list} described below;
465 any symbol can serve both purposes.
469 @defmac push newelt listname
470 This macro provides an alternative way to write
471 @code{(setq @var{listname} (cons @var{newelt} @var{listname}))}.
472 It is new in Emacs 21.
475 @defun list &rest objects
476 This function creates a list with @var{objects} as its elements. The
477 resulting list is always @code{nil}-terminated. If no @var{objects}
478 are given, the empty list is returned.
483 @result{} (1 2 3 4 5)
486 (list 1 2 '(3 4 5) 'foo)
487 @result{} (1 2 (3 4 5) foo)
496 @defun make-list length object
497 This function creates a list of length @var{length}, in which all the
498 elements have the identical value @var{object}. Compare
499 @code{make-list} with @code{make-string} (@pxref{Creating Strings}).
504 @result{} (pigs pigs pigs)
513 @defun append &rest sequences
514 @cindex copying lists
515 This function returns a list containing all the elements of
516 @var{sequences}. The @var{sequences} may be lists, vectors,
517 bool-vectors, or strings, but the last one should usually be a list.
518 All arguments except the last one are copied, so none of the arguments
519 is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join
520 lists with no copying.)
522 More generally, the final argument to @code{append} may be any Lisp
523 object. The final argument is not copied or converted; it becomes the
524 @sc{cdr} of the last cons cell in the new list. If the final argument
525 is itself a list, then its elements become in effect elements of the
526 result list. If the final element is not a list, the result is a
527 ``dotted list'' since its final @sc{cdr} is not @code{nil} as required
530 The @code{append} function also allows integers as arguments. It
531 converts them to strings of digits, making up the decimal print
532 representation of the integer, and then uses the strings instead of the
533 original integers. @strong{Don't use this feature; we plan to eliminate
534 it. If you already use this feature, change your programs now!} The
535 proper way to convert an integer to a decimal number in this way is with
536 @code{format} (@pxref{Formatting Strings}) or @code{number-to-string}
537 (@pxref{String Conversion}).
540 Here is an example of using @code{append}:
544 (setq trees '(pine oak))
546 (setq more-trees (append '(maple birch) trees))
547 @result{} (maple birch pine oak)
554 @result{} (maple birch pine oak)
557 (eq trees (cdr (cdr more-trees)))
562 You can see how @code{append} works by looking at a box diagram. The
563 variable @code{trees} is set to the list @code{(pine oak)} and then the
564 variable @code{more-trees} is set to the list @code{(maple birch pine
565 oak)}. However, the variable @code{trees} continues to refer to the
572 | --- --- --- --- -> --- --- --- ---
573 --> | | |--> | | |--> | | |--> | | |--> nil
574 --- --- --- --- --- --- --- ---
577 --> maple -->birch --> pine --> oak
581 An empty sequence contributes nothing to the value returned by
582 @code{append}. As a consequence of this, a final @code{nil} argument
583 forces a copy of the previous argument:
591 (setq wood (append trees nil))
605 This once was the usual way to copy a list, before the function
606 @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}.
608 Here we show the use of vectors and strings as arguments to @code{append}:
612 (append [a b] "cd" nil)
613 @result{} (a b 99 100)
617 With the help of @code{apply} (@pxref{Calling Functions}), we can append
618 all the lists in a list of lists:
622 (apply 'append '((a b c) nil (x y z) nil))
623 @result{} (a b c x y z)
627 If no @var{sequences} are given, @code{nil} is returned:
636 Here are some examples where the final argument is not a list:
642 @result{} (x y . [z])
646 The second example shows that when the final argument is a sequence but
647 not a list, the sequence's elements do not become elements of the
648 resulting list. Instead, the sequence becomes the final @sc{cdr}, like
649 any other non-list final argument.
652 This function creates a new list whose elements are the elements of
653 @var{list}, but in reverse order. The original argument @var{list} is
670 @node Modifying Lists
671 @section Modifying Existing List Structure
672 @cindex destructive list operations
674 You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the
675 primitives @code{setcar} and @code{setcdr}. We call these ``destructive''
676 operations because they change existing list structure.
678 @cindex CL note---@code{rplaca} vrs @code{setcar}
682 @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and
683 @code{rplacd} to alter list structure; they change structure the same
684 way as @code{setcar} and @code{setcdr}, but the Common Lisp functions
685 return the cons cell while @code{setcar} and @code{setcdr} return the
686 new @sc{car} or @sc{cdr}.
690 * Setcar:: Replacing an element in a list.
691 * Setcdr:: Replacing part of the list backbone.
692 This can be used to remove or add elements.
693 * Rearrangement:: Reordering the elements in a list; combining lists.
697 @subsection Altering List Elements with @code{setcar}
699 Changing the @sc{car} of a cons cell is done with @code{setcar}. When
700 used on a list, @code{setcar} replaces one element of a list with a
703 @defun setcar cons object
704 This function stores @var{object} as the new @sc{car} of @var{cons},
705 replacing its previous @sc{car}. In other words, it changes the
706 @sc{car} slot of @var{cons} to refer to @var{object}. It returns the
707 value @var{object}. For example:
725 When a cons cell is part of the shared structure of several lists,
726 storing a new @sc{car} into the cons changes one element of each of
727 these lists. Here is an example:
731 ;; @r{Create two lists that are partly shared.}
734 (setq x2 (cons 'z (cdr x1)))
739 ;; @r{Replace the @sc{car} of a shared link.}
740 (setcar (cdr x1) 'foo)
742 x1 ; @r{Both lists are changed.}
749 ;; @r{Replace the @sc{car} of a link that is not shared.}
752 x1 ; @r{Only one list is changed.}
753 @result{} (baz foo c)
759 Here is a graphical depiction of the shared structure of the two lists
760 in the variables @code{x1} and @code{x2}, showing why replacing @code{b}
765 --- --- --- --- --- ---
766 x1---> | | |----> | | |--> | | |--> nil
767 --- --- --- --- --- ---
781 Here is an alternative form of box diagram, showing the same relationship:
786 -------------- -------------- --------------
787 | car | cdr | | car | cdr | | car | cdr |
788 | a | o------->| b | o------->| c | nil |
790 -------------- | -------------- --------------
802 @subsection Altering the CDR of a List
804 The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}:
806 @defun setcdr cons object
807 This function stores @var{object} as the new @sc{cdr} of @var{cons},
808 replacing its previous @sc{cdr}. In other words, it changes the
809 @sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the
813 Here is an example of replacing the @sc{cdr} of a list with a
814 different list. All but the first element of the list are removed in
815 favor of a different sequence of elements. The first element is
816 unchanged, because it resides in the @sc{car} of the list, and is not
817 reached via the @sc{cdr}.
834 You can delete elements from the middle of a list by altering the
835 @sc{cdr}s of the cons cells in the list. For example, here we delete
836 the second element, @code{b}, from the list @code{(a b c)}, by changing
837 the @sc{cdr} of the first cons cell:
843 (setcdr x1 (cdr (cdr x1)))
851 Here is the result in box notation:
857 -------------- | -------------- | --------------
858 | car | cdr | | | car | cdr | -->| car | cdr |
859 | a | o----- | b | o-------->| c | nil |
861 -------------- -------------- --------------
866 The second cons cell, which previously held the element @code{b}, still
867 exists and its @sc{car} is still @code{b}, but it no longer forms part
870 It is equally easy to insert a new element by changing @sc{cdr}s:
876 (setcdr x1 (cons 'd (cdr x1)))
883 Here is this result in box notation:
887 -------------- ------------- -------------
888 | car | cdr | | car | cdr | | car | cdr |
889 | a | o | -->| b | o------->| c | nil |
890 | | | | | | | | | | |
891 --------- | -- | ------------- -------------
904 @subsection Functions that Rearrange Lists
905 @cindex rearrangement of lists
906 @cindex modification of lists
908 Here are some functions that rearrange lists ``destructively'' by
909 modifying the @sc{cdr}s of their component cons cells. We call these
910 functions ``destructive'' because they chew up the original lists passed
911 to them as arguments, relinking their cons cells to form a new list that
912 is the returned value.
915 See @code{delq}, in @ref{Sets And Lists}, for another function
916 that modifies cons cells.
919 The function @code{delq} in the following section is another example
920 of destructive list manipulation.
923 @defun nconc &rest lists
924 @cindex concatenating lists
925 @cindex joining lists
926 This function returns a list containing all the elements of @var{lists}.
927 Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are
928 @emph{not} copied. Instead, the last @sc{cdr} of each of the
929 @var{lists} is changed to refer to the following list. The last of the
930 @var{lists} is not altered. For example:
939 @result{} (1 2 3 4 5)
943 @result{} (1 2 3 4 5)
947 Since the last argument of @code{nconc} is not itself modified, it is
948 reasonable to use a constant list, such as @code{'(4 5)}, as in the
949 above example. For the same reason, the last argument need not be a
959 @result{} (1 2 3 . z)
963 @result{} (1 2 3 . z)
967 However, the other arguments (all but the last) must be lists.
969 A common pitfall is to use a quoted constant list as a non-last
970 argument to @code{nconc}. If you do this, your program will change
971 each time you run it! Here is what happens:
975 (defun add-foo (x) ; @r{We want this function to add}
976 (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.}
980 (symbol-function 'add-foo)
981 @result{} (lambda (x) (nconc (quote (foo)) x))
985 (setq xx (add-foo '(1 2))) ; @r{It seems to work.}
989 (setq xy (add-foo '(3 4))) ; @r{What happened?}
990 @result{} (foo 1 2 3 4)
998 (symbol-function 'add-foo)
999 @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
1004 @defun nreverse list
1005 @cindex reversing a list
1006 This function reverses the order of the elements of @var{list}.
1007 Unlike @code{reverse}, @code{nreverse} alters its argument by reversing
1008 the @sc{cdr}s in the cons cells forming the list. The cons cell that
1009 used to be the last one in @var{list} becomes the first cons cell of the
1026 ;; @r{The cons cell that was first is now last.}
1032 To avoid confusion, we usually store the result of @code{nreverse}
1033 back in the same variable which held the original list:
1036 (setq x (nreverse x))
1039 Here is the @code{nreverse} of our favorite example, @code{(a b c)},
1040 presented graphically:
1044 @r{Original list head:} @r{Reversed list:}
1045 ------------- ------------- ------------
1046 | car | cdr | | car | cdr | | car | cdr |
1047 | a | nil |<-- | b | o |<-- | c | o |
1048 | | | | | | | | | | | | |
1049 ------------- | --------- | - | -------- | -
1051 ------------- ------------
1056 @defun sort list predicate
1058 @cindex sorting lists
1059 This function sorts @var{list} stably, though destructively, and
1060 returns the sorted list. It compares elements using @var{predicate}. A
1061 stable sort is one in which elements with equal sort keys maintain their
1062 relative order before and after the sort. Stability is important when
1063 successive sorts are used to order elements according to different
1066 The argument @var{predicate} must be a function that accepts two
1067 arguments. It is called with two elements of @var{list}. To get an
1068 increasing order sort, the @var{predicate} should return @code{t} if the
1069 first element is ``less than'' the second, or @code{nil} if not.
1071 The comparison function @var{predicate} must give reliable results for
1072 any given pair of arguments, at least within a single call to
1073 @code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is
1074 less than @var{b}, @var{b} must not be less than @var{a}. It must be
1075 @dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b}
1076 is less than @var{c}, then @var{a} must be less than @var{c}. If you
1077 use a comparison function which does not meet these requirements, the
1078 result of @code{sort} is unpredictable.
1080 The destructive aspect of @code{sort} is that it rearranges the cons
1081 cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort
1082 function would create new cons cells to store the elements in their
1083 sorted order. If you wish to make a sorted copy without destroying the
1084 original, copy it first with @code{copy-sequence} and then sort.
1086 Sorting does not change the @sc{car}s of the cons cells in @var{list};
1087 the cons cell that originally contained the element @code{a} in
1088 @var{list} still has @code{a} in its @sc{car} after sorting, but it now
1089 appears in a different position in the list due to the change of
1090 @sc{cdr}s. For example:
1094 (setq nums '(1 3 2 6 5 4 0))
1095 @result{} (1 3 2 6 5 4 0)
1099 @result{} (0 1 2 3 4 5 6)
1103 @result{} (1 2 3 4 5 6)
1108 @strong{Warning}: Note that the list in @code{nums} no longer contains
1109 0; this is the same cons cell that it was before, but it is no longer
1110 the first one in the list. Don't assume a variable that formerly held
1111 the argument now holds the entire sorted list! Instead, save the result
1112 of @code{sort} and use that. Most often we store the result back into
1113 the variable that held the original list:
1116 (setq nums (sort nums '<))
1119 @xref{Sorting}, for more functions that perform sorting.
1120 See @code{documentation} in @ref{Accessing Documentation}, for a
1121 useful example of @code{sort}.
1124 @node Sets And Lists
1125 @section Using Lists as Sets
1126 @cindex lists as sets
1129 A list can represent an unordered mathematical set---simply consider a
1130 value an element of a set if it appears in the list, and ignore the
1131 order of the list. To form the union of two sets, use @code{append} (as
1132 long as you don't mind having duplicate elements). Other useful
1133 functions for sets include @code{memq} and @code{delq}, and their
1134 @code{equal} versions, @code{member} and @code{delete}.
1136 @cindex CL note---lack @code{union}, @code{intersection}
1138 @b{Common Lisp note:} Common Lisp has functions @code{union} (which
1139 avoids duplicate elements) and @code{intersection} for set operations,
1140 but GNU Emacs Lisp does not have them. You can write them in Lisp if
1144 @defun memq object list
1145 @cindex membership in a list
1146 This function tests to see whether @var{object} is a member of
1147 @var{list}. If it is, @code{memq} returns a list starting with the
1148 first occurrence of @var{object}. Otherwise, it returns @code{nil}.
1149 The letter @samp{q} in @code{memq} says that it uses @code{eq} to
1150 compare @var{object} against the elements of the list. For example:
1154 (memq 'b '(a b c b a))
1158 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1164 @defun delq object list
1165 @cindex deletion of elements
1166 This function destructively removes all elements @code{eq} to
1167 @var{object} from @var{list}. The letter @samp{q} in @code{delq} says
1168 that it uses @code{eq} to compare @var{object} against the elements of
1169 the list, like @code{memq}.
1172 When @code{delq} deletes elements from the front of the list, it does so
1173 simply by advancing down the list and returning a sublist that starts
1174 after those elements:
1178 (delq 'a '(a b c)) @equiv{} (cdr '(a b c))
1182 When an element to be deleted appears in the middle of the list,
1183 removing it involves changing the @sc{cdr}s (@pxref{Setcdr}).
1187 (setq sample-list '(a b c (4)))
1188 @result{} (a b c (4))
1191 (delq 'a sample-list)
1196 @result{} (a b c (4))
1199 (delq 'c sample-list)
1208 Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to
1209 splice out the third element, but @code{(delq 'a sample-list)} does not
1210 splice anything---it just returns a shorter list. Don't assume that a
1211 variable which formerly held the argument @var{list} now has fewer
1212 elements, or that it still holds the original list! Instead, save the
1213 result of @code{delq} and use that. Most often we store the result back
1214 into the variable that held the original list:
1217 (setq flowers (delq 'rose flowers))
1220 In the following example, the @code{(4)} that @code{delq} attempts to match
1221 and the @code{(4)} in the @code{sample-list} are not @code{eq}:
1225 (delq '(4) sample-list)
1230 The following two functions are like @code{memq} and @code{delq} but use
1231 @code{equal} rather than @code{eq} to compare elements. @xref{Equality
1234 @defun member object list
1235 The function @code{member} tests to see whether @var{object} is a member
1236 of @var{list}, comparing members with @var{object} using @code{equal}.
1237 If @var{object} is a member, @code{member} returns a list starting with
1238 its first occurrence in @var{list}. Otherwise, it returns @code{nil}.
1240 Compare this with @code{memq}:
1244 (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.}
1248 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1252 ;; @r{Two strings with the same contents are @code{equal}.}
1253 (member "foo" '("foo" "bar"))
1254 @result{} ("foo" "bar")
1259 @defun delete object list
1260 This function destructively removes all elements @code{equal} to
1261 @var{object} from @var{list}. It is to @code{delq} as @code{member} is
1262 to @code{memq}: it uses @code{equal} to compare elements with
1263 @var{object}, like @code{member}; when it finds an element that matches,
1264 it removes the element just as @code{delq} would. For example:
1268 (delete '(2) '((2) (1) (2)))
1275 @b{Common Lisp note:} The functions @code{member} and @code{delete} in
1276 GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common
1277 Lisp versions do not use @code{equal} to compare elements.
1280 See also the function @code{add-to-list}, in @ref{Setting Variables},
1281 for another way to add an element to a list stored in a variable.
1283 @node Association Lists
1284 @section Association Lists
1285 @cindex association list
1288 An @dfn{association list}, or @dfn{alist} for short, records a mapping
1289 from keys to values. It is a list of cons cells called
1290 @dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the
1291 @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key''
1292 is not related to the term ``key sequence''; it means a value used to
1293 look up an item in a table. In this case, the table is the alist, and
1294 the alist associations are the items.}
1296 Here is an example of an alist. The key @code{pine} is associated with
1297 the value @code{cones}; the key @code{oak} is associated with
1298 @code{acorns}; and the key @code{maple} is associated with @code{seeds}.
1308 The associated values in an alist may be any Lisp objects; so may the
1309 keys. For example, in the following alist, the symbol @code{a} is
1310 associated with the number @code{1}, and the string @code{"b"} is
1311 associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of
1318 Sometimes it is better to design an alist to store the associated
1319 value in the @sc{car} of the @sc{cdr} of the element. Here is an
1323 '((rose red) (lily white) (buttercup yellow))
1327 Here we regard @code{red} as the value associated with @code{rose}. One
1328 advantage of this kind of alist is that you can store other related
1329 information---even a list of other items---in the @sc{cdr} of the
1330 @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see
1331 below) to find the element containing a given value. When neither of
1332 these considerations is important, the choice is a matter of taste, as
1333 long as you are consistent about it for any given alist.
1335 Note that the same alist shown above could be regarded as having the
1336 associated value in the @sc{cdr} of the element; the value associated
1337 with @code{rose} would be the list @code{(red)}.
1339 Association lists are often used to record information that you might
1340 otherwise keep on a stack, since new associations may be added easily to
1341 the front of the list. When searching an association list for an
1342 association with a given key, the first one found is returned, if there
1345 In Emacs Lisp, it is @emph{not} an error if an element of an
1346 association list is not a cons cell. The alist search functions simply
1347 ignore such elements. Many other versions of Lisp signal errors in such
1350 Note that property lists are similar to association lists in several
1351 respects. A property list behaves like an association list in which
1352 each key can occur only once. @xref{Property Lists}, for a comparison
1353 of property lists and association lists.
1355 @defun assoc key alist
1356 This function returns the first association for @var{key} in
1357 @var{alist}. It compares @var{key} against the alist elements using
1358 @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no
1359 association in @var{alist} has a @sc{car} @code{equal} to @var{key}.
1363 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1364 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1366 @result{} (oak . acorns)
1367 (cdr (assoc 'oak trees))
1369 (assoc 'birch trees)
1373 Here is another example, in which the keys and values are not symbols:
1376 (setq needles-per-cluster
1377 '((2 "Austrian Pine" "Red Pine")
1381 (cdr (assoc 3 needles-per-cluster))
1382 @result{} ("Pitch Pine")
1383 (cdr (assoc 2 needles-per-cluster))
1384 @result{} ("Austrian Pine" "Red Pine")
1388 The functions @code{assoc-ignore-representation} and
1389 @code{assoc-ignore-case} are much like @code{assoc} except using
1390 @code{compare-strings} to do the comparison. @xref{Text Comparison}.
1392 @defun rassoc value alist
1393 This function returns the first association with value @var{value} in
1394 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1395 a @sc{cdr} @code{equal} to @var{value}.
1397 @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of
1398 each @var{alist} association instead of the @sc{car}. You can think of
1399 this as ``reverse @code{assoc}'', finding the key for a given value.
1402 @defun assq key alist
1403 This function is like @code{assoc} in that it returns the first
1404 association for @var{key} in @var{alist}, but it makes the comparison
1405 using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil}
1406 if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}.
1407 This function is used more often than @code{assoc}, since @code{eq} is
1408 faster than @code{equal} and most alists use symbols as keys.
1409 @xref{Equality Predicates}.
1412 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1413 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1415 @result{} (pine . cones)
1418 On the other hand, @code{assq} is not usually useful in alists where the
1419 keys may not be symbols:
1423 '(("simple leaves" . oak)
1424 ("compound leaves" . horsechestnut)))
1426 (assq "simple leaves" leaves)
1428 (assoc "simple leaves" leaves)
1429 @result{} ("simple leaves" . oak)
1433 @defun rassq value alist
1434 This function returns the first association with value @var{value} in
1435 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1436 a @sc{cdr} @code{eq} to @var{value}.
1438 @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of
1439 each @var{alist} association instead of the @sc{car}. You can think of
1440 this as ``reverse @code{assq}'', finding the key for a given value.
1445 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1447 (rassq 'acorns trees)
1448 @result{} (oak . acorns)
1449 (rassq 'spores trees)
1453 Note that @code{rassq} cannot search for a value stored in the @sc{car}
1454 of the @sc{cdr} of an element:
1457 (setq colors '((rose red) (lily white) (buttercup yellow)))
1459 (rassq 'white colors)
1463 In this case, the @sc{cdr} of the association @code{(lily white)} is not
1464 the symbol @code{white}, but rather the list @code{(white)}. This
1465 becomes clearer if the association is written in dotted pair notation:
1468 (lily white) @equiv{} (lily . (white))
1472 @tindex assoc-default
1473 @defun assoc-default key alist test default
1474 This function searches @var{alist} for a match for @var{key}. For each
1475 element of @var{alist}, it compares the element (if it is an atom) or
1476 the element's @sc{car} (if it is a cons) against @var{key}, by calling
1477 @var{test} with two arguments: the element or its @sc{car}, and
1478 @var{key}. The arguments are passed in that order so that you can get
1479 useful results using @code{string-match} with an alist that contains
1480 regular expressions (@pxref{Regexp Search}). If @var{test} is omitted
1481 or @code{nil}, @code{equal} is used for comparison.
1483 If an alist element matches @var{key} by this criterion,
1484 then @code{assoc-default} returns a value based on this element.
1485 If the element is a cons, then the value is the element's @sc{cdr}.
1486 Otherwise, the return value is @var{default}.
1488 If no alist element matches @var{key}, @code{assoc-default} returns
1492 @defun copy-alist alist
1493 @cindex copying alists
1494 This function returns a two-level deep copy of @var{alist}: it creates a
1495 new copy of each association, so that you can alter the associations of
1496 the new alist without changing the old one.
1500 (setq needles-per-cluster
1501 '((2 . ("Austrian Pine" "Red Pine"))
1502 (3 . ("Pitch Pine"))
1504 (5 . ("White Pine"))))
1506 ((2 "Austrian Pine" "Red Pine")
1510 (setq copy (copy-alist needles-per-cluster))
1512 ((2 "Austrian Pine" "Red Pine")
1516 (eq needles-per-cluster copy)
1518 (equal needles-per-cluster copy)
1520 (eq (car needles-per-cluster) (car copy))
1522 (cdr (car (cdr needles-per-cluster)))
1523 @result{} ("Pitch Pine")
1525 (eq (cdr (car (cdr needles-per-cluster)))
1526 (cdr (car (cdr copy))))
1531 This example shows how @code{copy-alist} makes it possible to change
1532 the associations of one copy without affecting the other:
1536 (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
1537 (cdr (assq 3 needles-per-cluster))
1538 @result{} ("Pitch Pine")
1543 @defun assoc-delete-all key alist
1544 @tindex assoc-delete-all
1545 This function deletes from @var{alist} all the elements whose @sc{car}
1546 is @var{key}. It returns the modified alist.
1549 (assoc-delete-all 'foo
1550 '((foo 1) (bar 2) (foo 3) (lose 4)))
1551 @result{} ((bar 2) (lose 4))