2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
4 @c Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../info/lists
7 @node Lists, Sequences Arrays Vectors, Strings and Characters, Top
10 @cindex element (of list)
12 A @dfn{list} represents a sequence of zero or more elements (which may
13 be any Lisp objects). The important difference between lists and
14 vectors is that two or more lists can share part of their structure; in
15 addition, you can insert or delete elements in a list without copying
19 * Cons Cells:: How lists are made out of cons cells.
20 * Lists as Boxes:: Graphical notation to explain lists.
21 * List-related Predicates:: Is this object a list? Comparing two lists.
22 * List Elements:: Extracting the pieces of a list.
23 * Building Lists:: Creating list structure.
24 * Modifying Lists:: Storing new pieces into an existing list.
25 * Sets And Lists:: A list can represent a finite mathematical set.
26 * Association Lists:: A list can represent a finite relation or mapping.
30 @section Lists and Cons Cells
31 @cindex lists and cons cells
32 @cindex @code{nil} and lists
34 Lists in Lisp are not a primitive data type; they are built up from
35 @dfn{cons cells}. A cons cell is a data object that represents an
36 ordered pair. That is, it has two slots, and each slot @dfn{holds}, or
37 @dfn{refers to}, some Lisp object. One slot is known as the @sc{car},
38 and the other is known as the @sc{cdr}. (These names are traditional;
39 see @ref{Cons Cell Type}.) @sc{cdr} is pronounced ``could-er.''
41 We say that ``the @sc{car} of this cons cell is'' whatever object
42 its @sc{car} slot currently holds, and likewise for the @sc{cdr}.
44 A list is a series of cons cells ``chained together,'' so that each
45 cell refers to the next one. There is one cons cell for each element of
46 the list. By convention, the @sc{car}s of the cons cells hold the
47 elements of the list, and the @sc{cdr}s are used to chain the list: the
48 @sc{cdr} slot of each cons cell refers to the following cons cell. The
49 @sc{cdr} of the last cons cell is @code{nil}. This asymmetry between
50 the @sc{car} and the @sc{cdr} is entirely a matter of convention; at the
51 level of cons cells, the @sc{car} and @sc{cdr} slots have the same
54 @cindex list structure
55 Because most cons cells are used as part of lists, the phrase
56 @dfn{list structure} has come to mean any structure made out of cons
59 The symbol @code{nil} is considered a list as well as a symbol; it is
60 the list with no elements. For convenience, the symbol @code{nil} is
61 considered to have @code{nil} as its @sc{cdr} (and also as its
64 The @sc{cdr} of any nonempty list @var{l} is a list containing all the
65 elements of @var{l} except the first.
68 @comment node-name, next, previous, up
69 @section Lists as Linked Pairs of Boxes
70 @cindex box representation for lists
71 @cindex lists represented as boxes
72 @cindex cons cell as box
74 A cons cell can be illustrated as a pair of boxes. The first box
75 represents the @sc{car} and the second box represents the @sc{cdr}.
76 Here is an illustration of the two-element list, @code{(tulip lily)},
77 made from two cons cells:
81 --------------- ---------------
82 | car | cdr | | car | cdr |
83 | tulip | o---------->| lily | nil |
85 --------------- ---------------
89 Each pair of boxes represents a cons cell. Each box ``refers to'',
90 ``points to'' or ``holds'' a Lisp object. (These terms are
91 synonymous.) The first box, which describes the @sc{car} of the first
92 cons cell, contains the symbol @code{tulip}. The arrow from the
93 @sc{cdr} box of the first cons cell to the second cons cell indicates
94 that the @sc{cdr} of the first cons cell is the second cons cell.
96 The same list can be illustrated in a different sort of box notation
102 | | |--> | | |--> nil
110 Here is a more complex illustration, showing the three-element list,
111 @code{((pine needles) oak maple)}, the first element of which is a
116 --- --- --- --- --- ---
117 | | |--> | | |--> | | |--> nil
118 --- --- --- --- --- ---
124 --> | | |--> | | |--> nil
132 The same list represented in the first box notation looks like this:
136 -------------- -------------- --------------
137 | car | cdr | | car | cdr | | car | cdr |
138 | o | o------->| oak | o------->| maple | nil |
140 -- | --------- -------------- --------------
143 | -------------- ----------------
144 | | car | cdr | | car | cdr |
145 ------>| pine | o------->| needles | nil |
147 -------------- ----------------
151 @xref{Cons Cell Type}, for the read and print syntax of cons cells and
152 lists, and for more ``box and arrow'' illustrations of lists.
154 @node List-related Predicates
155 @section Predicates on Lists
157 The following predicates test whether a Lisp object is an atom, is a
158 cons cell or is a list, or whether it is the distinguished object
159 @code{nil}. (Many of these predicates can be defined in terms of the
160 others, but they are used so often that it is worth having all of them.)
163 This function returns @code{t} if @var{object} is a cons cell, @code{nil}
164 otherwise. @code{nil} is not a cons cell, although it @emph{is} a list.
169 This function returns @code{t} if @var{object} is an atom, @code{nil}
170 otherwise. All objects except cons cells are atoms. The symbol
171 @code{nil} is an atom and is also a list; it is the only Lisp object
175 (atom @var{object}) @equiv{} (not (consp @var{object}))
180 This function returns @code{t} if @var{object} is a cons cell or
181 @code{nil}. Otherwise, it returns @code{nil}.
196 This function is the opposite of @code{listp}: it returns @code{t} if
197 @var{object} is not a list. Otherwise, it returns @code{nil}.
200 (listp @var{object}) @equiv{} (not (nlistp @var{object}))
205 This function returns @code{t} if @var{object} is @code{nil}, and
206 returns @code{nil} otherwise. This function is identical to @code{not},
207 but as a matter of clarity we use @code{null} when @var{object} is
208 considered a list and @code{not} when it is considered a truth value
209 (see @code{not} in @ref{Combining Conditions}).
226 @section Accessing Elements of Lists
227 @cindex list elements
230 This function returns the value referred to by the first slot of the
231 cons cell @var{cons-cell}. Expressed another way, this function
232 returns the @sc{car} of @var{cons-cell}.
234 As a special case, if @var{cons-cell} is @code{nil}, then @code{car}
235 is defined to return @code{nil}; therefore, any list is a valid argument
236 for @code{car}. An error is signaled if the argument is not a cons cell
252 This function returns the value referred to by the second slot of
253 the cons cell @var{cons-cell}. Expressed another way, this function
254 returns the @sc{cdr} of @var{cons-cell}.
256 As a special case, if @var{cons-cell} is @code{nil}, then @code{cdr}
257 is defined to return @code{nil}; therefore, any list is a valid argument
258 for @code{cdr}. An error is signaled if the argument is not a cons cell
273 @defun car-safe object
274 This function lets you take the @sc{car} of a cons cell while avoiding
275 errors for other data types. It returns the @sc{car} of @var{object} if
276 @var{object} is a cons cell, @code{nil} otherwise. This is in contrast
277 to @code{car}, which signals an error if @var{object} is not a list.
281 (car-safe @var{object})
283 (let ((x @var{object}))
291 @defun cdr-safe object
292 This function lets you take the @sc{cdr} of a cons cell while
293 avoiding errors for other data types. It returns the @sc{cdr} of
294 @var{object} if @var{object} is a cons cell, @code{nil} otherwise.
295 This is in contrast to @code{cdr}, which signals an error if
296 @var{object} is not a list.
300 (cdr-safe @var{object})
302 (let ((x @var{object}))
312 This macro is a way of examining the @sc{car} of a list,
313 and taking it off the list, all at once. It is new in Emacs 21.
315 It operates on the list which is stored in the symbol @var{listname}.
316 It removes this element from the list by setting @var{listname}
317 to the @sc{cdr} of its old value---but it also returns the @sc{car}
318 of that list, which is the element being removed.
331 This function returns the @var{n}th element of @var{list}. Elements
332 are numbered starting with zero, so the @sc{car} of @var{list} is
333 element number zero. If the length of @var{list} is @var{n} or less,
334 the value is @code{nil}.
336 If @var{n} is negative, @code{nth} returns the first element of
352 (nth n x) @equiv{} (car (nthcdr n x))
356 The function @code{elt} is similar, but applies to any kind of sequence.
357 For historical reasons, it takes its arguments in the opposite order.
358 @xref{Sequence Functions}.
362 This function returns the @var{n}th @sc{cdr} of @var{list}. In other
363 words, it skips past the first @var{n} links of @var{list} and returns
366 If @var{n} is zero or negative, @code{nthcdr} returns all of
367 @var{list}. If the length of @var{list} is @var{n} or less,
368 @code{nthcdr} returns @code{nil}.
372 (nthcdr 1 '(1 2 3 4))
376 (nthcdr 10 '(1 2 3 4))
380 (nthcdr -3 '(1 2 3 4))
386 @defun last list &optional n
387 This function returns the last link of @var{list}. The @code{car} of
388 this link is the list's last element. If @var{list} is null,
389 @code{nil} is returned. If @var{n} is non-@code{nil}, the
390 @var{n}th-to-last link is returned instead, or the whole of @var{list}
391 if @var{n} is bigger than @var{list}'s length.
394 @defun safe-length list
395 This function returns the length of @var{list}, with no risk
396 of either an error or an infinite loop.
398 If @var{list} is not really a list, @code{safe-length} returns 0. If
399 @var{list} is circular, it returns a finite value which is at least the
400 number of distinct elements.
403 The most common way to compute the length of a list, when you are not
404 worried that it may be circular, is with @code{length}. @xref{Sequence
407 @defun caar cons-cell
408 This is the same as @code{(car (car @var{cons-cell}))}.
411 @defun cadr cons-cell
412 This is the same as @code{(car (cdr @var{cons-cell}))}
413 or @code{(nth 1 @var{cons-cell})}.
416 @defun cdar cons-cell
417 This is the same as @code{(cdr (car @var{cons-cell}))}.
420 @defun cddr cons-cell
421 This is the same as @code{(cdr (cdr @var{cons-cell}))}
422 or @code{(nthcdr 2 @var{cons-cell})}.
425 @defun butlast x &optional n
426 This function returns the list @var{x} with the last element,
427 or the last @var{n} elements, removed. If @var{n} is greater
428 than zero it makes a copy of the list so as not to damage the
429 original list. In general, @code{(append (butlast @var{x} @var{n})
430 (last @var{x} @var{n}))} will return a list equal to @var{x}.
433 @defun nbutlast x &optional n
434 This is a version of @code{butlast} that works by destructively
435 modifying the @code{cdr} of the appropriate element, rather than
436 making a copy of the list.
440 @comment node-name, next, previous, up
441 @section Building Cons Cells and Lists
443 @cindex building lists
445 Many functions build lists, as lists reside at the very heart of Lisp.
446 @code{cons} is the fundamental list-building function; however, it is
447 interesting to note that @code{list} is used more times in the source
448 code for Emacs than @code{cons}.
450 @defun cons object1 object2
451 This function is the fundamental function used to build new list
452 structure. It creates a new cons cell, making @var{object1} the
453 @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new cons
454 cell. The arguments @var{object1} and @var{object2} may be any Lisp
455 objects, but most often @var{object2} is a list.
473 @code{cons} is often used to add a single element to the front of a
474 list. This is called @dfn{consing the element onto the list}.
475 @footnote{There is no strictly equivalent way to add an element to
476 the end of a list. You can use @code{(append @var{listname} (list
477 @var{newelt}))}, which creates a whole new list by copying @var{listname}
478 and adding @var{newelt} to its end. Or you can use @code{(nconc
479 @var{listname} (list @var{newelt}))}, which modifies @var{listname}
480 by following all the @sc{cdr}s and then replacing the terminating
481 @code{nil}. Compare this to adding an element to the beginning of a
482 list with @code{cons}, which neither copies nor modifies the list.}
486 (setq list (cons newelt list))
489 Note that there is no conflict between the variable named @code{list}
490 used in this example and the function named @code{list} described below;
491 any symbol can serve both purposes.
495 @defmac push newelt listname
496 This macro provides an alternative way to write
497 @code{(setq @var{listname} (cons @var{newelt} @var{listname}))}.
498 It is new in Emacs 21.
510 @defun list &rest objects
511 This function creates a list with @var{objects} as its elements. The
512 resulting list is always @code{nil}-terminated. If no @var{objects}
513 are given, the empty list is returned.
518 @result{} (1 2 3 4 5)
521 (list 1 2 '(3 4 5) 'foo)
522 @result{} (1 2 (3 4 5) foo)
531 @defun make-list length object
532 This function creates a list of @var{length} elements, in which each
533 element is @var{object}. Compare @code{make-list} with
534 @code{make-string} (@pxref{Creating Strings}).
539 @result{} (pigs pigs pigs)
546 (setq l (make-list 3 '(a b))
547 @result{} ((a b) (a b) (a b))
548 (eq (car l) (cadr l))
554 @defun append &rest sequences
555 @cindex copying lists
556 This function returns a list containing all the elements of
557 @var{sequences}. The @var{sequences} may be lists, vectors,
558 bool-vectors, or strings, but the last one should usually be a list.
559 All arguments except the last one are copied, so none of the arguments
560 is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join
561 lists with no copying.)
563 More generally, the final argument to @code{append} may be any Lisp
564 object. The final argument is not copied or converted; it becomes the
565 @sc{cdr} of the last cons cell in the new list. If the final argument
566 is itself a list, then its elements become in effect elements of the
567 result list. If the final element is not a list, the result is a
568 ``dotted list'' since its final @sc{cdr} is not @code{nil} as required
571 In Emacs 20 and before, the @code{append} function also allowed
572 integers as (non last) arguments. It converted them to strings of
573 digits, making up the decimal print representation of the integer, and
574 then used the strings instead of the original integers. This obsolete
575 usage no longer works. The proper way to convert an integer to a
576 decimal number in this way is with @code{format} (@pxref{Formatting
577 Strings}) or @code{number-to-string} (@pxref{String Conversion}).
580 Here is an example of using @code{append}:
584 (setq trees '(pine oak))
586 (setq more-trees (append '(maple birch) trees))
587 @result{} (maple birch pine oak)
594 @result{} (maple birch pine oak)
597 (eq trees (cdr (cdr more-trees)))
602 You can see how @code{append} works by looking at a box diagram. The
603 variable @code{trees} is set to the list @code{(pine oak)} and then the
604 variable @code{more-trees} is set to the list @code{(maple birch pine
605 oak)}. However, the variable @code{trees} continues to refer to the
612 | --- --- --- --- -> --- --- --- ---
613 --> | | |--> | | |--> | | |--> | | |--> nil
614 --- --- --- --- --- --- --- ---
617 --> maple -->birch --> pine --> oak
621 An empty sequence contributes nothing to the value returned by
622 @code{append}. As a consequence of this, a final @code{nil} argument
623 forces a copy of the previous argument:
631 (setq wood (append trees nil))
645 This once was the usual way to copy a list, before the function
646 @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}.
648 Here we show the use of vectors and strings as arguments to @code{append}:
652 (append [a b] "cd" nil)
653 @result{} (a b 99 100)
657 With the help of @code{apply} (@pxref{Calling Functions}), we can append
658 all the lists in a list of lists:
662 (apply 'append '((a b c) nil (x y z) nil))
663 @result{} (a b c x y z)
667 If no @var{sequences} are given, @code{nil} is returned:
676 Here are some examples where the final argument is not a list:
682 @result{} (x y . [z])
686 The second example shows that when the final argument is a sequence but
687 not a list, the sequence's elements do not become elements of the
688 resulting list. Instead, the sequence becomes the final @sc{cdr}, like
689 any other non-list final argument.
692 This function creates a new list whose elements are the elements of
693 @var{list}, but in reverse order. The original argument @var{list} is
710 @defun remq object list
711 This function returns a copy of @var{list}, with all elements removed
712 which are @code{eq} to @var{object}. The letter @samp{q} in @code{remq}
713 says that it uses @code{eq} to compare @var{object} against the elements
718 (setq sample-list '(a b c a b c))
719 @result{} (a b c a b c)
722 (remq 'a sample-list)
727 @result{} (a b c a b c)
731 The function @code{delq} offers a way to perform this operation
732 destructively. See @ref{Sets And Lists}.
735 @defun copy-tree tree &optional vecp
736 This function returns a copy the tree @code{tree}. If @var{tree} is a
737 cons cell, this makes a new cons cell with the same @sc{car} and
738 @sc{cdr}, then recursively copies the @sc{car} and @sc{cdr} in the
741 Normally, when @var{tree} is anything other than a cons cell,
742 @code{copy-tree} simply returns @var{tree}. However, if @var{vecp} is
743 non-@code{nil}, it copies vectors too (and operates recursively on
747 @defun number-sequence from &optional to separation
748 This returns a list of numbers starting with @var{from} and
749 incrementing by @var{separation}, and ending at or just before
750 @var{to}. @var{separation} can be positive or negative and defaults
751 to 1. If @var{to} is @code{nil} or numerically equal to @var{from},
752 the one element list @code{(from)} is returned. If @var{separation}
753 is 0 and @var{to} is neither @code{nil} nor numerically equal to
754 @var{from}, an error is signaled.
756 All arguments can be integers or floating point numbers. However,
757 floating point arguments can be tricky, because floating point
758 arithmetic is inexact. For instance, depending on the machine, it may
759 quite well happen that @code{(number-sequence 0.4 0.6 0.2)} returns
760 the one element list @code{(0.4)}, whereas
761 @code{(number-sequence 0.4 0.8 0.2)} returns a list with three
762 elements. The @var{n}th element of the list is computed by the exact
763 formula @code{(+ @var{from} (* @var{n} @var{separation}))}. Thus, if
764 one wants to make sure that @var{to} is included in the list, one can
765 pass an expression of this exact type for @var{to}. Alternatively,
766 one can replace @var{to} with a slightly larger value (or a slightly
767 more negative value if @var{separation} is negative).
772 (number-sequence 4 9)
773 @result{} (4 5 6 7 8 9)
774 (number-sequence 9 4 -1)
775 @result{} (9 8 7 6 5 4)
776 (number-sequence 9 4 -2)
780 (number-sequence 8 5)
782 (number-sequence 5 8 -1)
784 (number-sequence 1.5 6 2)
785 @result{} (1.5 3.5 5.5)
789 @node Modifying Lists
790 @section Modifying Existing List Structure
791 @cindex destructive list operations
793 You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the
794 primitives @code{setcar} and @code{setcdr}. We call these ``destructive''
795 operations because they change existing list structure.
797 @cindex CL note---@code{rplaca} vrs @code{setcar}
801 @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and
802 @code{rplacd} to alter list structure; they change structure the same
803 way as @code{setcar} and @code{setcdr}, but the Common Lisp functions
804 return the cons cell while @code{setcar} and @code{setcdr} return the
805 new @sc{car} or @sc{cdr}.
809 * Setcar:: Replacing an element in a list.
810 * Setcdr:: Replacing part of the list backbone.
811 This can be used to remove or add elements.
812 * Rearrangement:: Reordering the elements in a list; combining lists.
816 @subsection Altering List Elements with @code{setcar}
818 Changing the @sc{car} of a cons cell is done with @code{setcar}. When
819 used on a list, @code{setcar} replaces one element of a list with a
822 @defun setcar cons object
823 This function stores @var{object} as the new @sc{car} of @var{cons},
824 replacing its previous @sc{car}. In other words, it changes the
825 @sc{car} slot of @var{cons} to refer to @var{object}. It returns the
826 value @var{object}. For example:
844 When a cons cell is part of the shared structure of several lists,
845 storing a new @sc{car} into the cons changes one element of each of
846 these lists. Here is an example:
850 ;; @r{Create two lists that are partly shared.}
853 (setq x2 (cons 'z (cdr x1)))
858 ;; @r{Replace the @sc{car} of a shared link.}
859 (setcar (cdr x1) 'foo)
861 x1 ; @r{Both lists are changed.}
868 ;; @r{Replace the @sc{car} of a link that is not shared.}
871 x1 ; @r{Only one list is changed.}
872 @result{} (baz foo c)
878 Here is a graphical depiction of the shared structure of the two lists
879 in the variables @code{x1} and @code{x2}, showing why replacing @code{b}
884 --- --- --- --- --- ---
885 x1---> | | |----> | | |--> | | |--> nil
886 --- --- --- --- --- ---
900 Here is an alternative form of box diagram, showing the same relationship:
905 -------------- -------------- --------------
906 | car | cdr | | car | cdr | | car | cdr |
907 | a | o------->| b | o------->| c | nil |
909 -------------- | -------------- --------------
921 @subsection Altering the CDR of a List
923 The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}:
925 @defun setcdr cons object
926 This function stores @var{object} as the new @sc{cdr} of @var{cons},
927 replacing its previous @sc{cdr}. In other words, it changes the
928 @sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the
932 Here is an example of replacing the @sc{cdr} of a list with a
933 different list. All but the first element of the list are removed in
934 favor of a different sequence of elements. The first element is
935 unchanged, because it resides in the @sc{car} of the list, and is not
936 reached via the @sc{cdr}.
953 You can delete elements from the middle of a list by altering the
954 @sc{cdr}s of the cons cells in the list. For example, here we delete
955 the second element, @code{b}, from the list @code{(a b c)}, by changing
956 the @sc{cdr} of the first cons cell:
962 (setcdr x1 (cdr (cdr x1)))
970 Here is the result in box notation:
976 -------------- | -------------- | --------------
977 | car | cdr | | | car | cdr | -->| car | cdr |
978 | a | o----- | b | o-------->| c | nil |
980 -------------- -------------- --------------
985 The second cons cell, which previously held the element @code{b}, still
986 exists and its @sc{car} is still @code{b}, but it no longer forms part
989 It is equally easy to insert a new element by changing @sc{cdr}s:
995 (setcdr x1 (cons 'd (cdr x1)))
1002 Here is this result in box notation:
1006 -------------- ------------- -------------
1007 | car | cdr | | car | cdr | | car | cdr |
1008 | a | o | -->| b | o------->| c | nil |
1009 | | | | | | | | | | |
1010 --------- | -- | ------------- -------------
1023 @subsection Functions that Rearrange Lists
1024 @cindex rearrangement of lists
1025 @cindex modification of lists
1027 Here are some functions that rearrange lists ``destructively'' by
1028 modifying the @sc{cdr}s of their component cons cells. We call these
1029 functions ``destructive'' because they chew up the original lists passed
1030 to them as arguments, relinking their cons cells to form a new list that
1031 is the returned value.
1034 See @code{delq}, in @ref{Sets And Lists}, for another function
1035 that modifies cons cells.
1038 The function @code{delq} in the following section is another example
1039 of destructive list manipulation.
1042 @defun nconc &rest lists
1043 @cindex concatenating lists
1044 @cindex joining lists
1045 This function returns a list containing all the elements of @var{lists}.
1046 Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are
1047 @emph{not} copied. Instead, the last @sc{cdr} of each of the
1048 @var{lists} is changed to refer to the following list. The last of the
1049 @var{lists} is not altered. For example:
1058 @result{} (1 2 3 4 5)
1062 @result{} (1 2 3 4 5)
1066 Since the last argument of @code{nconc} is not itself modified, it is
1067 reasonable to use a constant list, such as @code{'(4 5)}, as in the
1068 above example. For the same reason, the last argument need not be a
1078 @result{} (1 2 3 . z)
1082 @result{} (1 2 3 . z)
1086 However, the other arguments (all but the last) must be lists.
1088 A common pitfall is to use a quoted constant list as a non-last
1089 argument to @code{nconc}. If you do this, your program will change
1090 each time you run it! Here is what happens:
1094 (defun add-foo (x) ; @r{We want this function to add}
1095 (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.}
1099 (symbol-function 'add-foo)
1100 @result{} (lambda (x) (nconc (quote (foo)) x))
1104 (setq xx (add-foo '(1 2))) ; @r{It seems to work.}
1108 (setq xy (add-foo '(3 4))) ; @r{What happened?}
1109 @result{} (foo 1 2 3 4)
1117 (symbol-function 'add-foo)
1118 @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
1123 @defun nreverse list
1124 @cindex reversing a list
1125 This function reverses the order of the elements of @var{list}.
1126 Unlike @code{reverse}, @code{nreverse} alters its argument by reversing
1127 the @sc{cdr}s in the cons cells forming the list. The cons cell that
1128 used to be the last one in @var{list} becomes the first cons cell of the
1145 ;; @r{The cons cell that was first is now last.}
1151 To avoid confusion, we usually store the result of @code{nreverse}
1152 back in the same variable which held the original list:
1155 (setq x (nreverse x))
1158 Here is the @code{nreverse} of our favorite example, @code{(a b c)},
1159 presented graphically:
1163 @r{Original list head:} @r{Reversed list:}
1164 ------------- ------------- ------------
1165 | car | cdr | | car | cdr | | car | cdr |
1166 | a | nil |<-- | b | o |<-- | c | o |
1167 | | | | | | | | | | | | |
1168 ------------- | --------- | - | -------- | -
1170 ------------- ------------
1175 @defun sort list predicate
1177 @cindex sorting lists
1178 This function sorts @var{list} stably, though destructively, and
1179 returns the sorted list. It compares elements using @var{predicate}. A
1180 stable sort is one in which elements with equal sort keys maintain their
1181 relative order before and after the sort. Stability is important when
1182 successive sorts are used to order elements according to different
1185 The argument @var{predicate} must be a function that accepts two
1186 arguments. It is called with two elements of @var{list}. To get an
1187 increasing order sort, the @var{predicate} should return @code{t} if the
1188 first element is ``less than'' the second, or @code{nil} if not.
1190 The comparison function @var{predicate} must give reliable results for
1191 any given pair of arguments, at least within a single call to
1192 @code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is
1193 less than @var{b}, @var{b} must not be less than @var{a}. It must be
1194 @dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b}
1195 is less than @var{c}, then @var{a} must be less than @var{c}. If you
1196 use a comparison function which does not meet these requirements, the
1197 result of @code{sort} is unpredictable.
1199 The destructive aspect of @code{sort} is that it rearranges the cons
1200 cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort
1201 function would create new cons cells to store the elements in their
1202 sorted order. If you wish to make a sorted copy without destroying the
1203 original, copy it first with @code{copy-sequence} and then sort.
1205 Sorting does not change the @sc{car}s of the cons cells in @var{list};
1206 the cons cell that originally contained the element @code{a} in
1207 @var{list} still has @code{a} in its @sc{car} after sorting, but it now
1208 appears in a different position in the list due to the change of
1209 @sc{cdr}s. For example:
1213 (setq nums '(1 3 2 6 5 4 0))
1214 @result{} (1 3 2 6 5 4 0)
1218 @result{} (0 1 2 3 4 5 6)
1222 @result{} (1 2 3 4 5 6)
1227 @strong{Warning}: Note that the list in @code{nums} no longer contains
1228 0; this is the same cons cell that it was before, but it is no longer
1229 the first one in the list. Don't assume a variable that formerly held
1230 the argument now holds the entire sorted list! Instead, save the result
1231 of @code{sort} and use that. Most often we store the result back into
1232 the variable that held the original list:
1235 (setq nums (sort nums '<))
1238 @xref{Sorting}, for more functions that perform sorting.
1239 See @code{documentation} in @ref{Accessing Documentation}, for a
1240 useful example of @code{sort}.
1243 @node Sets And Lists
1244 @section Using Lists as Sets
1245 @cindex lists as sets
1248 A list can represent an unordered mathematical set---simply consider a
1249 value an element of a set if it appears in the list, and ignore the
1250 order of the list. To form the union of two sets, use @code{append} (as
1251 long as you don't mind having duplicate elements). Other useful
1252 functions for sets include @code{memq} and @code{delq}, and their
1253 @code{equal} versions, @code{member} and @code{delete}.
1255 @cindex CL note---lack @code{union}, @code{intersection}
1257 @b{Common Lisp note:} Common Lisp has functions @code{union} (which
1258 avoids duplicate elements) and @code{intersection} for set operations,
1259 but GNU Emacs Lisp does not have them. You can write them in Lisp if
1263 @defun memq object list
1264 @cindex membership in a list
1265 This function tests to see whether @var{object} is a member of
1266 @var{list}. If it is, @code{memq} returns a list starting with the
1267 first occurrence of @var{object}. Otherwise, it returns @code{nil}.
1268 The letter @samp{q} in @code{memq} says that it uses @code{eq} to
1269 compare @var{object} against the elements of the list. For example:
1273 (memq 'b '(a b c b a))
1277 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1283 @defun delq object list
1284 @cindex deletion of elements
1285 This function destructively removes all elements @code{eq} to
1286 @var{object} from @var{list}. The letter @samp{q} in @code{delq} says
1287 that it uses @code{eq} to compare @var{object} against the elements of
1288 the list, like @code{memq} and @code{remq}.
1291 When @code{delq} deletes elements from the front of the list, it does so
1292 simply by advancing down the list and returning a sublist that starts
1293 after those elements:
1297 (delq 'a '(a b c)) @equiv{} (cdr '(a b c))
1301 When an element to be deleted appears in the middle of the list,
1302 removing it involves changing the @sc{cdr}s (@pxref{Setcdr}).
1306 (setq sample-list '(a b c (4)))
1307 @result{} (a b c (4))
1310 (delq 'a sample-list)
1315 @result{} (a b c (4))
1318 (delq 'c sample-list)
1327 Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to
1328 splice out the third element, but @code{(delq 'a sample-list)} does not
1329 splice anything---it just returns a shorter list. Don't assume that a
1330 variable which formerly held the argument @var{list} now has fewer
1331 elements, or that it still holds the original list! Instead, save the
1332 result of @code{delq} and use that. Most often we store the result back
1333 into the variable that held the original list:
1336 (setq flowers (delq 'rose flowers))
1339 In the following example, the @code{(4)} that @code{delq} attempts to match
1340 and the @code{(4)} in the @code{sample-list} are not @code{eq}:
1344 (delq '(4) sample-list)
1349 The following two functions are like @code{memq} and @code{delq} but use
1350 @code{equal} rather than @code{eq} to compare elements. @xref{Equality
1353 @defun member object list
1354 The function @code{member} tests to see whether @var{object} is a member
1355 of @var{list}, comparing members with @var{object} using @code{equal}.
1356 If @var{object} is a member, @code{member} returns a list starting with
1357 its first occurrence in @var{list}. Otherwise, it returns @code{nil}.
1359 Compare this with @code{memq}:
1363 (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.}
1367 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1371 ;; @r{Two strings with the same contents are @code{equal}.}
1372 (member "foo" '("foo" "bar"))
1373 @result{} ("foo" "bar")
1378 @defun delete object sequence
1379 If @code{sequence} is a list, this function destructively removes all
1380 elements @code{equal} to @var{object} from @var{sequence}. For lists,
1381 @code{delete} is to @code{delq} as @code{member} is to @code{memq}: it
1382 uses @code{equal} to compare elements with @var{object}, like
1383 @code{member}; when it finds an element that matches, it removes the
1384 element just as @code{delq} would.
1386 If @code{sequence} is a vector or string, @code{delete} returns a copy
1387 of @code{sequence} with all elements @code{equal} to @code{object}
1394 (delete '(2) '((2) (1) (2)))
1398 (delete '(2) [(2) (1) (2)])
1404 @defun remove object sequence
1405 This function is the non-destructive counterpart of @code{delete}. If
1406 returns a copy of @code{sequence}, a list, vector, or string, with
1407 elements @code{equal} to @code{object} removed. For example:
1411 (remove '(2) '((2) (1) (2)))
1415 (remove '(2) [(2) (1) (2)])
1422 @b{Common Lisp note:} The functions @code{member}, @code{delete} and
1423 @code{remove} in GNU Emacs Lisp are derived from Maclisp, not Common
1424 Lisp. The Common Lisp versions do not use @code{equal} to compare
1428 @defun member-ignore-case object list
1429 This function is like @code{member}, except that @var{object} should
1430 be a string and that it ignores differences in letter-case and text
1431 representation: upper-case and lower-case letters are treated as
1432 equal, and unibyte strings are converted to multibyte prior to
1436 See also the function @code{add-to-list}, in @ref{Setting Variables},
1437 for another way to add an element to a list stored in a variable.
1439 @node Association Lists
1440 @section Association Lists
1441 @cindex association list
1444 An @dfn{association list}, or @dfn{alist} for short, records a mapping
1445 from keys to values. It is a list of cons cells called
1446 @dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the
1447 @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key''
1448 is not related to the term ``key sequence''; it means a value used to
1449 look up an item in a table. In this case, the table is the alist, and
1450 the alist associations are the items.}
1452 Here is an example of an alist. The key @code{pine} is associated with
1453 the value @code{cones}; the key @code{oak} is associated with
1454 @code{acorns}; and the key @code{maple} is associated with @code{seeds}.
1464 The associated values in an alist may be any Lisp objects; so may the
1465 keys. For example, in the following alist, the symbol @code{a} is
1466 associated with the number @code{1}, and the string @code{"b"} is
1467 associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of
1474 Sometimes it is better to design an alist to store the associated
1475 value in the @sc{car} of the @sc{cdr} of the element. Here is an
1476 example of such an alist:
1479 ((rose red) (lily white) (buttercup yellow))
1483 Here we regard @code{red} as the value associated with @code{rose}. One
1484 advantage of this kind of alist is that you can store other related
1485 information---even a list of other items---in the @sc{cdr} of the
1486 @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see
1487 below) to find the element containing a given value. When neither of
1488 these considerations is important, the choice is a matter of taste, as
1489 long as you are consistent about it for any given alist.
1491 Note that the same alist shown above could be regarded as having the
1492 associated value in the @sc{cdr} of the element; the value associated
1493 with @code{rose} would be the list @code{(red)}.
1495 Association lists are often used to record information that you might
1496 otherwise keep on a stack, since new associations may be added easily to
1497 the front of the list. When searching an association list for an
1498 association with a given key, the first one found is returned, if there
1501 In Emacs Lisp, it is @emph{not} an error if an element of an
1502 association list is not a cons cell. The alist search functions simply
1503 ignore such elements. Many other versions of Lisp signal errors in such
1506 Note that property lists are similar to association lists in several
1507 respects. A property list behaves like an association list in which
1508 each key can occur only once. @xref{Property Lists}, for a comparison
1509 of property lists and association lists.
1511 @defun assoc key alist
1512 This function returns the first association for @var{key} in
1513 @var{alist}. It compares @var{key} against the alist elements using
1514 @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no
1515 association in @var{alist} has a @sc{car} @code{equal} to @var{key}.
1519 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1520 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1522 @result{} (oak . acorns)
1523 (cdr (assoc 'oak trees))
1525 (assoc 'birch trees)
1529 Here is another example, in which the keys and values are not symbols:
1532 (setq needles-per-cluster
1533 '((2 "Austrian Pine" "Red Pine")
1537 (cdr (assoc 3 needles-per-cluster))
1538 @result{} ("Pitch Pine")
1539 (cdr (assoc 2 needles-per-cluster))
1540 @result{} ("Austrian Pine" "Red Pine")
1544 The functions @code{assoc-ignore-representation} and
1545 @code{assoc-ignore-case} are much like @code{assoc} except using
1546 @code{compare-strings} to do the comparison. @xref{Text Comparison}.
1548 @defun rassoc value alist
1549 This function returns the first association with value @var{value} in
1550 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1551 a @sc{cdr} @code{equal} to @var{value}.
1553 @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of
1554 each @var{alist} association instead of the @sc{car}. You can think of
1555 this as ``reverse @code{assoc}'', finding the key for a given value.
1558 @defun assq key alist
1559 This function is like @code{assoc} in that it returns the first
1560 association for @var{key} in @var{alist}, but it makes the comparison
1561 using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil}
1562 if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}.
1563 This function is used more often than @code{assoc}, since @code{eq} is
1564 faster than @code{equal} and most alists use symbols as keys.
1565 @xref{Equality Predicates}.
1568 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1569 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1571 @result{} (pine . cones)
1574 On the other hand, @code{assq} is not usually useful in alists where the
1575 keys may not be symbols:
1579 '(("simple leaves" . oak)
1580 ("compound leaves" . horsechestnut)))
1582 (assq "simple leaves" leaves)
1584 (assoc "simple leaves" leaves)
1585 @result{} ("simple leaves" . oak)
1589 @defun rassq value alist
1590 This function returns the first association with value @var{value} in
1591 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1592 a @sc{cdr} @code{eq} to @var{value}.
1594 @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of
1595 each @var{alist} association instead of the @sc{car}. You can think of
1596 this as ``reverse @code{assq}'', finding the key for a given value.
1601 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1603 (rassq 'acorns trees)
1604 @result{} (oak . acorns)
1605 (rassq 'spores trees)
1609 Note that @code{rassq} cannot search for a value stored in the @sc{car}
1610 of the @sc{cdr} of an element:
1613 (setq colors '((rose red) (lily white) (buttercup yellow)))
1615 (rassq 'white colors)
1619 In this case, the @sc{cdr} of the association @code{(lily white)} is not
1620 the symbol @code{white}, but rather the list @code{(white)}. This
1621 becomes clearer if the association is written in dotted pair notation:
1624 (lily white) @equiv{} (lily . (white))
1628 @defun assoc-default key alist &optional test default
1629 This function searches @var{alist} for a match for @var{key}. For each
1630 element of @var{alist}, it compares the element (if it is an atom) or
1631 the element's @sc{car} (if it is a cons) against @var{key}, by calling
1632 @var{test} with two arguments: the element or its @sc{car}, and
1633 @var{key}. The arguments are passed in that order so that you can get
1634 useful results using @code{string-match} with an alist that contains
1635 regular expressions (@pxref{Regexp Search}). If @var{test} is omitted
1636 or @code{nil}, @code{equal} is used for comparison.
1638 If an alist element matches @var{key} by this criterion,
1639 then @code{assoc-default} returns a value based on this element.
1640 If the element is a cons, then the value is the element's @sc{cdr}.
1641 Otherwise, the return value is @var{default}.
1643 If no alist element matches @var{key}, @code{assoc-default} returns
1647 @defun copy-alist alist
1648 @cindex copying alists
1649 This function returns a two-level deep copy of @var{alist}: it creates a
1650 new copy of each association, so that you can alter the associations of
1651 the new alist without changing the old one.
1655 (setq needles-per-cluster
1656 '((2 . ("Austrian Pine" "Red Pine"))
1657 (3 . ("Pitch Pine"))
1659 (5 . ("White Pine"))))
1661 ((2 "Austrian Pine" "Red Pine")
1665 (setq copy (copy-alist needles-per-cluster))
1667 ((2 "Austrian Pine" "Red Pine")
1671 (eq needles-per-cluster copy)
1673 (equal needles-per-cluster copy)
1675 (eq (car needles-per-cluster) (car copy))
1677 (cdr (car (cdr needles-per-cluster)))
1678 @result{} ("Pitch Pine")
1680 (eq (cdr (car (cdr needles-per-cluster)))
1681 (cdr (car (cdr copy))))
1686 This example shows how @code{copy-alist} makes it possible to change
1687 the associations of one copy without affecting the other:
1691 (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
1692 (cdr (assq 3 needles-per-cluster))
1693 @result{} ("Pitch Pine")
1698 @defun assq-delete-all key alist
1699 @tindex assq-delete-all
1700 This function deletes from @var{alist} all the elements whose @sc{car}
1701 is @code{eq} to @var{key}, much as if you used @code{delq} to delete
1702 each such element one by one. It returns the shortened alist, and
1703 often modifies the original list structure of @var{alist}. For
1704 correct results, use the return value of @code{assq-delete-all} rather
1705 than looking at the saved value of @var{alist}.
1708 (setq alist '((foo 1) (bar 2) (foo 3) (lose 4)))
1709 @result{} ((foo 1) (bar 2) (foo 3) (lose 4))
1710 (assq-delete-all 'foo alist)
1711 @result{} ((bar 2) (lose 4))
1713 @result{} ((foo 1) (bar 2) (lose 4))
1718 arch-tag: 31fb8a4e-4aa8-4a74-a206-aa00451394d4