2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2012 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
8 @cindex element (of list)
10 A @dfn{list} represents a sequence of zero or more elements (which may
11 be any Lisp objects). The important difference between lists and
12 vectors is that two or more lists can share part of their structure; in
13 addition, you can insert or delete elements in a list without copying
17 * Cons Cells:: How lists are made out of cons cells.
18 * List-related Predicates:: Is this object a list? Comparing two lists.
19 * List Elements:: Extracting the pieces of a list.
20 * Building Lists:: Creating list structure.
21 * List Variables:: Modifying lists stored in variables.
22 * Modifying Lists:: Storing new pieces into an existing list.
23 * Sets And Lists:: A list can represent a finite mathematical set.
24 * Association Lists:: A list can represent a finite relation or mapping.
28 @section Lists and Cons Cells
29 @cindex lists and cons cells
31 Lists in Lisp are not a primitive data type; they are built up from
32 @dfn{cons cells} (@pxref{Cons Cell Type}). A cons cell is a data
33 object that represents an ordered pair. That is, it has two slots,
34 and each slot @dfn{holds}, or @dfn{refers to}, some Lisp object. One
35 slot is known as the @sc{car}, and the other is known as the @sc{cdr}.
36 (These names are traditional; see @ref{Cons Cell Type}.) @sc{cdr} is
37 pronounced ``could-er''.
39 We say that ``the @sc{car} of this cons cell is'' whatever object
40 its @sc{car} slot currently holds, and likewise for the @sc{cdr}.
42 A list is a series of cons cells ``chained together'', so that each
43 cell refers to the next one. There is one cons cell for each element
44 of the list. By convention, the @sc{car}s of the cons cells hold the
45 elements of the list, and the @sc{cdr}s are used to chain the list
46 (this asymmetry between @sc{car} and @sc{cdr} is entirely a matter of
47 convention; at the level of cons cells, the @sc{car} and @sc{cdr}
48 slots have similar properties). Hence, the @sc{cdr} slot of each cons
49 cell in a list refers to the following cons cell.
52 Also by convention, the @sc{cdr} of the last cons cell in a list is
53 @code{nil}. We call such a @code{nil}-terminated structure a
54 @dfn{true list}. In Emacs Lisp, the symbol @code{nil} is both a
55 symbol and a list with no elements. For convenience, the symbol
56 @code{nil} is considered to have @code{nil} as its @sc{cdr} (and also
59 Hence, the @sc{cdr} of a true list is always a true list. The
60 @sc{cdr} of a nonempty true list is a true list containing all the
61 elements except the first.
65 If the @sc{cdr} of a list's last cons cell is some value other than
66 @code{nil}, we call the structure a @dfn{dotted list}, since its
67 printed representation would use dotted pair notation (@pxref{Dotted
68 Pair Notation}). There is one other possibility: some cons cell's
69 @sc{cdr} could point to one of the previous cons cells in the list.
70 We call that structure a @dfn{circular list}.
72 For some purposes, it does not matter whether a list is true,
73 circular or dotted. If a program doesn't look far enough down the
74 list to see the @sc{cdr} of the final cons cell, it won't care.
75 However, some functions that operate on lists demand true lists and
76 signal errors if given a dotted list. Most functions that try to find
77 the end of a list enter infinite loops if given a circular list.
79 @cindex list structure
80 Because most cons cells are used as part of lists, we refer to any
81 structure made out of cons cells as a @dfn{list structure}.
83 @node List-related Predicates
84 @section Predicates on Lists
86 The following predicates test whether a Lisp object is an atom,
87 whether it is a cons cell or is a list, or whether it is the
88 distinguished object @code{nil}. (Many of these predicates can be
89 defined in terms of the others, but they are used so often that it is
93 This function returns @code{t} if @var{object} is a cons cell, @code{nil}
94 otherwise. @code{nil} is not a cons cell, although it @emph{is} a list.
98 This function returns @code{t} if @var{object} is an atom, @code{nil}
99 otherwise. All objects except cons cells are atoms. The symbol
100 @code{nil} is an atom and is also a list; it is the only Lisp object
104 (atom @var{object}) @equiv{} (not (consp @var{object}))
109 This function returns @code{t} if @var{object} is a cons cell or
110 @code{nil}. Otherwise, it returns @code{nil}.
125 This function is the opposite of @code{listp}: it returns @code{t} if
126 @var{object} is not a list. Otherwise, it returns @code{nil}.
129 (listp @var{object}) @equiv{} (not (nlistp @var{object}))
134 This function returns @code{t} if @var{object} is @code{nil}, and
135 returns @code{nil} otherwise. This function is identical to @code{not},
136 but as a matter of clarity we use @code{null} when @var{object} is
137 considered a list and @code{not} when it is considered a truth value
138 (see @code{not} in @ref{Combining Conditions}).
154 @section Accessing Elements of Lists
155 @cindex list elements
158 This function returns the value referred to by the first slot of the
159 cons cell @var{cons-cell}. In other words, it returns the @sc{car} of
162 As a special case, if @var{cons-cell} is @code{nil}, this function
163 returns @code{nil}. Therefore, any list is a valid argument. An
164 error is signaled if the argument is not a cons cell or @code{nil}.
179 This function returns the value referred to by the second slot of the
180 cons cell @var{cons-cell}. In other words, it returns the @sc{cdr} of
183 As a special case, if @var{cons-cell} is @code{nil}, this function
184 returns @code{nil}; therefore, any list is a valid argument. An error
185 is signaled if the argument is not a cons cell or @code{nil}.
199 @defun car-safe object
200 This function lets you take the @sc{car} of a cons cell while avoiding
201 errors for other data types. It returns the @sc{car} of @var{object} if
202 @var{object} is a cons cell, @code{nil} otherwise. This is in contrast
203 to @code{car}, which signals an error if @var{object} is not a list.
207 (car-safe @var{object})
209 (let ((x @var{object}))
217 @defun cdr-safe object
218 This function lets you take the @sc{cdr} of a cons cell while
219 avoiding errors for other data types. It returns the @sc{cdr} of
220 @var{object} if @var{object} is a cons cell, @code{nil} otherwise.
221 This is in contrast to @code{cdr}, which signals an error if
222 @var{object} is not a list.
226 (cdr-safe @var{object})
228 (let ((x @var{object}))
237 This macro is a way of examining the @sc{car} of a list,
238 and taking it off the list, all at once.
239 @c FIXME I don't think is a particularly good way to do it,
240 @c but generalized variables have not been introduced yet.
241 (In fact, this macro can act on generalized variables, not just lists.
242 @xref{Generalized Variables}.)
244 It operates on the list which is stored in the symbol @var{listname}.
245 It removes this element from the list by setting @var{listname}
246 to the @sc{cdr} of its old value---but it also returns the @sc{car}
247 of that list, which is the element being removed.
259 For the @code{push} macro, which adds an element to a list,
260 @xref{List Variables}.
264 @anchor{Definition of nth}
265 This function returns the @var{n}th element of @var{list}. Elements
266 are numbered starting with zero, so the @sc{car} of @var{list} is
267 element number zero. If the length of @var{list} is @var{n} or less,
268 the value is @code{nil}.
270 If @var{n} is negative, @code{nth} returns the first element of
286 (nth n x) @equiv{} (car (nthcdr n x))
290 The function @code{elt} is similar, but applies to any kind of sequence.
291 For historical reasons, it takes its arguments in the opposite order.
292 @xref{Sequence Functions}.
296 This function returns the @var{n}th @sc{cdr} of @var{list}. In other
297 words, it skips past the first @var{n} links of @var{list} and returns
300 If @var{n} is zero or negative, @code{nthcdr} returns all of
301 @var{list}. If the length of @var{list} is @var{n} or less,
302 @code{nthcdr} returns @code{nil}.
306 (nthcdr 1 '(1 2 3 4))
310 (nthcdr 10 '(1 2 3 4))
314 (nthcdr -3 '(1 2 3 4))
320 @defun last list &optional n
321 This function returns the last link of @var{list}. The @code{car} of
322 this link is the list's last element. If @var{list} is null,
323 @code{nil} is returned. If @var{n} is non-@code{nil}, the
324 @var{n}th-to-last link is returned instead, or the whole of @var{list}
325 if @var{n} is bigger than @var{list}'s length.
328 @defun safe-length list
329 @anchor{Definition of safe-length}
330 This function returns the length of @var{list}, with no risk of either
331 an error or an infinite loop. It generally returns the number of
332 distinct cons cells in the list. However, for circular lists,
333 the value is just an upper bound; it is often too large.
335 If @var{list} is not @code{nil} or a cons cell, @code{safe-length}
339 The most common way to compute the length of a list, when you are not
340 worried that it may be circular, is with @code{length}. @xref{Sequence
343 @defun caar cons-cell
344 This is the same as @code{(car (car @var{cons-cell}))}.
347 @defun cadr cons-cell
348 This is the same as @code{(car (cdr @var{cons-cell}))}
349 or @code{(nth 1 @var{cons-cell})}.
352 @defun cdar cons-cell
353 This is the same as @code{(cdr (car @var{cons-cell}))}.
356 @defun cddr cons-cell
357 This is the same as @code{(cdr (cdr @var{cons-cell}))}
358 or @code{(nthcdr 2 @var{cons-cell})}.
361 @defun butlast x &optional n
362 This function returns the list @var{x} with the last element,
363 or the last @var{n} elements, removed. If @var{n} is greater
364 than zero it makes a copy of the list so as not to damage the
365 original list. In general, @code{(append (butlast @var{x} @var{n})
366 (last @var{x} @var{n}))} will return a list equal to @var{x}.
369 @defun nbutlast x &optional n
370 This is a version of @code{butlast} that works by destructively
371 modifying the @code{cdr} of the appropriate element, rather than
372 making a copy of the list.
376 @section Building Cons Cells and Lists
378 @cindex building lists
380 Many functions build lists, as lists reside at the very heart of Lisp.
381 @code{cons} is the fundamental list-building function; however, it is
382 interesting to note that @code{list} is used more times in the source
383 code for Emacs than @code{cons}.
385 @defun cons object1 object2
386 This function is the most basic function for building new list
387 structure. It creates a new cons cell, making @var{object1} the
388 @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new
389 cons cell. The arguments @var{object1} and @var{object2} may be any
390 Lisp objects, but most often @var{object2} is a list.
408 @code{cons} is often used to add a single element to the front of a
409 list. This is called @dfn{consing the element onto the list}.
410 @footnote{There is no strictly equivalent way to add an element to
411 the end of a list. You can use @code{(append @var{listname} (list
412 @var{newelt}))}, which creates a whole new list by copying @var{listname}
413 and adding @var{newelt} to its end. Or you can use @code{(nconc
414 @var{listname} (list @var{newelt}))}, which modifies @var{listname}
415 by following all the @sc{cdr}s and then replacing the terminating
416 @code{nil}. Compare this to adding an element to the beginning of a
417 list with @code{cons}, which neither copies nor modifies the list.}
421 (setq list (cons newelt list))
424 Note that there is no conflict between the variable named @code{list}
425 used in this example and the function named @code{list} described below;
426 any symbol can serve both purposes.
429 @defun list &rest objects
430 This function creates a list with @var{objects} as its elements. The
431 resulting list is always @code{nil}-terminated. If no @var{objects}
432 are given, the empty list is returned.
437 @result{} (1 2 3 4 5)
440 (list 1 2 '(3 4 5) 'foo)
441 @result{} (1 2 (3 4 5) foo)
450 @defun make-list length object
451 This function creates a list of @var{length} elements, in which each
452 element is @var{object}. Compare @code{make-list} with
453 @code{make-string} (@pxref{Creating Strings}).
458 @result{} (pigs pigs pigs)
465 (setq l (make-list 3 '(a b)))
466 @result{} ((a b) (a b) (a b))
467 (eq (car l) (cadr l))
473 @defun append &rest sequences
474 @cindex copying lists
475 This function returns a list containing all the elements of
476 @var{sequences}. The @var{sequences} may be lists, vectors,
477 bool-vectors, or strings, but the last one should usually be a list.
478 All arguments except the last one are copied, so none of the arguments
479 is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join
480 lists with no copying.)
482 More generally, the final argument to @code{append} may be any Lisp
483 object. The final argument is not copied or converted; it becomes the
484 @sc{cdr} of the last cons cell in the new list. If the final argument
485 is itself a list, then its elements become in effect elements of the
486 result list. If the final element is not a list, the result is a
487 dotted list since its final @sc{cdr} is not @code{nil} as required
491 Here is an example of using @code{append}:
495 (setq trees '(pine oak))
497 (setq more-trees (append '(maple birch) trees))
498 @result{} (maple birch pine oak)
505 @result{} (maple birch pine oak)
508 (eq trees (cdr (cdr more-trees)))
513 You can see how @code{append} works by looking at a box diagram. The
514 variable @code{trees} is set to the list @code{(pine oak)} and then the
515 variable @code{more-trees} is set to the list @code{(maple birch pine
516 oak)}. However, the variable @code{trees} continues to refer to the
523 | --- --- --- --- -> --- --- --- ---
524 --> | | |--> | | |--> | | |--> | | |--> nil
525 --- --- --- --- --- --- --- ---
528 --> maple -->birch --> pine --> oak
532 An empty sequence contributes nothing to the value returned by
533 @code{append}. As a consequence of this, a final @code{nil} argument
534 forces a copy of the previous argument:
542 (setq wood (append trees nil))
556 This once was the usual way to copy a list, before the function
557 @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}.
559 Here we show the use of vectors and strings as arguments to @code{append}:
563 (append [a b] "cd" nil)
564 @result{} (a b 99 100)
568 With the help of @code{apply} (@pxref{Calling Functions}), we can append
569 all the lists in a list of lists:
573 (apply 'append '((a b c) nil (x y z) nil))
574 @result{} (a b c x y z)
578 If no @var{sequences} are given, @code{nil} is returned:
587 Here are some examples where the final argument is not a list:
593 @result{} (x y . [z])
597 The second example shows that when the final argument is a sequence but
598 not a list, the sequence's elements do not become elements of the
599 resulting list. Instead, the sequence becomes the final @sc{cdr}, like
600 any other non-list final argument.
603 This function creates a new list whose elements are the elements of
604 @var{list}, but in reverse order. The original argument @var{list} is
621 @defun copy-tree tree &optional vecp
622 This function returns a copy of the tree @code{tree}. If @var{tree} is a
623 cons cell, this makes a new cons cell with the same @sc{car} and
624 @sc{cdr}, then recursively copies the @sc{car} and @sc{cdr} in the
627 Normally, when @var{tree} is anything other than a cons cell,
628 @code{copy-tree} simply returns @var{tree}. However, if @var{vecp} is
629 non-@code{nil}, it copies vectors too (and operates recursively on
633 @defun number-sequence from &optional to separation
634 This returns a list of numbers starting with @var{from} and
635 incrementing by @var{separation}, and ending at or just before
636 @var{to}. @var{separation} can be positive or negative and defaults
637 to 1. If @var{to} is @code{nil} or numerically equal to @var{from},
638 the value is the one-element list @code{(@var{from})}. If @var{to} is
639 less than @var{from} with a positive @var{separation}, or greater than
640 @var{from} with a negative @var{separation}, the value is @code{nil}
641 because those arguments specify an empty sequence.
643 If @var{separation} is 0 and @var{to} is neither @code{nil} nor
644 numerically equal to @var{from}, @code{number-sequence} signals an
645 error, since those arguments specify an infinite sequence.
647 All arguments can be integers or floating point numbers. However,
648 floating point arguments can be tricky, because floating point
649 arithmetic is inexact. For instance, depending on the machine, it may
650 quite well happen that @code{(number-sequence 0.4 0.6 0.2)} returns
651 the one element list @code{(0.4)}, whereas
652 @code{(number-sequence 0.4 0.8 0.2)} returns a list with three
653 elements. The @var{n}th element of the list is computed by the exact
654 formula @code{(+ @var{from} (* @var{n} @var{separation}))}. Thus, if
655 one wants to make sure that @var{to} is included in the list, one can
656 pass an expression of this exact type for @var{to}. Alternatively,
657 one can replace @var{to} with a slightly larger value (or a slightly
658 more negative value if @var{separation} is negative).
663 (number-sequence 4 9)
664 @result{} (4 5 6 7 8 9)
665 (number-sequence 9 4 -1)
666 @result{} (9 8 7 6 5 4)
667 (number-sequence 9 4 -2)
671 (number-sequence 8 5)
673 (number-sequence 5 8 -1)
675 (number-sequence 1.5 6 2)
676 @result{} (1.5 3.5 5.5)
681 @section Modifying List Variables
683 These functions, and one macro, provide convenient ways
684 to modify a list which is stored in a variable.
686 @defmac push newelt listname
687 This macro provides an alternative way to write
688 @code{(setq @var{listname} (cons @var{newelt} @var{listname}))}.
689 @c FIXME I don't think is a particularly good way to do it,
690 @c but generalized variables have not been introduced yet.
691 (In fact, this macro can act on generalized variables, not just lists.
692 @xref{Generalized Variables}.)
704 For the @code{pop} macro, which removes the first element from a list,
705 @xref{List Elements}.
708 Two functions modify lists that are the values of variables.
710 @defun add-to-list symbol element &optional append compare-fn
711 This function sets the variable @var{symbol} by consing @var{element}
712 onto the old value, if @var{element} is not already a member of that
713 value. It returns the resulting list, whether updated or not. The
714 value of @var{symbol} had better be a list already before the call.
715 @code{add-to-list} uses @var{compare-fn} to compare @var{element}
716 against existing list members; if @var{compare-fn} is @code{nil}, it
719 Normally, if @var{element} is added, it is added to the front of
720 @var{symbol}, but if the optional argument @var{append} is
721 non-@code{nil}, it is added at the end.
723 The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
724 is an ordinary function, like @code{set} and unlike @code{setq}. Quote
725 the argument yourself if that is what you want.
728 Here's a scenario showing how to use @code{add-to-list}:
734 (add-to-list 'foo 'c) ;; @r{Add @code{c}.}
737 (add-to-list 'foo 'b) ;; @r{No effect.}
740 foo ;; @r{@code{foo} was changed.}
744 An equivalent expression for @code{(add-to-list '@var{var}
745 @var{value})} is this:
748 (or (member @var{value} @var{var})
749 (setq @var{var} (cons @var{value} @var{var})))
752 @defun add-to-ordered-list symbol element &optional order
753 This function sets the variable @var{symbol} by inserting
754 @var{element} into the old value, which must be a list, at the
755 position specified by @var{order}. If @var{element} is already a
756 member of the list, its position in the list is adjusted according
757 to @var{order}. Membership is tested using @code{eq}.
758 This function returns the resulting list, whether updated or not.
760 The @var{order} is typically a number (integer or float), and the
761 elements of the list are sorted in non-decreasing numerical order.
763 @var{order} may also be omitted or @code{nil}. Then the numeric order
764 of @var{element} stays unchanged if it already has one; otherwise,
765 @var{element} has no numeric order. Elements without a numeric list
766 order are placed at the end of the list, in no particular order.
768 Any other value for @var{order} removes the numeric order of @var{element}
769 if it already has one; otherwise, it is equivalent to @code{nil}.
771 The argument @var{symbol} is not implicitly quoted;
772 @code{add-to-ordered-list} is an ordinary function, like @code{set}
773 and unlike @code{setq}. Quote the argument yourself if necessary.
775 The ordering information is stored in a hash table on @var{symbol}'s
776 @code{list-order} property.
779 Here's a scenario showing how to use @code{add-to-ordered-list}:
785 (add-to-ordered-list 'foo 'a 1) ;; @r{Add @code{a}.}
788 (add-to-ordered-list 'foo 'c 3) ;; @r{Add @code{c}.}
791 (add-to-ordered-list 'foo 'b 2) ;; @r{Add @code{b}.}
794 (add-to-ordered-list 'foo 'b 4) ;; @r{Move @code{b}.}
797 (add-to-ordered-list 'foo 'd) ;; @r{Append @code{d}.}
800 (add-to-ordered-list 'foo 'e) ;; @r{Add @code{e}}.
801 @result{} (a c b e d)
803 foo ;; @r{@code{foo} was changed.}
804 @result{} (a c b e d)
807 @node Modifying Lists
808 @section Modifying Existing List Structure
809 @cindex destructive list operations
811 You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the
812 primitives @code{setcar} and @code{setcdr}. We call these ``destructive''
813 operations because they change existing list structure.
815 @cindex CL note---@code{rplaca} vs @code{setcar}
819 @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and
820 @code{rplacd} to alter list structure; they change structure the same
821 way as @code{setcar} and @code{setcdr}, but the Common Lisp functions
822 return the cons cell while @code{setcar} and @code{setcdr} return the
823 new @sc{car} or @sc{cdr}.
827 * Setcar:: Replacing an element in a list.
828 * Setcdr:: Replacing part of the list backbone.
829 This can be used to remove or add elements.
830 * Rearrangement:: Reordering the elements in a list; combining lists.
834 @subsection Altering List Elements with @code{setcar}
836 Changing the @sc{car} of a cons cell is done with @code{setcar}. When
837 used on a list, @code{setcar} replaces one element of a list with a
840 @defun setcar cons object
841 This function stores @var{object} as the new @sc{car} of @var{cons},
842 replacing its previous @sc{car}. In other words, it changes the
843 @sc{car} slot of @var{cons} to refer to @var{object}. It returns the
844 value @var{object}. For example:
862 When a cons cell is part of the shared structure of several lists,
863 storing a new @sc{car} into the cons changes one element of each of
864 these lists. Here is an example:
868 ;; @r{Create two lists that are partly shared.}
871 (setq x2 (cons 'z (cdr x1)))
876 ;; @r{Replace the @sc{car} of a shared link.}
877 (setcar (cdr x1) 'foo)
879 x1 ; @r{Both lists are changed.}
886 ;; @r{Replace the @sc{car} of a link that is not shared.}
889 x1 ; @r{Only one list is changed.}
890 @result{} (baz foo c)
896 Here is a graphical depiction of the shared structure of the two lists
897 in the variables @code{x1} and @code{x2}, showing why replacing @code{b}
902 --- --- --- --- --- ---
903 x1---> | | |----> | | |--> | | |--> nil
904 --- --- --- --- --- ---
918 Here is an alternative form of box diagram, showing the same relationship:
923 -------------- -------------- --------------
924 | car | cdr | | car | cdr | | car | cdr |
925 | a | o------->| b | o------->| c | nil |
927 -------------- | -------------- --------------
939 @subsection Altering the CDR of a List
941 The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}:
943 @defun setcdr cons object
944 This function stores @var{object} as the new @sc{cdr} of @var{cons},
945 replacing its previous @sc{cdr}. In other words, it changes the
946 @sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the
950 Here is an example of replacing the @sc{cdr} of a list with a
951 different list. All but the first element of the list are removed in
952 favor of a different sequence of elements. The first element is
953 unchanged, because it resides in the @sc{car} of the list, and is not
954 reached via the @sc{cdr}.
971 You can delete elements from the middle of a list by altering the
972 @sc{cdr}s of the cons cells in the list. For example, here we delete
973 the second element, @code{b}, from the list @code{(a b c)}, by changing
974 the @sc{cdr} of the first cons cell:
980 (setcdr x1 (cdr (cdr x1)))
987 Here is the result in box notation:
993 -------------- | -------------- | --------------
994 | car | cdr | | | car | cdr | -->| car | cdr |
995 | a | o----- | b | o-------->| c | nil |
997 -------------- -------------- --------------
1002 The second cons cell, which previously held the element @code{b}, still
1003 exists and its @sc{car} is still @code{b}, but it no longer forms part
1006 It is equally easy to insert a new element by changing @sc{cdr}s:
1012 (setcdr x1 (cons 'd (cdr x1)))
1019 Here is this result in box notation:
1023 -------------- ------------- -------------
1024 | car | cdr | | car | cdr | | car | cdr |
1025 | a | o | -->| b | o------->| c | nil |
1026 | | | | | | | | | | |
1027 --------- | -- | ------------- -------------
1040 @subsection Functions that Rearrange Lists
1041 @cindex rearrangement of lists
1042 @cindex modification of lists
1044 Here are some functions that rearrange lists ``destructively'' by
1045 modifying the @sc{cdr}s of their component cons cells. We call these
1046 functions ``destructive'' because they chew up the original lists passed
1047 to them as arguments, relinking their cons cells to form a new list that
1048 is the returned value.
1051 See @code{delq}, in @ref{Sets And Lists}, for another function
1052 that modifies cons cells.
1055 The function @code{delq} in the following section is another example
1056 of destructive list manipulation.
1059 @defun nconc &rest lists
1060 @cindex concatenating lists
1061 @cindex joining lists
1062 This function returns a list containing all the elements of @var{lists}.
1063 Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are
1064 @emph{not} copied. Instead, the last @sc{cdr} of each of the
1065 @var{lists} is changed to refer to the following list. The last of the
1066 @var{lists} is not altered. For example:
1075 @result{} (1 2 3 4 5)
1079 @result{} (1 2 3 4 5)
1083 Since the last argument of @code{nconc} is not itself modified, it is
1084 reasonable to use a constant list, such as @code{'(4 5)}, as in the
1085 above example. For the same reason, the last argument need not be a
1095 @result{} (1 2 3 . z)
1099 @result{} (1 2 3 . z)
1103 However, the other arguments (all but the last) must be lists.
1105 A common pitfall is to use a quoted constant list as a non-last
1106 argument to @code{nconc}. If you do this, your program will change
1107 each time you run it! Here is what happens:
1111 (defun add-foo (x) ; @r{We want this function to add}
1112 (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.}
1116 (symbol-function 'add-foo)
1117 @result{} (lambda (x) (nconc (quote (foo)) x))
1121 (setq xx (add-foo '(1 2))) ; @r{It seems to work.}
1125 (setq xy (add-foo '(3 4))) ; @r{What happened?}
1126 @result{} (foo 1 2 3 4)
1134 (symbol-function 'add-foo)
1135 @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
1140 @defun nreverse list
1141 @cindex reversing a list
1142 This function reverses the order of the elements of @var{list}.
1143 Unlike @code{reverse}, @code{nreverse} alters its argument by reversing
1144 the @sc{cdr}s in the cons cells forming the list. The cons cell that
1145 used to be the last one in @var{list} becomes the first cons cell of the
1162 ;; @r{The cons cell that was first is now last.}
1168 To avoid confusion, we usually store the result of @code{nreverse}
1169 back in the same variable which held the original list:
1172 (setq x (nreverse x))
1175 Here is the @code{nreverse} of our favorite example, @code{(a b c)},
1176 presented graphically:
1180 @r{Original list head:} @r{Reversed list:}
1181 ------------- ------------- ------------
1182 | car | cdr | | car | cdr | | car | cdr |
1183 | a | nil |<-- | b | o |<-- | c | o |
1184 | | | | | | | | | | | | |
1185 ------------- | --------- | - | -------- | -
1187 ------------- ------------
1192 @defun sort list predicate
1194 @cindex sorting lists
1195 This function sorts @var{list} stably, though destructively, and
1196 returns the sorted list. It compares elements using @var{predicate}. A
1197 stable sort is one in which elements with equal sort keys maintain their
1198 relative order before and after the sort. Stability is important when
1199 successive sorts are used to order elements according to different
1202 The argument @var{predicate} must be a function that accepts two
1203 arguments. It is called with two elements of @var{list}. To get an
1204 increasing order sort, the @var{predicate} should return non-@code{nil} if the
1205 first element is ``less than'' the second, or @code{nil} if not.
1207 The comparison function @var{predicate} must give reliable results for
1208 any given pair of arguments, at least within a single call to
1209 @code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is
1210 less than @var{b}, @var{b} must not be less than @var{a}. It must be
1211 @dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b}
1212 is less than @var{c}, then @var{a} must be less than @var{c}. If you
1213 use a comparison function which does not meet these requirements, the
1214 result of @code{sort} is unpredictable.
1216 The destructive aspect of @code{sort} is that it rearranges the cons
1217 cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort
1218 function would create new cons cells to store the elements in their
1219 sorted order. If you wish to make a sorted copy without destroying the
1220 original, copy it first with @code{copy-sequence} and then sort.
1222 Sorting does not change the @sc{car}s of the cons cells in @var{list};
1223 the cons cell that originally contained the element @code{a} in
1224 @var{list} still has @code{a} in its @sc{car} after sorting, but it now
1225 appears in a different position in the list due to the change of
1226 @sc{cdr}s. For example:
1230 (setq nums '(1 3 2 6 5 4 0))
1231 @result{} (1 3 2 6 5 4 0)
1235 @result{} (0 1 2 3 4 5 6)
1239 @result{} (1 2 3 4 5 6)
1244 @strong{Warning}: Note that the list in @code{nums} no longer contains
1245 0; this is the same cons cell that it was before, but it is no longer
1246 the first one in the list. Don't assume a variable that formerly held
1247 the argument now holds the entire sorted list! Instead, save the result
1248 of @code{sort} and use that. Most often we store the result back into
1249 the variable that held the original list:
1252 (setq nums (sort nums '<))
1255 @xref{Sorting}, for more functions that perform sorting.
1256 See @code{documentation} in @ref{Accessing Documentation}, for a
1257 useful example of @code{sort}.
1260 @node Sets And Lists
1261 @section Using Lists as Sets
1262 @cindex lists as sets
1265 A list can represent an unordered mathematical set---simply consider a
1266 value an element of a set if it appears in the list, and ignore the
1267 order of the list. To form the union of two sets, use @code{append} (as
1268 long as you don't mind having duplicate elements). You can remove
1269 @code{equal} duplicates using @code{delete-dups}. Other useful
1270 functions for sets include @code{memq} and @code{delq}, and their
1271 @code{equal} versions, @code{member} and @code{delete}.
1273 @cindex CL note---lack @code{union}, @code{intersection}
1275 @b{Common Lisp note:} Common Lisp has functions @code{union} (which
1276 avoids duplicate elements) and @code{intersection} for set operations.
1277 Although standard GNU Emacs Lisp does not have them, the @file{cl-lib}
1278 library provides versions.
1279 @xref{Lists as Sets,,, cl, Common Lisp Extensions}.
1282 @defun memq object list
1283 @cindex membership in a list
1284 This function tests to see whether @var{object} is a member of
1285 @var{list}. If it is, @code{memq} returns a list starting with the
1286 first occurrence of @var{object}. Otherwise, it returns @code{nil}.
1287 The letter @samp{q} in @code{memq} says that it uses @code{eq} to
1288 compare @var{object} against the elements of the list. For example:
1292 (memq 'b '(a b c b a))
1296 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1302 @defun delq object list
1303 @cindex deleting list elements
1304 This function destructively removes all elements @code{eq} to
1305 @var{object} from @var{list}, and returns the resulting list. The
1306 letter @samp{q} in @code{delq} says that it uses @code{eq} to compare
1307 @var{object} against the elements of the list, like @code{memq} and
1310 Typically, when you invoke @code{delq}, you should use the return
1311 value by assigning it to the variable which held the original list.
1312 The reason for this is explained below.
1315 The @code{delq} function deletes elements from the front of the list
1316 by simply advancing down the list, and returning a sublist that starts
1317 after those elements. For example:
1321 (delq 'a '(a b c)) @equiv{} (cdr '(a b c))
1326 When an element to be deleted appears in the middle of the list,
1327 removing it involves changing the @sc{cdr}s (@pxref{Setcdr}).
1331 (setq sample-list '(a b c (4)))
1332 @result{} (a b c (4))
1335 (delq 'a sample-list)
1340 @result{} (a b c (4))
1343 (delq 'c sample-list)
1352 Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to
1353 splice out the third element, but @code{(delq 'a sample-list)} does not
1354 splice anything---it just returns a shorter list. Don't assume that a
1355 variable which formerly held the argument @var{list} now has fewer
1356 elements, or that it still holds the original list! Instead, save the
1357 result of @code{delq} and use that. Most often we store the result back
1358 into the variable that held the original list:
1361 (setq flowers (delq 'rose flowers))
1364 In the following example, the @code{(4)} that @code{delq} attempts to match
1365 and the @code{(4)} in the @code{sample-list} are not @code{eq}:
1369 (delq '(4) sample-list)
1374 If you want to delete elements that are @code{equal} to a given value,
1375 use @code{delete} (see below).
1377 @defun remq object list
1378 This function returns a copy of @var{list}, with all elements removed
1379 which are @code{eq} to @var{object}. The letter @samp{q} in @code{remq}
1380 says that it uses @code{eq} to compare @var{object} against the elements
1385 (setq sample-list '(a b c a b c))
1386 @result{} (a b c a b c)
1389 (remq 'a sample-list)
1394 @result{} (a b c a b c)
1399 @defun memql object list
1400 The function @code{memql} tests to see whether @var{object} is a member
1401 of @var{list}, comparing members with @var{object} using @code{eql},
1402 so floating point elements are compared by value.
1403 If @var{object} is a member, @code{memql} returns a list starting with
1404 its first occurrence in @var{list}. Otherwise, it returns @code{nil}.
1406 Compare this with @code{memq}:
1410 (memql 1.2 '(1.1 1.2 1.3)) ; @r{@code{1.2} and @code{1.2} are @code{eql}.}
1414 (memq 1.2 '(1.1 1.2 1.3)) ; @r{@code{1.2} and @code{1.2} are not @code{eq}.}
1420 The following three functions are like @code{memq}, @code{delq} and
1421 @code{remq}, but use @code{equal} rather than @code{eq} to compare
1422 elements. @xref{Equality Predicates}.
1424 @defun member object list
1425 The function @code{member} tests to see whether @var{object} is a member
1426 of @var{list}, comparing members with @var{object} using @code{equal}.
1427 If @var{object} is a member, @code{member} returns a list starting with
1428 its first occurrence in @var{list}. Otherwise, it returns @code{nil}.
1430 Compare this with @code{memq}:
1434 (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.}
1438 (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.}
1442 ;; @r{Two strings with the same contents are @code{equal}.}
1443 (member "foo" '("foo" "bar"))
1444 @result{} ("foo" "bar")
1449 @defun delete object sequence
1450 This function removes all elements @code{equal} to @var{object} from
1451 @var{sequence}, and returns the resulting sequence.
1453 If @var{sequence} is a list, @code{delete} is to @code{delq} as
1454 @code{member} is to @code{memq}: it uses @code{equal} to compare
1455 elements with @var{object}, like @code{member}; when it finds an
1456 element that matches, it cuts the element out just as @code{delq}
1457 would. As with @code{delq}, you should typically use the return value
1458 by assigning it to the variable which held the original list.
1460 If @code{sequence} is a vector or string, @code{delete} returns a copy
1461 of @code{sequence} with all elements @code{equal} to @code{object}
1468 (setq l '((2) (1) (2)))
1473 ;; @r{If you want to change @code{l} reliably,}
1474 ;; @r{write @code{(setq l (delete '(2) l))}.}
1477 (setq l '((2) (1) (2)))
1482 ;; @r{In this case, it makes no difference whether you set @code{l},}
1483 ;; @r{but you should do so for the sake of the other case.}
1486 (delete '(2) [(2) (1) (2)])
1492 @defun remove object sequence
1493 This function is the non-destructive counterpart of @code{delete}. It
1494 returns a copy of @code{sequence}, a list, vector, or string, with
1495 elements @code{equal} to @code{object} removed. For example:
1499 (remove '(2) '((2) (1) (2)))
1503 (remove '(2) [(2) (1) (2)])
1510 @b{Common Lisp note:} The functions @code{member}, @code{delete} and
1511 @code{remove} in GNU Emacs Lisp are derived from Maclisp, not Common
1512 Lisp. The Common Lisp versions do not use @code{equal} to compare
1516 @defun member-ignore-case object list
1517 This function is like @code{member}, except that @var{object} should
1518 be a string and that it ignores differences in letter-case and text
1519 representation: upper-case and lower-case letters are treated as
1520 equal, and unibyte strings are converted to multibyte prior to
1524 @defun delete-dups list
1525 This function destructively removes all @code{equal} duplicates from
1526 @var{list}, stores the result in @var{list} and returns it. Of
1527 several @code{equal} occurrences of an element in @var{list},
1528 @code{delete-dups} keeps the first one.
1531 See also the function @code{add-to-list}, in @ref{List Variables},
1532 for a way to add an element to a list stored in a variable and used as a
1535 @node Association Lists
1536 @section Association Lists
1537 @cindex association list
1540 An @dfn{association list}, or @dfn{alist} for short, records a mapping
1541 from keys to values. It is a list of cons cells called
1542 @dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the
1543 @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key''
1544 is not related to the term ``key sequence''; it means a value used to
1545 look up an item in a table. In this case, the table is the alist, and
1546 the alist associations are the items.}
1548 Here is an example of an alist. The key @code{pine} is associated with
1549 the value @code{cones}; the key @code{oak} is associated with
1550 @code{acorns}; and the key @code{maple} is associated with @code{seeds}.
1560 Both the values and the keys in an alist may be any Lisp objects.
1561 For example, in the following alist, the symbol @code{a} is
1562 associated with the number @code{1}, and the string @code{"b"} is
1563 associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of
1570 Sometimes it is better to design an alist to store the associated
1571 value in the @sc{car} of the @sc{cdr} of the element. Here is an
1572 example of such an alist:
1575 ((rose red) (lily white) (buttercup yellow))
1579 Here we regard @code{red} as the value associated with @code{rose}. One
1580 advantage of this kind of alist is that you can store other related
1581 information---even a list of other items---in the @sc{cdr} of the
1582 @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see
1583 below) to find the element containing a given value. When neither of
1584 these considerations is important, the choice is a matter of taste, as
1585 long as you are consistent about it for any given alist.
1587 The same alist shown above could be regarded as having the
1588 associated value in the @sc{cdr} of the element; the value associated
1589 with @code{rose} would be the list @code{(red)}.
1591 Association lists are often used to record information that you might
1592 otherwise keep on a stack, since new associations may be added easily to
1593 the front of the list. When searching an association list for an
1594 association with a given key, the first one found is returned, if there
1597 In Emacs Lisp, it is @emph{not} an error if an element of an
1598 association list is not a cons cell. The alist search functions simply
1599 ignore such elements. Many other versions of Lisp signal errors in such
1602 Note that property lists are similar to association lists in several
1603 respects. A property list behaves like an association list in which
1604 each key can occur only once. @xref{Property Lists}, for a comparison
1605 of property lists and association lists.
1607 @defun assoc key alist
1608 This function returns the first association for @var{key} in
1609 @var{alist}, comparing @var{key} against the alist elements using
1610 @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no
1611 association in @var{alist} has a @sc{car} @code{equal} to @var{key}.
1615 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1616 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1618 @result{} (oak . acorns)
1619 (cdr (assoc 'oak trees))
1621 (assoc 'birch trees)
1625 Here is another example, in which the keys and values are not symbols:
1628 (setq needles-per-cluster
1629 '((2 "Austrian Pine" "Red Pine")
1633 (cdr (assoc 3 needles-per-cluster))
1634 @result{} ("Pitch Pine")
1635 (cdr (assoc 2 needles-per-cluster))
1636 @result{} ("Austrian Pine" "Red Pine")
1640 The function @code{assoc-string} is much like @code{assoc} except
1641 that it ignores certain differences between strings. @xref{Text
1644 @defun rassoc value alist
1645 This function returns the first association with value @var{value} in
1646 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1647 a @sc{cdr} @code{equal} to @var{value}.
1649 @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of
1650 each @var{alist} association instead of the @sc{car}. You can think of
1651 this as ``reverse @code{assoc}'', finding the key for a given value.
1654 @defun assq key alist
1655 This function is like @code{assoc} in that it returns the first
1656 association for @var{key} in @var{alist}, but it makes the comparison
1657 using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil}
1658 if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}.
1659 This function is used more often than @code{assoc}, since @code{eq} is
1660 faster than @code{equal} and most alists use symbols as keys.
1661 @xref{Equality Predicates}.
1664 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1665 @result{} ((pine . cones) (oak . acorns) (maple . seeds))
1667 @result{} (pine . cones)
1670 On the other hand, @code{assq} is not usually useful in alists where the
1671 keys may not be symbols:
1675 '(("simple leaves" . oak)
1676 ("compound leaves" . horsechestnut)))
1678 (assq "simple leaves" leaves)
1680 (assoc "simple leaves" leaves)
1681 @result{} ("simple leaves" . oak)
1685 @defun rassq value alist
1686 This function returns the first association with value @var{value} in
1687 @var{alist}. It returns @code{nil} if no association in @var{alist} has
1688 a @sc{cdr} @code{eq} to @var{value}.
1690 @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of
1691 each @var{alist} association instead of the @sc{car}. You can think of
1692 this as ``reverse @code{assq}'', finding the key for a given value.
1697 (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
1699 (rassq 'acorns trees)
1700 @result{} (oak . acorns)
1701 (rassq 'spores trees)
1705 @code{rassq} cannot search for a value stored in the @sc{car}
1706 of the @sc{cdr} of an element:
1709 (setq colors '((rose red) (lily white) (buttercup yellow)))
1711 (rassq 'white colors)
1715 In this case, the @sc{cdr} of the association @code{(lily white)} is not
1716 the symbol @code{white}, but rather the list @code{(white)}. This
1717 becomes clearer if the association is written in dotted pair notation:
1720 (lily white) @equiv{} (lily . (white))
1724 @defun assoc-default key alist &optional test default
1725 This function searches @var{alist} for a match for @var{key}. For each
1726 element of @var{alist}, it compares the element (if it is an atom) or
1727 the element's @sc{car} (if it is a cons) against @var{key}, by calling
1728 @var{test} with two arguments: the element or its @sc{car}, and
1729 @var{key}. The arguments are passed in that order so that you can get
1730 useful results using @code{string-match} with an alist that contains
1731 regular expressions (@pxref{Regexp Search}). If @var{test} is omitted
1732 or @code{nil}, @code{equal} is used for comparison.
1734 If an alist element matches @var{key} by this criterion,
1735 then @code{assoc-default} returns a value based on this element.
1736 If the element is a cons, then the value is the element's @sc{cdr}.
1737 Otherwise, the return value is @var{default}.
1739 If no alist element matches @var{key}, @code{assoc-default} returns
1743 @defun copy-alist alist
1744 @cindex copying alists
1745 This function returns a two-level deep copy of @var{alist}: it creates a
1746 new copy of each association, so that you can alter the associations of
1747 the new alist without changing the old one.
1751 (setq needles-per-cluster
1752 '((2 . ("Austrian Pine" "Red Pine"))
1753 (3 . ("Pitch Pine"))
1755 (5 . ("White Pine"))))
1757 ((2 "Austrian Pine" "Red Pine")
1761 (setq copy (copy-alist needles-per-cluster))
1763 ((2 "Austrian Pine" "Red Pine")
1767 (eq needles-per-cluster copy)
1769 (equal needles-per-cluster copy)
1771 (eq (car needles-per-cluster) (car copy))
1773 (cdr (car (cdr needles-per-cluster)))
1774 @result{} ("Pitch Pine")
1776 (eq (cdr (car (cdr needles-per-cluster)))
1777 (cdr (car (cdr copy))))
1782 This example shows how @code{copy-alist} makes it possible to change
1783 the associations of one copy without affecting the other:
1787 (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
1788 (cdr (assq 3 needles-per-cluster))
1789 @result{} ("Pitch Pine")
1794 @defun assq-delete-all key alist
1795 This function deletes from @var{alist} all the elements whose @sc{car}
1796 is @code{eq} to @var{key}, much as if you used @code{delq} to delete
1797 each such element one by one. It returns the shortened alist, and
1798 often modifies the original list structure of @var{alist}. For
1799 correct results, use the return value of @code{assq-delete-all} rather
1800 than looking at the saved value of @var{alist}.
1803 (setq alist '((foo 1) (bar 2) (foo 3) (lose 4)))
1804 @result{} ((foo 1) (bar 2) (foo 3) (lose 4))
1805 (assq-delete-all 'foo alist)
1806 @result{} ((bar 2) (lose 4))
1808 @result{} ((foo 1) (bar 2) (lose 4))
1812 @defun rassq-delete-all value alist
1813 This function deletes from @var{alist} all the elements whose @sc{cdr}
1814 is @code{eq} to @var{value}. It returns the shortened alist, and
1815 often modifies the original list structure of @var{alist}.
1816 @code{rassq-delete-all} is like @code{assq-delete-all} except that it
1817 compares the @sc{cdr} of each @var{alist} association instead of the