Next: Modifying Lists, Previous: List Elements, Up: Lists
Many functions build lists, as lists reside at the very heart of Lisp.
cons
is the fundamental list-building function; however, it is
interesting to note that list
is used more times in the source
code for Emacs than cons
.
This function is the fundamental function used to build new list structure. It creates a new cons cell, making object1 the car, and object2 the cdr. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list.
(cons 1 '(2)) => (1 2) (cons 1 '()) => (1) (cons 1 2) => (1 . 2)
cons
is often used to add a single element to the front of a list. This is called consing the element onto the list. 1 For example:(setq list (cons newelt list))Note that there is no conflict between the variable named
list
used in this example and the function namedlist
described below; any symbol can serve both purposes.
This macro provides an alternative way to write
(setq
listname(cons
newelt listname))
. It is new in Emacs 21.(setq l '(a b)) => (a b) (push 'c l) => (c a b) l => (c a b)
This function creates a list with objects as its elements. The resulting list is always
nil
-terminated. If no objects are given, the empty list is returned.(list 1 2 3 4 5) => (1 2 3 4 5) (list 1 2 '(3 4 5) 'foo) => (1 2 (3 4 5) foo) (list) => nil
This function creates a list of length elements, in which each element is object. Compare
make-list
withmake-string
(see Creating Strings).(make-list 3 'pigs) => (pigs pigs pigs) (make-list 0 'pigs) => nil (setq l (make-list 3 '(a b)) => ((a b) (a b) (a b)) (eq (car l) (cadr l)) => t
This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, bool-vectors, or strings, but the last one should usually be a list. All arguments except the last one are copied, so none of the arguments is altered. (See
nconc
in Rearrangement, for a way to join lists with no copying.)More generally, the final argument to
append
may be any Lisp object. The final argument is not copied or converted; it becomes the cdr of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a “dotted list” since its final cdr is notnil
as required in a true list.The
append
function also allows integers as arguments. It converts them to strings of digits, making up the decimal print representation of the integer, and then uses the strings instead of the original integers. Don't use this feature; we plan to eliminate it. If you already use this feature, change your programs now! The proper way to convert an integer to a decimal number in this way is withformat
(see Formatting Strings) ornumber-to-string
(see String Conversion).
Here is an example of using append
:
(setq trees '(pine oak)) => (pine oak) (setq more-trees (append '(maple birch) trees)) => (maple birch pine oak) trees => (pine oak) more-trees => (maple birch pine oak) (eq trees (cdr (cdr more-trees))) => t
You can see how append
works by looking at a box diagram. The
variable trees
is set to the list (pine oak)
and then the
variable more-trees
is set to the list (maple birch pine
oak)
. However, the variable trees
continues to refer to the
original list:
more-trees trees | | | --- --- --- --- -> --- --- --- --- --> | | |--> | | |--> | | |--> | | |--> nil --- --- --- --- --- --- --- --- | | | | | | | | --> maple -->birch --> pine --> oak
An empty sequence contributes nothing to the value returned by
append
. As a consequence of this, a final nil
argument
forces a copy of the previous argument:
trees => (pine oak) (setq wood (append trees nil)) => (pine oak) wood => (pine oak) (eq wood trees) => nil
This once was the usual way to copy a list, before the function
copy-sequence
was invented. See Sequences Arrays Vectors.
Here we show the use of vectors and strings as arguments to append
:
(append [a b] "cd" nil) => (a b 99 100)
With the help of apply
(see Calling Functions), we can append
all the lists in a list of lists:
(apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z)
If no sequences are given, nil
is returned:
(append) => nil
Here are some examples where the final argument is not a list:
(append '(x y) 'z) => (x y . z) (append '(x y) [z]) => (x y . [z])
The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument.
This function creates a new list whose elements are the elements of list, but in reverse order. The original argument list is not altered.
(setq x '(1 2 3 4)) => (1 2 3 4) (reverse x) => (4 3 2 1) x => (1 2 3 4)
This function returns a copy of list, with all elements removed which are
eq
to object. The letter ‘q’ inremq
says that it useseq
to compare object against the elements oflist
.(setq sample-list '(a b c a b c)) => (a b c a b c) (remq 'a sample-list) => (b c b c) sample-list => (a b c a b c)The function
delq
offers a way to perform this operation destructively. See Sets And Lists.
[1] There is no strictly equivalent way to add an element to
the end of a list. You can use (append
listname (list
newelt))
, which creates a whole new list by copying listname
and adding newelt to its end. Or you can use (nconc
listname (list
newelt))
, which modifies listname
by following all the cdrs and then replacing the terminating
nil
. Compare this to adding an element to the beginning of a
list with cons
, which neither copies nor modifies the list.