Common Lisp Extensions
11.1 List Functions
This section describes a number of simple operations on lists,
i.e., chains of cons cells.
- Function: caddr x
- This function is equivalent to
(car (cdr (cdr x))).
Likewise, this package defines all 28 cxxxr functions
where xxx is up to four `a's and/or `d's.
All of these functions are setf-able, and calls to them
are expanded inline by the byte-compiler for maximum efficiency.
- Function: first x
- This function is a synonym for
(car x). Likewise,
the functions second, third, ..., through
tenth return the given element of the list x.
- Function: rest x
- This function is a synonym for
(cdr x).
- Function: endp x
- Common Lisp defines this function to act like
null, but
signaling an error if x is neither a nil nor a
cons cell. This package simply defines endp as a synonym
for null.
- Function: list-length x
- This function returns the length of list x, exactly like
(length x), except that if x is a circular
list (where the cdr-chain forms a loop rather than terminating
with nil), this function returns nil. (The regular
length function would get stuck if given a circular list.)
- Function: list* arg &rest others
- This function constructs a list of its arguments. The final
argument becomes the
cdr of the last cell constructed.
Thus, (list* a b c) is equivalent to
(cons a (cons b c)), and
(list* a b nil) is equivalent to
(list a b).
(Note that this function really is called list* in Common
Lisp; it is not a name invented for this package like member*
or defun*.)
- Function: ldiff list sublist
- If sublist is a sublist of list, i.e., is
eq to
one of the cons cells of list, then this function returns
a copy of the part of list up to but not including
sublist. For example, (ldiff x (cddr x)) returns
the first two elements of the list x. The result is a
copy; the original list is not modified. If sublist
is not a sublist of list, a copy of the entire list
is returned.
- Function: copy-list list
- This function returns a copy of the list list. It copies
dotted lists like
(1 2 . 3) correctly.
- Function: copy-tree x &optional vecp
- This function returns a copy of the tree of cons cells x.
Unlike
copy-sequence (and its alias copy-list),
which copies only along the cdr direction, this function
copies (recursively) along both the car and the cdr
directions. If x is not a cons cell, the function simply
returns x unchanged. If the optional vecp argument
is true, this function copies vectors (recursively) as well as
cons cells.
- Function: tree-equal x y &key :test :test-not :key
- This function compares two trees of cons cells. If x and
y are both cons cells, their
cars and cdrs are
compared recursively. If neither x nor y is a cons
cell, they are compared by eql, or according to the
specified test. The :key function, if specified, is
applied to the elements of both trees. See section 10. Sequences.
