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  ANSI Common Lisp   14 Conses   14.2 Dictionary of Conses

14.2.36 assoc, assoc-if, assoc-if-not Function

Syntax:
assoc item alist &key key test test-not    entry

assoc-if predicate alist &key key    entry

assoc-if-not predicate alist &key key    entry

Arguments and Values:
item - an object.

alist - an association list.

predicate - a designator for a function of one argument that returns a generalized boolean.

test - a designator for a function of two arguments that returns a generalized boolean.

test-not - a designator for a function of two arguments that returns a generalized boolean.

key - a designator for a function of one argument, or nil.

entry - a cons that is an element of alist, or nil.

Description:
assoc, assoc-if, and assoc-if-not return the first cons in alist whose car satisfies the test, or nil if no such cons is found.

For assoc, assoc-if, and assoc-if-not, if nil appears in alist in place of a pair, it is ignored.

Examples:
 (setq values '((x . 100) (y . 200) (z . 50)))  ((X . 100) (Y . 200) (Z . 50))
 (assoc 'y values)  (Y . 200)
 (rplacd (assoc 'y values) 201)  (Y . 201)
 (assoc 'y values)  (Y . 201)
 (setq alist '((1 . "one")(2 . "two")(3 . "three"))) 
 ((1 . "one") (2 . "two") (3 . "three"))
 (assoc 2 alist)  (2 . "two")
 (assoc-if #'evenp alist)  (2 . "two")
 (assoc-if-not #'(lambda(x) (< x 3)) alist)  (3 . "three")
 (setq alist '(("one" . 1)("two" . 2)))  (("one" . 1) ("two" . 2))
 (assoc "one" alist)   NIL
 (assoc "one" alist :test #'equalp)  ("one" . 1)
 (assoc "two" alist :key #'(lambda(x) (char x 2)))   NIL 
 (assoc #\o alist :key #'(lambda(x) (char x 2)))  ("two" . 2)
 (assoc 'r '((a . b) (c . d) (r . x) (s . y) (r . z)))   (R . X)
 (assoc 'goo '((foo . bar) (zoo . goo)))   NIL
 (assoc '2 '((1 a b c) (2 b c d) (-7 x y z)))  (2 B C D)
 (setq alist '(("one" . 1) ("2" . 2) ("three" . 3)))
 (("one" . 1) ("2" . 2) ("three" . 3))
 (assoc-if-not #'alpha-char-p alist
               :key #'(lambda (x) (char x 0)))  ("2" . 2)

Exceptional Situations:
Should be prepared to signal an error of type type-error if alist is not an association list.

See Also:
rassoc, find, member, position, Section 3.6 Traversal Rules and Side Effects

Notes:
The :test-not parameter is deprecated.

The function assoc-if-not is deprecated.

It is possible to rplacd the result of assoc, provided that it is not nil, in order to "update" alist.

The two expressions

 (assoc item list :test fn)
and

 (find item list :test fn :key #'car)
are equivalent in meaning with one exception: if nil appears in alist in place of a pair, and item is nil, find will compute the car of the nil in alist, find that it is equal to item, and return nil, whereas assoc will ignore the nil in alist and continue to search for an actual cons whose car is nil.

Allegro CL Implementation Details:
None.

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