ASSOC

look up key in association list
Major Section:  PROGRAMMING

General Forms:
(assoc x alist)
(assoc x alist :test 'eql)   ; same as above (eql as equality test)
(assoc x alist :test 'eq)    ; same, but eq is equality test
(assoc x alist :test 'equal) ; same, but equal is equality test

(Assoc x alist) is the first member of alist whose car is x, or nil if no such member exists. The optional keyword, :TEST, has no effect logically, but provides the test (default eql) used for comparing x with the cars of successive elements of alist.

The guard for a call of assoc depends on the test. In all cases, the second argument must satisfy alistp. If the test is eql, then either the first argument must be suitable for eql (see eqlablep) or the second argument must satisfy eqlable-alistp. If the test is eq, then either the first argument must be a symbol or the second argument must satisfy symbol-alistp.

See equality-variants for a discussion of the relation between assoc and its variants:

(assoc-eq x alist) is equivalent to (assoc x alist :test 'eq);

(assoc-equal x alist) is equivalent to (assoc x alist :test 'equal).

In particular, reasoning about any of these primitives reduces to reasoning about the function assoc-equal.

Assoc is defined by Common Lisp. See any Common Lisp documentation for more information.