An association list or alist is a list of pairs, in which the first element ( car) of each pair is a key value and the remainder of it is associated data. Association lists are a simple way to implement small databases.
(assoc x alist) searches alist, a list of pairs, for a pair whose car equals x. The result is the pair that was found, or #f if no matching pair is found.
(assoc 'two '((one 1) (two 2) (three 3)))
= (two 2)
An additional operation (in this case, cadr)
on the result of assoc is used to get the desired data.
(cadr (assoc 'two '((one 1) (two 2) ...)))
= 2
Three assoc functions use different equality predicates: