assoc

(assoc key alist &key test key)

Searches an association list (a list of (key . value) pairs) and returns the first pair whose car matches key, or nil if none matches. The comparison is eql by default; the optional :test keyword takes a function designator to use a different comparison (e.g. #'equal for string keys), and the optional :key keyword takes a selector function applied to each pair's car before the comparison. The returned pair shares structure with the alist. Use rassoc to search by value instead of by key.