gethash

(gethash key table &optional default)

Looks up key in table and returns the associated value, or default (nil when omitted) if the key is absent. Keys are compared structurally as if by equal, so an equal list, string, or number key matches. To store a value, use gethash as a setf place: (setf (gethash key table) value), which also cooperates with incf/decf/push. The place's key, table and default are evaluated left to right, then the value; the default of a write is evaluated but never stored. Under multiple-value-bind (and the other multiple-value consumers) a gethash call also supplies a second present-p value distinguishing a stored nil from a missing key. The function object #'gethash takes the same optional default and answers the same two values. A table that is no hash table signals a type-error whose expected type is hash-table: GETHASH: The value 5 is not of type HASH-TABLE.