KeyMemberQ[assoc,form] yields True if a key in the association assoc matches form, and False otherwise.
KeyMemberQ[form]represents an operator form of KeyMemberQ that can be applied to an expression.
Test whether "x" is a key in the association:
Test whether the association contains a string key:
KeyMemberQ works with lists of rules:
Determine which of the associations does not contain the key:
KeyFreeQ MemberQ KeySelect KeyTake