Bidirectional map

In computer science, a bidirectional map, or hash bag, is an associative data structure in which the ( k e y , v a l u e ) pairs form a one-to-one correspondence. Thus the binary relation is functional in each direction: v a l u e can also act as a key to k e y . A pair ( a , b ) thus provides a unique coupling between a and b so that b can be found when a is used as a key and a can be found when b is used as a key.