Multimap (data structure)

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For the provider of online mapping services, see Multimap.com.

A multimap is a generalization of a map or associative array abstract data type in which more than one value may be associated with and returned for a given key. Both map and multimap are particular cases of containers (see for example C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.

[edit] Examples

In a student enrollment system, where students may be enrolled in multiple classes simultaneously, there might be an association for each enrollment of a student in a course, where the key is the student ID and the value is the course ID. If a student is enrolled in three courses, there will be three associations containing the same key.
The index of a book may report any number of references for a given index term, and thus may be coded as a multimap from index terms to any number of reference locations.

[edit] Language Support

C++'s Standard Template Library provides the "multimap" container for the sorted multimap using a self-balancing binary search tree [1], and SGI's STL extension provides the "hash_multimap" container, which implements a multimap using a hash table [2].

Apache Commons Collections provides a MultiMap interface for Java. It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.

Google Collections also provides an interface Multimap and implementations.