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In computer science, a multimap (sometimes also multihash, multidict or multidictionary) 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 (for example, see C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.

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 or pages.
• Querystrings may have multiple values associated with a single field. This is commonly generated when a web form allows multiple check boxes or selections to be chosen in response to a single form element.

Language support

C++

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

As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap.

Dart

Quiver provides a Multimap for Dart.

Java

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 Guava provides a Multimap interface and implementations of it.

Python

Python provides a collections.defaultdict class that can be used to create a multimap. The user can instantiate the class as collections.defaultdict(list).

OCaml

OCaml's standard library module Hashtbl implements a hash table where it's possible to store multiple values for a key.

Scala

The Scala programming language's API also provides Multimap and implementations.

See also

• Multiset for the case where same item can appear several times

References

1. "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
2. "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
3. "Working Draft, Standard for Programming Language C++" (PDF). p. 7807.
4. "Multimap". Quiver API docs.
5. "Interface MultiMap". Commons Collections 3.2.2 API, Apache Commons.
6. "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons.
7. "Interface Multimap<K,V>". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01.
8. "Scala.collection.mutable.MultiMap". Scala stable API.

• Associative arrays
• Abstract data types