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In computer science, a multimap (sometimes also multihash or multidict) 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.


  • 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++'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]

As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap.[3]


Quiver provides a Multimap for Dart.[4]


Apache Commons Collections provides a MultiMap interface for Java.[5] It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.[6]

Google Guava provides a Multimap interface and implementations of it. [7]


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


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


The Scala programming language's API also provides Multimap and implementations[8]

See also


  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.
  8. ^ "Scala.collection.mutable.MultiMap". Scala stable API.

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