Multimap

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title: "Multimap" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["associative-arrays", "abstract-data-types"] description: "Data structure in computer science" topic_path: "general/associative-arrays" source: "https://en.wikipedia.org/wiki/Multimap" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Data structure in computer science ::

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.

Kotlin

Kotlin does not have explicit support for multimaps, but can implement them using Maps with containers for the value type. E.g. a Map can associate each User with a list of Books.

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.

References

References

1. "multimap<Key, Data, Compare, Alloc>". [[Silicon Graphics International]].
2. "hash_multimap<Key, HashFcn, EqualKey, Alloc>". [[Silicon Graphics International]].
3. "Working Draft, Standard for Programming Language C++".
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. (5 December 2023). "Implement a MultiMap in Kotlin | Baeldung on Kotlin".
9. "Accessing data using Room DAOs".
10. "Scala.collection.mutable.MultiMap". Scala stable API.

::callout[type=info title="Wikipedia Source"] This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page. ::