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This article is about the data type. For the mathematical concept, see Multivalued function. For the mapping website, see Multimap.com.

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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. 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++

The C++ standard library provides the std::multimap container for the sorted multimap using a self-balancing binary search tree.^[1].

As of C++11, the Standard Template Library provides the std::unordered_multimap for the unordered multimap implemented with a hash table^[2], itself based on SGI's STL hash_multimap.^[3]

Dart

Quiver provides a Multimap for Dart.^[4]

Java

Apache Commons Collections provides a org.apache.commons.collections4.MultiMap interface for Java.^[5] It also provides a org.apache.commons.collections4.map.MultiValueMap implementing class that makes a org.apache.commons.collections4.MultiMap out of a java.util.Map object and a type of java.util.Collection.^[6]

Google Guava provides a com.google.common.collect.Multimap interface and implementations of it.^[7]

Kotlin

Kotlin does not have explicit support for multimaps,^[8] but can implement them using kotlin.collections.Maps with containers^[9] for the value type. E.g. a Map<User, List<Book>> 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 is possible to store multiple values for a key.

Scala

The Scala programming language's API also provides scala.collection.mutable.MultiMap and implementations.^[10]

References

↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
↑ "Working Draft, Standard for Programming Language C++" (PDF). p. 7807.
↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
↑ "Multimap". Quiver API docs.
↑ "Interface MultiMap". Commons Collections 3.2.2 API, Apache Commons.
↑ "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons.
↑ "Interface Multimap<K,V>". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01.
↑ "Implement a MultiMap in Kotlin | Baeldung on Kotlin". 5 December 2023.
↑ "Accessing data using Room DAOs".
↑ "Scala.collection.mutable.MultiMap". Scala stable API.

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