Multimap

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 <code>multimap</code> container for the sorted multimap using a self-balancing binary search tree, and SGI's STL extension provides the <code>hash_multimap</code> container, which implements a multimap using a hash table.

As of C++11, the Standard Template Library provides the <code>unordered_multimap</code> 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 <code>Map<User, List<Book>></code> can associate each User with a list of Books.

Python

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

OCaml

OCaml's standard library module <code>Hashtbl</code> 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.