Multimap
# 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 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.

