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.

C++
C++'s Standard Template Library provides the  container for the sorted multimap using a self-balancing binary search tree, and SGI's STL extension provides the   container, which implements a multimap using a hash table.

As of C++11, the Standard Template Library provides the  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  class that can be used to create a multimap. The user can instantiate the class as.

OCaml
OCaml's standard library module  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.