Multimap

In computer science, a multimap (sometimes also multihash or multidict) 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,^[1] and SGI's STL extension provides the hash_multimap container, which implements a multimap using a hash table.^[2]

As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap.^[3]

Dart

Quiver provides a Multimap for Dart.^[4]

Java

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

Google Guava provides an interface Multimap and implementations. ^[7]

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^[8]

References

↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
↑ "Working Draft, Standard for Programming Language C++" (PDF). p. 7807.
↑ "Multimap". Quiver API docs.
↑ "Interface MultiMap". Commons Collections 3.2.1 API, Apache Commons.
↑ "Class MultiValueMap". Commons Collections 3.2.1 API, Apache Commons.
↑ "Interface Multimap<K,V>". Guava Library 2.0.
↑ "Scala.collection.mutable.MultiMap". Scala stable API.

Data structures
Types	Collection Container
Abstract	Associative array Multimap List Stack Queue Double-ended queue Priority queue Double-ended priority queue Set Multiset Disjoint-set
Arrays	Bit array Circular buffer Dynamic array Hash table Hashed array tree Sparse matrix
Linked	Association list Linked list Skip list Unrolled linked list XOR linked list
Trees	B-tree Binary search tree AA tree AVL tree Red–black tree Self-balancing tree Splay tree Heap Binary heap Binomial heap Fibonacci heap R-tree R* tree R+ tree Hilbert R-tree Trie Hash tree
Graphs	Binary decision diagram Directed acyclic graph Directed acyclic word graph
List of data structures

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