Multimap Explained

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,^[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 a Multimap interface and implementations of it.^[7]

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.^[8]

See also

• Multiset for the case where same item can appear several times

References

1. Web site: multimap . Standard Template Library Programmer's Guide . Silicon Graphics International.
2. Web site: hash_multimap . Standard Template Library Programmer's Guide . Silicon Graphics International.
3. Web site: Working Draft, Standard for Programming Language C++ . 7807.
4. Web site: Multimap . Quiver API docs.
5. Web site: Interface MultiMap . Commons Collections 3.2.2 API, Apache Commons.
6. Web site: Class MultiValueMap . Commons Collections 3.2.2 API, Apache Commons.
7. Web site: Interface Multimap . Guava Library 2.0 . 2013-01-01 . https://web.archive.org/web/20130115105942/http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html . 2013-01-15 . dead .
8. Web site: Scala.collection.mutable.MultiMap . Scala stable API.