Multimap: Difference between revisions
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In [[computer science]], a '''multimap''' (sometimes also '''multihash''', '''multidict''' or '''multidictionary''') is a generalization of a [[associative array|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 [[container (data structure)|container]]s (for example, see [[C++]] [[Standard Template Library]] [[Standard Template Library#Containers|containers]]). Often the multimap is implemented as a map with [[List (computing)|list]]s or [[Set (computer science)|set]]s as the map values. |
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==Examples== |
==Examples== |
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* 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. |
* 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. |
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* 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. |
* 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. |
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* [[Querystring |
* [[Querystring]]s may have multiple values associated with a single field. This is commonly generated when a [[web form]] allows multiple [[check box]]es or selections to be chosen in response to a single form element. |
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==Language support== |
==Language support== |
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| ⚫ | [[C++]]'s [[Standard Template Library]] provides the <code>multimap</code> [[Standard Template Library#Containers|container]] for the sorted multimap using a [[self-balancing binary search tree]],<ref>{{cite web | url = http://www.sgi.com/tech/stl/Multimap.html | title = multimap<Key, Data, Compare, Alloc> | work = Standard Template Library Programmer's Guide | publisher = [[Silicon Graphics International]]}}</ref> and [[Silicon Graphics International|SGI]]'s STL extension provides the <code>hash_multimap</code> container, which implements a multimap using a [[hash table]].<ref>{{cite web | url = http://www.sgi.com/tech/stl/ |
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===C++=== |
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| ⚫ | [[Apache Commons]] Collections provides a MultiMap interface for [[Java (programming language)|Java]].<ref>{{cite web | url = |
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| ⚫ | [[C++]]'s [[Standard Template Library]] provides the <code>multimap</code> [[Standard Template Library#Containers|container]] for the sorted multimap using a [[self-balancing binary search tree]],<ref>{{cite web | url = http://www.sgi.com/tech/stl/Multimap.html | title = multimap<Key, Data, Compare, Alloc> | work = Standard Template Library Programmer's Guide | publisher = [[Silicon Graphics International]]}}</ref> and [[Silicon Graphics International|SGI]]'s STL extension provides the <code>hash_multimap</code> container, which implements a multimap using a [[hash table]].<ref>{{cite web | url = http://www.sgi.com/tech/stl/hash_multimap.html | title = hash_multimap<Key, HashFcn, EqualKey, Alloc> | work = Standard Template Library Programmer's Guide | publisher = [[Silicon Graphics International]]}}</ref> |
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As of C++11, the [[Standard Template Library]] provides the <code>unordered_multimap</code> for the unordered multimap.<ref>{{cite web |url= http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf |title = Working Draft, Standard for Programming Language C++ | page= 7807}}</ref> |
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===Dart=== |
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Quiver provides a Multimap for [[Dart (programming language)|Dart]].<ref>{{cite web | url = https://pub.dev/documentation/quiver/latest/quiver.collection/Multimap-class.html | title = Multimap | work = Quiver API docs}}</ref> |
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===Java=== |
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| ⚫ | [[Apache Commons]] Collections provides a MultiMap interface for [[Java (programming language)|Java]].<ref>{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/MultiMap.html | title = Interface MultiMap | work = Commons Collections 3.2.2 API, [[Apache Commons]]}}</ref> It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.<ref>{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/map/MultiValueMap.html | title = Class MultiValueMap | work = Commons Collections 3.2.2 API, [[Apache Commons]]}}</ref> |
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* [[Abstract data type]] for the type of concept in general |
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* [[Associative array]] for the more fundamental abstract data type |
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[[Google Guava]] provides a Multimap interface and implementations of it.<ref>{{cite web | url = http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | title = Interface Multimap<K,V> | work = Guava Library 2.0 | access-date = 2013-01-01 | archive-url = https://web.archive.org/web/20130115105942/http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | archive-date = 2013-01-15 | url-status = dead }}</ref> |
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===Python=== |
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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>. |
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===OCaml=== |
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[[OCaml]]'s standard library module <code>Hashtbl</code> implements a hash table where it's possible to store multiple values for a key. |
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===Scala=== |
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==See also== |
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* [[Multiset]] for the case where same item can appear several times |
* [[Multiset]] for the case where same item can appear several times |
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[[Category:Associative arrays]] |
[[Category:Associative arrays]] |
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[[Category:Abstract data types]] |
[[Category:Abstract data types]] |
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{{comp-sci-stub}} |
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Latest revision as of 18:55, 13 May 2024
 | This article needs additional citations for verification. (February 2022) |
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
[edit]- 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
[edit]C++
[edit]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
[edit]Quiver provides a Multimap for Dart.[4]
Java
[edit]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
[edit]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
[edit]OCaml's standard library module Hashtbl implements a hash table where it's possible to store multiple values for a key.
Scala
[edit]The Scala programming language's API also provides Multimap and implementations.[8]
See also
[edit]- Multiset for the case where same item can appear several times
References
[edit]- ^ "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 3.2.2 API, Apache Commons.
- ^ "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons.
- ^ "Interface Multimap<K,V>". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01.
- ^ "Scala.collection.mutable.MultiMap". Scala stable API.