Multimap
In computer science, a multimap (sometimes also multihash) 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 (see for example C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.
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[edit] 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.
- 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.
[edit] Language support
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]
Apache Commons Collections provides a MultiMap interface for Java.[3] It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.[4]
Google Collections also provides an interface Multimap and implementations.[5]
Scala language's API also provides Multimap and implementations[6]
[edit] See also
- Abstract data type for the type of concept in general
- Associative array for the more fundamental abstract data type
- Multiset for the case where same item can appear several times
[edit] References
- ^ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/Multimap.html.
- ^ "hash_multiset<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/hash_multiset.html.
- ^ "Interface MultiMap". Commons Collections 3.2.1 API, Apache Commons. http://commons.apache.org/collections/api-release/org/apache/commons/collections/MultiMap.html.
- ^ "Class MultiValueMap". Commons Collections 3.2.1 API, Apache Commons. http://commons.apache.org/collections/api-release/org/apache/commons/collections/map/MultiValueMap.html.
- ^ "Interface Multimap<K,V>". Google Collections Library 1.0. http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Multimap.html.
- ^ "Scala.collection.mutable.MultiMap". Scala stable API. http://www.scala-lang.org/api/current/scala/collection/mutable/MultiMap.html.
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