Multimap
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.
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.
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]
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
References
- ^ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
- ^ "hash_multiset<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
- ^ "Interface MultiMap". Commons Collections 3.2.1 API, Apache Commons.
- ^ "Class MultiValueMap". Commons Collections 3.2.1 API, Apache Commons.
- ^ "Interface Multimap<K,V>". Google Collections Library 1.0.
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