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.

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 Guava (previously named Google Collections) also provides an interface Multimap and implementations. ^[5] The Scala programming language's API also provides Multimap and implementations^[6]

OCaml's standard library module Hashtbl implements a hash table where it's possible to store multiple values for a key.

See also

• Abstract data type for the concept of type in general
• Associative array for the more fundamental abstract data type
• Multiset for the case where same item can appear several times

References

1. ↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. 
2. ↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. 
3. ↑ "Interface MultiMap". Commons Collections 3.2.1 API, Apache Commons. 
4. ↑ "Class MultiValueMap". Commons Collections 3.2.1 API, Apache Commons. 
5. ↑ "Interface Multimap<K,V>". Guava Library 2.0. 
6. ↑ "Scala.collection.mutable.MultiMap". Scala stable API.