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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.
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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]
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 provides a collections.defaultdict
class that can be used to create a multimap. The user can instantiate the class as collections.defaultdict(list)
.
OCaml's standard library module Hashtbl
implements a hash table where it's possible to store multiple values for a key.
The Scala programming language's API also provides Multimap and implementations.[8]
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