Multimap

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. Often the multimap is implemented as a map with lists or sets as the map values.

Examples

Language support

C++

The C++ standard library provides the std::multimap container for the sorted multimap using a self-balancing binary search tree..

As of C++11, the Standard Template Library provides the std::unordered_multimap for the unordered multimap implemented with a hash table, itself based on SGI's STL hash_multimap.

Dart

Quiver provides a Multimap for Dart.

Java

Apache Commons Collections provides a org.apache.commons.collections4.MultiMap interface for Java. It also provides a org.apache.commons.collections4.map.MultiValueMap implementing class that makes a org.apache.commons.collections4.MultiMap out of a java.util.Map object and a type of java.util.Collection.

Google Guava provides a com.google.common.collect.Multimap interface and implementations of it.

Kotlin

Kotlin does not have explicit support for multimaps, but can implement them using kotlin.collections.Maps with containers for the value type. E.g. a Map<User, List<Book>> can associate each User with a list of Books.

Python

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

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

Scala

The Scala programming language's API also provides scala.collection.mutable.MultiMap and implementations.

See also

References

  1. ↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
  2. ↑ "Working Draft, Standard for Programming Language C++" (PDF). p. 7807.
  3. ↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
  4. ↑ "Multimap". Quiver API docs.
  5. ↑ "Interface MultiMap". Commons Collections 3.2.2 API, Apache Commons.
  6. ↑ "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons.
  7. ↑ "Interface Multimap<K,V>". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01.
  8. ↑ "Implement a MultiMap in Kotlin | Baeldung on Kotlin". 5 December 2023.
  9. ↑ "Accessing data using Room DAOs".
  10. ↑ "Scala.collection.mutable.MultiMap". Scala stable API.