Help
Add this page to your book
Show book (500 pages)
Suggest pagesMultimap
 | This article needs more citations. (February 2022) |
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
[edit]- 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 or pages.
- 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
[edit]C++
[edit]The C++ standard library provides the std::multimap container for the sorted multimap using a self-balancing binary search tree.[1].
As of C++11, the Standard Template Library provides the std::unordered_multimap for the unordered multimap implemented with a hash table[2], itself based on SGI's STL hash_multimap.[3]
Dart
[edit]Java
[edit]Apache Commons Collections provides a org.apache.commons.collections4.MultiMap interface for Java.[5] 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.[6]
Google Guava provides a com.google.common.collect.Multimap interface and implementations of it.[7]
Kotlin
[edit]Kotlin does not have explicit support for multimaps,[8] but can implement them using kotlin.collections.Maps with containers[9] for the value type. E.g. a Map<User, List<Book>> can associate each User with a list of Books.
Python
[edit]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
[edit]OCaml's standard library module Hashtbl implements a hash table where it is possible to store multiple values for a key.
Scala
[edit]The Scala programming language's API also provides scala.collection.mutable.MultiMap and implementations.[10]
See also
[edit]- Multiset for the case where same item can appear several times
References
[edit]- ↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
- ↑ "Working Draft, Standard for Programming Language C++" (PDF). p. 7807.
- ↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International.
- ↑ "Multimap". Quiver API docs.
- ↑ "Interface MultiMap". Commons Collections 3.2.2 API, Apache Commons.
- ↑ "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons.
- ↑ "Interface Multimap<K,V>". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01.
- ↑ "Implement a MultiMap in Kotlin | Baeldung on Kotlin". 5 December 2023.
- ↑ "Accessing data using Room DAOs".
- ↑ "Scala.collection.mutable.MultiMap". Scala stable API.