Multimap

• 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.

C++'s Standard Template Library provides the multimap container for the sorted multimap using a self-balancing binary search tree,{{cite web | url = http://www.sgi.com/tech/stl/Multimap.html | title = multimap | work = Standard Template Library Programmer's Guide | publisher = Silicon Graphics International}} and SGI's STL extension provides the hash_multimap container, which implements a multimap using a hash table.{{cite web | url = http://www.sgi.com/tech/stl/hash_multimap.html | title = hash_multimap | work = Standard Template Library Programmer's Guide | publisher = Silicon Graphics International}}

As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap.{{cite web |url= http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf |title = Working Draft, Standard for Programming Language C++ | page= 7807}}

Quiver provides a Multimap for Dart.{{cite web | url = https://pub.dev/documentation/quiver/latest/quiver.collection/Multimap-class.html | title = Multimap | work = Quiver API docs}}

Apache Commons Collections provides a MultiMap interface for Java.{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/MultiMap.html | title = Interface MultiMap | work = Commons Collections 3.2.2 API, Apache Commons}} It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/map/MultiValueMap.html | title = Class MultiValueMap | work = Commons Collections 3.2.2 API, Apache Commons}}

Google Guava provides a Multimap interface and implementations of it.{{cite web | url = http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | title = Interface Multimap | work = Guava Library 2.0 | access-date = 2013-01-01 | archive-url = https://web.archive.org/web/20130115105942/http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | archive-date = 2013-01-15 | url-status = dead }}

Kotlin does not have explicit support for multimaps,{{cite web | url = https://www.baeldung.com/kotlin/multimap| title = Implement a MultiMap in Kotlin | Baeldung on Kotlin| date = 5 December 2023}} but can implement them using Maps with containers{{cite web | url = https://developer.android.com/training/data-storage/room/accessing-data#multimap| title = Accessing data using Room DAOs}} for the value type. E.g. a Map> can associate each User with a list of Books.

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.{{cite web | url = http://www.scala-lang.org/api/current/scala/collection/mutable/MultiMap.html | title = Scala.collection.mutable.MultiMap | work = Scala stable API}}

• Multiset for the case where same item can appear several times

{{Data structures}}

Category:Associative arrays

Category:Abstract data types