• Images
• Videos
• Documents
• Books
• Articles

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 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

C++

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

Dart

Quiver provides a Multimap for Dart.⁴

Java

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

Kotlin

Kotlin does not have explicit support for multimaps,⁸ but can implement them using Maps with containers⁹ 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's possible to store multiple values for a key.

Scala

The Scala programming language's API also provides Multimap and implementations.¹⁰

See also

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

References

1. "multimap". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/Multimap.html ↩

2. "hash_multimap". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/hash_multimap.html ↩

3. "Working Draft, Standard for Programming Language C++" (PDF). p. 7807. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf ↩

4. "Multimap". Quiver API docs. https://pub.dev/documentation/quiver/latest/quiver.collection/Multimap-class.html ↩

5. "Interface MultiMap". Commons Collections 3.2.2 API, Apache Commons. https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/MultiMap.html ↩

6. "Class MultiValueMap". Commons Collections 3.2.2 API, Apache Commons. https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/map/MultiValueMap.html ↩

7. "Interface Multimap". Guava Library 2.0. Archived from the original on 2013-01-15. Retrieved 2013-01-01. https://web.archive.org/web/20130115105942/http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html ↩

8. "Implement a MultiMap in Kotlin | Baeldung on Kotlin". 5 December 2023. https://www.baeldung.com/kotlin/multimap ↩

9. "Accessing data using Room DAOs". https://developer.android.com/training/data-storage/room/accessing-data#multimap ↩

10. "Scala.collection.mutable.MultiMap". Scala stable API. http://www.scala-lang.org/api/current/scala/collection/mutable/MultiMap.html ↩