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 (for example, see C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.
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 containers[9] 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
- ↑ "multimap<Key, Data, Compare, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/Multimap.html.
- ↑ "hash_multimap<Key, HashFcn, EqualKey, Alloc>". Standard Template Library Programmer's Guide. Silicon Graphics International. http://www.sgi.com/tech/stl/hash_multimap.html.
- ↑ "Working Draft, Standard for Programming Language C++". p. 7807. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf.
- ↑ "Multimap". Quiver API docs. https://pub.dev/documentation/quiver/latest/quiver.collection/Multimap-class.html.
- ↑ "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.
- ↑ "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.
- ↑ "Interface Multimap<K,V>". Guava Library 2.0. http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html.
- ↑ "Implement a MultiMap in Kotlin | Baeldung on Kotlin". 5 December 2023. https://www.baeldung.com/kotlin/multimap.
- ↑ "Accessing data using Room DAOs". https://developer.android.com/training/data-storage/room/accessing-data#multimap.
- ↑ "Scala.collection.mutable.MultiMap". Scala stable API. http://www.scala-lang.org/api/current/scala/collection/mutable/MultiMap.html.
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