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. ==Language support==

C++ C++'s Standard Template Library provides the multimap container for the sorted multimap using a self-balancing binary search tree, and SGI's STL extension provides the hash_multimap container, which implements a multimap using a hash table. As of C++11, the Standard Template Library provides the unordered_multimap for the unordered multimap. Dart Quiver provides a Multimap for Dart. Java Apache Commons Collections provides a MultiMap interface for Java. It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection. Google Guava provides a Multimap interface and implementations of it. Kotlin Kotlin does not have explicit support for multimaps, but can implement them using Maps with containers for the value type. E.g. a Map> 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. ==See also==