In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includin ...
, a multimap (sometimes also multihash, multidict or multidictionary) is a generalization of a
map or associative array abstract data type
In computer science, an abstract data type (ADT) is a mathematical model for data types. An abstract data type is defined by its behavior (semantics) from the point of view of a '' user'', of the data, specifically in terms of possible values, po ...
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
The Standard Template Library (STL) is a software library originally designed by Alexander Stepanov for the C++ programming language that influenced many parts of the C++ Standard Library. It provides four components called ''algorithms'', ''co ...
containers). Often the multimap is implemented as a map with
list
A ''list'' is any set of items in a row. List or lists may also refer to:
People
* List (surname)
Organizations
* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby uni ...
s 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.
*
Querystring
A query string is a part of a uniform resource locator (URL) that assigns values to specified parameters. A query string commonly includes fields added to a base URL by a Web browser or other client application, for example as part of an HTML, cho ...
s 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
The Standard Template Library (STL) is a software library originally designed by Alexander Stepanov for the C++ programming language that influenced many parts of the C++ Standard Library. It provides four components called ''algorithms'', ''co ...
provides the
multimap container for the sorted multimap using a
self-balancing binary search tree
In computer science, a self-balancing binary search tree (BST) is any node-based binary search tree that automatically keeps its height (maximal number of levels below the root) small in the face of arbitrary item insertions and deletions.Donald ...
, and
SGI's STL extension provides the
hash_multimap container, which implements a multimap using a
hash table
In computing, a hash table, also known as hash map, is a data structure that implements an associative array or dictionary. It is an abstract data type that maps keys to values. A hash table uses a hash function to compute an ''index'', ...
.
As of C++11, the
Standard Template Library
The Standard Template Library (STL) is a software library originally designed by Alexander Stepanov for the C++ programming language that influenced many parts of the C++ Standard Library. It provides four components called ''algorithms'', ''co ...
provides the
unordered_multimap for the unordered multimap.
Dart
Quiver provides a Multimap for
Dart
Dart or DART may refer to:
* Dart, the equipment in the game of darts
Arts, entertainment and media
* Dart (comics), an Image Comics superhero
* Dart, a character from ''G.I. Joe''
* Dart, a ''Thomas & Friends'' railway engine character
* D ...
.
Java
Apache Commons The Apache Commons is a project of the Apache Software Foundation, formerly under the Jakarta Project. The purpose of the Commons is to provide reusable, open source Java software. The Commons is composed of three parts: proper, sandbox, and dorma ...
Collections provides a MultiMap interface for
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
. 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.
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
OCaml ( , formerly Objective Caml) is a general-purpose, multi-paradigm programming language which extends the Caml dialect of ML with object-oriented features. OCaml was created in 1996 by Xavier Leroy, Jérôme Vouillon, Damien Doligez, D ...
'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
*
Abstract data type
In computer science, an abstract data type (ADT) is a mathematical model for data types. An abstract data type is defined by its behavior (semantics) from the point of view of a '' user'', of the data, specifically in terms of possible values, po ...
for the concept of type in general
*
Associative array
In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms an ...
for the more fundamental abstract data type
*
Multiset
In mathematics, a multiset (or bag, or mset) is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. The number of instances given for each element is called the multiplicity of that ...
for the case where same item can appear several times
References
{{Data structures
Associative arrays
Abstract data types