science

Multimap

Multimap is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Multimap rather than just read about it. In short: 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).

Key takeaways

Multimap belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Multimap to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Multimap from memory before moving on to harder problems.

Reference excerpt

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, 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<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.

See also Multiset for the case where same item can appear several times

References

Worked examples

Example 1 — a first encounter with Multimap

Start with the simplest possible case. Write down what Multimap claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Multimap before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Multimap ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Multimap

In research: Multimap appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Multimap in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Multimap is common in secondary-school and first-year university syllabi. It links to neighbouring topics Abstract data types, Associative arrays, so understanding it makes those chapters shorter.
In everyday life: Look for Multimap outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Multimap in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Multimap means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Multimap out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Multimap in simple terms?

Why does Multimap matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Multimap?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Multimap.