What Is the Difference Between a Map and a Dictionary?

Question

I know a map is a data structure that maps keys to values. Isn't a dictionary the same? What is the difference between a map and a dictionary¹?

_{1. I am not asking for how they are defined in language X or Y (which seems to be what generally people are asking here on SO), I want to know what is their difference in theory.}

Accepted Answer

Two terms for the same thing:

"Map" is used by Java, C++
"Dictionary" is used by .Net, Python
"Associative array" is used by PHP

"Map" is the correct mathematical term, but it is avoided because it has a separate meaning in functional programming.

Some languages use still other terms ("Object" in Javascript, "Hash" in Ruby, "Table" in Lua), but those all have separate meanings in programming too, so I'd avoid them.

See here for more info.

Answer 2

One is an older term for the other. Typically the term "dictionary" was used before the mathematical term "map" took hold. Also, dictionaries tend to have a key type of string, but that's not 100% true everywhere.

Answer 3

Summary of Computer Science terminology:

a dictionary is a data structure representing a set of elements, with insertion, deletion, and tests for membership; the elements may be, but are not necessarily, composed of distinct key and value parts
a map is an associative data structure able to store a set of keys, each associated with one (or sometimes more than one - e.g. C++ multimap) value, with the ability to access and erase existing entries given only the key.

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Discussion

Answering this question is complicated by programmers having seen the terms given more specific meanings in particular languages or systems they've used, but the question asks for a language agnostic comparison "in theory", which I'm taking to mean in Computing Science terms.

What Is the Difference Between a Map and a Dictionary?

12 Answers

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David Miller