Tutorial :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.


Solution:1

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.


Solution: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.


Solution:3

My 2 cents.

Dictionary is an abstract class in Java whereas Map is an interface. Since, Java does not support multiple inheritances, if a class extends Dictionary, it cannot extend any other class.

Therefore, the Map interface was introduced.

Dictionary class is obsolete and use of Map is preferred.


Solution:4

Typically I assume that a map is backed by a hash table; it connotes an unordered store. Dictionaries connote an ordered store.

There is a tree-based dictionary called a Trie.

In Lisp, it might look like this:

(a (n (d t)) n d )  

Which encapsulates the words:

  • a
  • and
  • ant
  • an
  • ad

The traversal from the top to the leaf yields a word.


Solution:5

Yes, they are the same, you may add "Associative Array" to the mix.

using Hashtable or a Hash ofter refers to the implementation.


Solution:6

so on a purely theoretical level.

A Dictionary is a value that can be used to locate a Linked Value. A Map is a Value that provides instructions on how to locate another values

all collections that allow non linear access (ie only get first or get last) are a Map, as even a simple Array has an index that maps to the correct value. So while a Dictionary is a Type of map, maps are a much broader range of possible function.

In Practice a its usually the mapping function that defines the name, so a HashMap is a mapped data structure that uses a hashing algorithm to link the key to the value, where as a Dictionary doesn't specify how the keys are linked to a value so could be stored via a linked list, tree or any other algorithm. from the usage end you usually don't care what the algorithm only that they work so you use a generic dictionary and only shift to one of the other structures only when you need to enfore the type of algorithm


Solution:7

Not really the same thing. Maps are a subset of dictionary. Dictionary is defined here as having the insert, delete, and find functions. Map as used by Java (according to this) is a dictionary with the requirement that keys mapping to values are strictly mapped as a one-to-one function. A dictionary might have more than one key map to one value, or one key map to several values (like chaining in a hasthtable), eg Twitter hashtag searches.

As a more "real world" example, looking up a word in a dictionary can give us a number of definitions for the same word, and when we find an entry that points us to another entry (see other word), a number of words for the same list of definitions. In the real world, maps are much broader, allowing us to have locations for names or names for coordinates, but also we can find a nearest neighbor or other attributes (populations, etc), so IMHO there could be argument for a greater expansion of the map type to possibly have graph based implementations, but it would be best to always assume just the key-value pair, especially since nearest neighbor and other attributes to the value could all just be data members of the value.

java maps, despite the one-to-one requirement, can implement something more like a generalized dictionary if the value is generalized as a collection itself, or if the values are merely references to collections stored elsewhere.

Remember that Java maintainers are not the maintainers of ADT definitions, and that Java decisions are specifically for Java.


Solution:8

Other terms for this concept that are fairly common: associative array and hash.


Solution:9

The main difference is that a Map, requires that all entries(value & key pair) have a unique key. If collisions occur, i.e. when a new entry has the same key as an entry already in the collection, then collision handling is required.

Usually, we handle collisions using either Separate Chaining. Or Linear Probing.

A Dictionary allows for multiple entries to be linked to the same key.

When a Map has implemented Separate Chaining, then it tends to resemble a Dictionary.


Solution:10

These are two different terms for the same concept.
Hashtable and HashMap also refer to the same concept.


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