CODEWITHSUNDEEP
c++dictionaryset
FJam
FJam

Reputation: 756

Why is a set used instead of a map? C++

Sets are used to get information of an object by providing all the information, usually used to check if the data exists. A map is used to get the information of an object by using a key (single data). Correct me if I am wrong. Now the question is why would we need a set in the first place, can't we a map to see if the data exist? why would we need to provide all the information just to see if it exist?

Upvotes: 1

Views: 992

Answers (3)

Escualo
Escualo

Reputation: 42072

A set serves to group items of the same type that are different among themselves (i.e., they are not equal). For example, the numbers 1 and 2 are both of int type, but 1!=2.

set containers are useful when you want to keep track of collections of homogeneous things as a group, and perform mathematical operations on such groups (like intersection, union, difference, etc). For example, imagine a set of search results containing all the documents mentioning the words cat and dog. And then another set containing all the documents mentioning the words pet. The union of those two sets would give you the group of documents containing the words cat, dog, and pet. Notice that such group will have no repetitions (i.e., if a document was in the both sets initially, it will be only once in the second set).

maps are most certainly not a set, but they can be seen as an arrangement which allows you to associate a value with every element of a set. They are used to represent relationships. For example, the set of people working for a company have an associated employee_number; in this case a map would be a useful structure to represent such relationship.

Going back to the previous example, if you wanted to know how many times has each page been accessed, you could probably create a map along the lines of std::map<Page, int>, that is, a relationship between the pages, and the number of times each has been visited.

Notice that the keys of a map form a set (probably this is what confuses many people), and an implication of this property is that you can only have a given key once (there are some esoteric containers where a key can be mapped to different values though).

So, if you need to interact with groups and collections as a whole, and with the members of the group itself, probably you want a set. If you need to associate certain things with members of a group or a collection, probably you want a map. If the association spans more than one dimension, probably you want a multi_map.

Important notice that in C++ std::set and std::map are ordered. C++11 offers alternative unordered containers called std::unordered_set and std::unordered_map.

Upvotes: 1

A. K.
A. K.

Reputation: 38126

There are many operations where you just need a set. Using a map would be just extra space.

  1. Set operations (Union, Intersection etc.).
  2. Keeping unique elements from a collection of numbers, objects etc.

Upvotes: 2

doublesharp
doublesharp

Reputation: 27599

A Set contains a unique list of ordered values, but a Map can contain a non unique set of unordered values accessed using a key.

Either could be used to determine if an object exists, it depends on your use case and how you need to be able to access that object - can you test to see if the Set contains an object that you have a reference to, or do you need to look it up by one or more keys to be able to compare it?

Upvotes: 0

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