CODEWITHSUNDEEP
c++
Poperton
Poperton

Reputation: 2148

How to get number of keys in std::map?

Couldn't find on stackoverflow.

std::map<T>.size()

gives number of elements, not keys.

So how to get the number of keys?

Upvotes: 4

Views: 23010

Answers (3)

mayank patel
mayank patel

Reputation: 36

Since the keys are not repeated in a map, the size gives the number of keys in the map.

Extra knowledge:- In the datatype multimap, the keys can be repeated, and over there the number of keys will not be equal to the size of the map as in the case of simple maps.

Upvotes: 2

Ted Lyngmo
Ted Lyngmo

Reputation: 117298

std::map<T>.size()

Gives the number of elements, and keys. It's a 1-to-1 match.

A map has a union of all keys you've tried to insert into the map. Insertion of an already existing key will be rejected if done via insert/emplace but the value-mapping will be replaced if the insertion is done using operator[].

In a map a key can only map to one value. It's a dictionary.

Variants:

There are multimap (and multiset) versions. In a multimap multiple equal keys may map to different entities.

A -> foo 
A -> apa
B -> bar
B -> bepa

In this multimap the size would be 4 which is the number of mapped elements, not the number of unique keys (which isn't a concern for multimaps).

The size() member function of a regular map and a multimap return the number of mapped elements, which for a regular map is the same as the number of unique key values.

Upvotes: 14

user11422223
user11422223

Reputation:

The number of keys is equal to the number of elements in a map, for which its size would be the same.

Upvotes: 6

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