std::unordered_map::count

size_type count ( const key_type& k ) const;

Count elements with a specific key

Searches the container for elements whose key is k and returns the number of elements found. Because unordered_map containers do not allow for duplicate keys, this means that the function actually returns 1 if an element with that key exists in the container, and zero otherwise.

Parameters

k: Key value to be searched for.
Member type key_type is the type of the keys for the elements in the container, defined in unordered_map as an alias of its first template parameter (Key).

Return value

1 if an element with a key equivalent to k is found, or zero otherwise.

Member type

size_type is an unsigned integral type.

Example

// unordered_map::count
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_map<std::string,double> mymap = {
     {"Burger",2.99},
     {"Fries",1.99},
     {"Soda",1.50} };

  for (auto& x: {"Burger","Pizza","Salad","Soda"}) {
    if (mymap.count(x)>0)
      std::cout << "mymap has " << x << std::endl;
    else
      std::cout << "mymap has no " << x << std::endl;
  }

  return 0;
}

Output:

mymap has Burger
mymap has no Pizza
mymap has no Salad
mymap has Soda

Complexity

Average case: linear in the number of elements counted.
Worst case: linear in container size.

Iterator validity

No changes.