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Showing content from https://cplusplus.com/reference/unordered_set/unordered_set/count/ below:

public member function

<unordered_set>

std::unordered_set::count 
size_type count ( const key_type& k ) const;

Count elements with a specific key

Searches the container for elements with a value of k and returns the number of elements found. Because unordered_set containers do not allow for duplicate values, this means that the function actually returns 1 if an element with that value exists in the container, and zero otherwise.

Parameters
k
Value of the elements to be counted.
Member type key_type is the type of the elements in the container. In unordered_set containers it is the same as value_type, defined as an alias of the class's first template parameter (Key).

Return value1 if an element with a value equivalent to k is found, or zero otherwise.

Member type size_type is an unsigned integral type.



Example 
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// unordered_set::count
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_set<std::string> myset = { "hat", "umbrella", "suit" };

  for (auto& x: {"hat","sunglasses","suit","t-shirt"}) {
    if (myset.count(x)>0)
      std::cout << "myset has " << x << std::endl;
    else
      std::cout << "myset has no " << x << std::endl;
  }

  return 0;
}

Output: 
myset has hat
myset has no sunglasses
myset has suit
myset has no t-shirt


Complexity Average case: constant.
Worst case: linear in container size.

Iterator validity No changes.

See also
unordered_set::find
Get iterator to element (public member function)
unordered_set::size
Return container size (public member function)

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