unordered_set count() function in C++ STL

The unordered_set::count() function is a built-in function in C++ STL which is used to count occurrences of a particular element in an unordered_set container. As the unordered_set container does not allows to store duplicate elements so this function is generally used to check if an element is present in the container or not. The function returns 1 if the element is present in the container otherwise it returns 0.
Syntax: 

unordered_set_name.count(element)

Parameter: This function accepts a single parameter element. This parameter represents the element which is needed to be checked if it is present in the container or not.
Return Value: This function returns 1 if the element is present in the container otherwise it returns 0.
Time Complexity: Time Complexity for unordered_set::count() method is O(1) in average cases, but in worst case, time complexity can be O(N) i.e. Linear, where N is size of container. 

Below programs illustrate the unordered_set::count() function:

Program 1: 

CPP

// CPP program to illustrate the
// unordered_set::count() function

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<int> sampleSet;

    // Inserting elements
    sampleSet.insert(5);
    sampleSet.insert(10);
    sampleSet.insert(15);
    sampleSet.insert(20);
    sampleSet.insert(25);

    // displaying all elements of sampleSet
    cout << "sampleSet contains: ";
    for (auto itr = sampleSet.begin(); itr != sampleSet.end(); itr++) {
        cout << *itr << " ";
    }

    // checking if element 20 is present in the set
    if (sampleSet.count(20) == 1) {
        cout << "\nElement 20 is present in the set";
    }
    else {
        cout << "\nElement 20 is not present in the set";
    }

    return 0;
}

Output:

sampleSet contains: 25 5 10 15 20 
Element 20 is present in the set

Program 2: 

CPP

// C++ program to illustrate the
// unordered_set::count() function

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<string> sampleSet;

    // Inserting elements
    sampleSet.insert("Welcome");
    sampleSet.insert("To");
    sampleSet.insert("GeeksforGeeks");
    sampleSet.insert("Computer Science Portal");
    sampleSet.insert("For Geeks");

    // displaying all elements of sampleSet
    cout << "sampleSet contains: ";
    for (auto itr = sampleSet.begin(); itr != sampleSet.end(); itr++) {
        cout << *itr << " ";
    }

    // checking if element GeeksforGeeks is
    // present in the set
    if (sampleSet.count("GeeksforGeeks") == 1) {
        cout << "\nGeeksforGeeks is present in the set";
    }
    else {
        cout << "\nGeeksforGeeks is not present in the set";
    }

    return 0;
}

Output:

sampleSet contains: Welcome To GeeksforGeeks For Geeks Computer Science Portal 
GeeksforGeeks is present in the set