unordered_set count() function in C++ STL

01 Jul, 2021
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;}
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;}
sampleSet contains: Welcome To GeeksforGeeks For Geeks Computer Science Portal 
GeeksforGeeks is present in the set
 
pragyeshtiwari29
CPP-Functions
cpp-unordered_set
cpp-unordered_set-functions
C++
CPP
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