C++ unordered_multiset::contains() Function

---

---

Description

The unordered_multiset::contains() function in C++ STL was introduced in C++20. It checks if an element exists in the unordered_multiset container or not. It searches for a key and returns a boolean value that represents whether the element is present or not in the unordered_multiset.

Syntax

Following is the syntax of unordered_multiset::contains() function −

// Check by key
bool contains(const key_type& key) const;
// Check by key-like object
template< class K >
bool contains(const K& keyObj) const;

Parameters

The contains() function accepts a single parameter. It can either be a key or a key-like object that you want to search in the unordered_multiset.

Return value

The contains() function returns a boolean value, i.e., true if element found and false if not found.

Exceptions

If an exception is thrown, the container remains unchanged.

Time complexity

The time complexity of unordered_multiset::contains() function is O(1) on average, and in the worst case, it can be O(N) due to frequent hash collisions, where N is the size of the unordered_multiset.

Examples of unordered_multiset::contains() Function

The following examples demonstrate the usage of unordered_multiset::contains() function in unordered_multiset −

Checking if Element Exists in Unordered_multiset

Below is an example to check if an element (30) exists in an unordered_multiset nums using the unordered_multiset::contains() function −

#include <iostream>
#include <unordered_set>
using namespace std;
int main() {
    unordered_multiset<int> nums = {10, 20, 30, 40, 50};
    cout << "Unordered_multiset elements:";
    for (const auto &num : nums)
        cout << " " << num;
    cout << endl;
    if (nums.contains(30))
        cout << "Element 30 is present in the unordered_multiset" << endl;
    else
        cout << "Element 30 is not present in the unordered_multiset" << endl;
    if (nums.contains(60))
        cout << "Element 60 is present in the unordered_multiset" << endl;
    else
        cout << "Element 60 is not present in the unordered_multiset" << endl;
    return 0;
}

The output of the above code is given below −

Unordered_multiset elements: 50 40 30 20 10
Element 30 is present in the unordered_multiset 
Element 60 is not present in the unordered_multiset

Checking Duplicate Elements

In this example, we have used the unordered_multiset::contains() function to check if an element (20) exists in the unordered_multiset nums that contains duplicate values.

#include <iostream>
#include <unordered_set>
using namespace std;
int main(){
    unordered_multiset<int> nums = {10, 20, 20, 20, 30, 40};
    cout << "Unordered_multiset elements:";
    for (const auto &num : nums)
        cout << " " << num;
    cout << endl;
    if (nums.contains(20)){
        cout << "Element 20 is present in the unordered_multiset" << endl;
        cout << "Count of element 20: " << nums.count(20) << endl;
    } else {
        cout << "Element 20 is not present in the unordered_multiset" << endl;
    }
    return 0;
}

The output of the above code is given below −

Unordered_multiset elements: 40 30 20 20 20 10
Element 20 is present in the unordered_multiset
Count of element 20: 3

Checking Elements in an Empty unordered_multiset

In this example, we are checking the result if we search an element in an empty unordered multiset −

#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
    // empty container
    unordered_multiset<int> ums;
    cout << "Size of unordered_multiset: " << ums.size() << endl;

    cout << "Is 10 present? : ";
    if (ums.contains(10))
        cout << "Yes\n";
    else
        cout << "No\n";

    cout << "Is 5 present? : ";
    if (ums.contains(5))
        cout << "Yes\n";
    else
        cout << "No\n";

    return 0;
}

The output of the above code is given below −

Size of unordered_multiset: 0
Is 10 present? : No
Is 5 present? : No

Using contains() with String Unordered_multiset

Here is an example to check the existence of string elements in an unordered_multiset words using the unordered_multiset::contains() function −

#include <iostream>
#include <unordered_set>
#include <string>
using namespace std;
int main() {
    unordered_multiset<string> words = {"apple", "banana", "cherry", "banana"};
    cout << "Unordered_multiset elements:";
    for (const auto &word : words)
        cout << " " << word;
    cout << endl;
    string search1 = "banana";
    string search2 = "mango";
    if (words.contains(search1))
        cout << "'" << search1 << "' is present in the unordered_multiset" << endl;
    else
        cout << "'" << search1 << "' is not present in the unordered_multiset" << endl;
    if (words.contains(search2))
        cout << "'" << search2 << "' is present in the unordered_multiset" << endl;
    else
        cout << "'" << search2 << "' is not present in the unordered_multiset" << endl;
    return 0;
}

The output of the above code is given below −

Unordered_multiset elements: banana banana cherry apple
'banana' is present in the unordered_multiset
'mango' is not present in the unordered_multiset

Run the above example codes in the compilers that supports C++20 and above.

Difference Between contains() and find() Function

The key differences between the unordered_multiset::contains() and unordered_multiset::find() functions in C++ STL unordered_multiset is given below −