unordered_set cbegin() function in C++ STL

Last Updated : 05 Jun, 2023

The unordered_set::cbegin() method is a built-in function in C++ STL which is used to return a const_iterator pointing to the first element in the unordered_set container. This iterator can point to either the very the first element or first element of any specified bucket in the unordered_set container.

Note: A const_iterator can only be used to access elements, it cannot modify the elements present in the container.

Syntax:

unordered_set_name.cbegin(n)

Parameter: This function accepts a single parameter n. This is an optional parameter and specifies the bucket number. If this parameter is not passed then the cbegin() method will return a const_iterator pointing to the first element of the container and if this parameter is passed then the begin() method will return a const_iterator pointing to the first element of specific bucket in the unordered_set container.

Return Value: This function returns a const_iterator pointing to the first element in the container or a specified bucket in the container. Below programs illustrate the unordered_set::cbegin() function:

Program 1:

CPP

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

#include <iostream> 
#include <unordered_set> 

using namespace std; 

int main() 
{ 

    unordered_set<int> sampleSet; 

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

    auto itr1 = sampleSet.cbegin(); 
    auto itr2 = sampleSet.cbegin(4); 

    cout << "First element in the container is: " << *itr1; 
    cout << "\nFirst element in the bucket 4 is: " << *itr2; 

    return 0; 
}

Output:

First element in the container is: 25
First element in the bucket 4 is: 15

Time complexity: O(1)
Auxiliary Space: O(1)

Program 2:

CPP

// C++ program to illustrate the 
// unordered_set::cbegin() 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"); 

    auto itr1 = sampleSet.cbegin(); 
    auto itr2 = sampleSet.cbegin(0); 

    cout << "First element in the container is: " << *itr1; 
    cout << "\nFirst element in the bucket 0 is: " << *itr2; 

    return 0; 
}

Output:

First element in the container is: Welcome
First element in the bucket 0 is: GeeksforGeeks

Time complexity: O(1)
Auxiliary Space: O(1)

unordered_set clear() function in C++ STL

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