unordered_set cbegin() function in C++ STL

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)