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
#include <iostream>
#include <unordered_set>
using namespace std;
int main()
{
unordered_set<int> sampleSet;
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;
}
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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
#include <iostream>
#include <unordered_set>
using namespace std;
int main()
{
unordered_set<string> sampleSet;
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;
}
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Output:
First element in the container is: Welcome
First element in the bucket 0 is: GeeksforGeeks
Time complexity: O(1)
Auxiliary Space: O(1)
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