std::string::crbegin() and std::string::crend() in C++ with Examples

std::string::crbegin()

The std::string::crbegin() is a string class built-in function that returns a constant reverse iterator referring to the last element in the string. Using this iterator starts the string traversal from the end of the string. Header File:

#include <string>

Template Class:

template <class C>
auto crbegin( const C& c ) 
        -> decltype(std::rbegin(c));

Syntax:

string_name.crbegin()

Parameters: This function doesn’t require any parameters. Return Value: This function std::string::crbegin() returns a constant reverse iterator referring to the last element in the string. Below is the program to illustrate string::crbegin(): Program 1:

CPP

// C++ program to illustrate
// std::string:crbegin()

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

// Driver Code
int main()
{

    // Given string
    string str("GeeksForGeeks");

    // Traverse the given string using
    // reverse iterator crbegin()
    for (auto it = str.crbegin();
        it != str.crend(); it++) {

        // Print the elements
        cout << *it;
    }
    return 0;
}

Output:

skeeGroFskeeG

Time Complexity: O(N) // Where N is the length of the string.
Auxiliary Space: O(1)

std::string::crend()

The std::string::crend() is a string class built-in function that returns a constant reverse iterator pointing to the theoretical element before the first element in the string. This iterator is used to reach the starting of the string while traversing the string in reverse order. Template Class:

template <class C>
auto crend( const C& c ) 
      -> decltype(std::rend(c));

Syntax:

string_name.crend()

Parameters: This function doesn’t require any parameters. Return Value: This function std::string::crend() returns a constant reverse iterator pointing to the element before the first element in the string. Below is the program to illustrate string::crend(): Program 2:

CPP

// C++ program to illustrate
// std::string:crend()

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

// Driver Code
int main()
{
    // Given string
    string str("GeeksForGeeks");

    // Find string length
    int N = str.length();

    // Given character
    char ch = 'k';

    // To check whether the char is
    // present or not
    bool a = true;

    // Traverse the given string using
    // reverse iterator crbegin() and
    // check if ch is present or not
    for (auto it = str.crbegin();
        it != str.crend(); it++) {

        if (*it == ch) {
            cout << "The last index is "
                << N - (it - str.crbegin() + 1)
                << endl;
            a = false;
            break;
        }
    }

    if (a) {
        cout << "Character is not present";
    }

    return 0;
}

Output:

The last index is 11

Time Complexity: O(N)  // Where N is the length of the string.
Auxiliary Space: O(1)

Let us see the differences in a tabular form -:

	std::string::crbegin()	std::string::crend()
1.	It is used to return a const_reverse_iterator pointing to the last character of the string	It is used to return a const_reverse_iterator pointing to the theoretical character preceding the first character of the string
2.	Its syntax is -: const_reverse_iterator crbegin()	Its syntax is -: const_reverse_iterator crend();
3.	It does not take any parameters.	It does not take any parameters.
4.	Its complexity is constant.	Its complexity is constant.
5.	Its iterator validity does not changes.	Its iterator validity does not changes.