How to Convert a C++ String to Uppercase?

Converting a string to uppercase means changing each character of the string that is in lowercase to an uppercase character. In the article, we will discuss how to convert a string to uppercase in C++.

Examples

Input: str = "Geeksforgeeks"
Output: GEEKSFORGEEKS
Explanation: Every character of string converted to uppercase.

Input: str = "Hello"
Output: HELLO
Explanation: Every character of string converted to uppercase.

Following are the different ways to convert the string to uppercase in C++:

Using std::transform() With std::toupper()

We can use std::transform() with toupper() function convert the string to uppercase. The std::transform() transforms the string by applying the given function to each character.

Syntax

std::transform(first, last, pos, toupper());

where first and last are the iterator to the beginning and the end of the string and pos the starting position of the destination.

Example

// C++ program to convert a string to uppercase
// using std::transform()
#include <bits/stdc++.h>
using namespace std;

int main() {
    string s = "Geeksforgeeks";

    // Using transform with toupper to convert
  	// string to uppercase
    transform(s.begin(), s.end(), s.begin(),
              ::toupper);

    cout << s;
    return 0;
}

GEEKSFORGEEKS

Time Complexity: O(n), where n is the number of characters in the string.
Auxiliary Space: O(1)

In this case, we have to use the scope resolution operator (::) to refer to the global toupper() function.

Using std::toupper() With Loop

In the above method, we can also use loop instead of std::stransform() function with toupper(). This loop will manually apply the toupper() function to each character of the string.

Example

// C++ program to convert a string to uppercase
// uisng toupper() with loop
#include <bits/stdc++.h>
using namespace std;

int main() {
    string s = "Geeksforgeeks";

    // Convert every character of string to
    // uppercase using toupper() method
    for (int i = 0; i < s.length(); i++)
        s[i] = toupper(s[i]);

    cout << s;
    return 0;
}

GEEKSFORGEEKS

Time Complexity: O(n), where n is the number of characters is string.
Auxiliary Space: O(1)

Manually Using ASCII Value Manipulation

In C++, lowercase and uppercase alphabets differ by a fixed value in the ASCII table. So, we can convert lowercase characters to uppercase by subtracting 32 from their ASCII value. The characters already in uppercase have to be handled separately either by using std::isupper() function or checking whether the ASCII value of character lies withing the range of uppercase characters.

Example

// C++ program to convert a string to uppercase
// using ASCII Value Manipulation
#include <bits/stdc++.h>
using namespace std;

int main() {
    string s = "Geeksforgeeks";

    // Convert the string to uppercase using
    // ASCII values
    for (int i = 0; i < s.length(); i++) {
        if (s[i] >= 'a' && s[i] <= 'z')
            s[i] = s[i] - 32;
    }

    cout << s;
    return 0;
}

GEEKSFORGEEKS

Time Complexity: O(n), where n is the number of characters in the string.
Auxiliary Space: O(1)