Write C program using isupper() function

In C, the isupper() function is used to check if a character is an uppercase letter (A-Z). It is defined in the ctype.h header file and returns a non-zero value if the character is uppercase, otherwise returns 0.

Syntax

int isupper(int ch);

Parameters: ch − The character to be checked (passed as int).

Return Value: Non-zero if the character is uppercase, 0 otherwise.

Example 1: Counting Uppercase Letters Using isupper()

This program counts the total number of uppercase letters in a string using the isupper() function −

#include <stdio.h>
#include <ctype.h>
#include <string.h>

int main() {
    char string[100];
    int counter = 0;
    int i;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    for (i = 0; string[i] != '\0'; i++) {
        if (isupper(string[i])) {
            counter++;
        }
    }
    
    printf("Capital letters in string: %d<br>", counter);
    return 0;
}

Enter the string: TutoRialsPoint CPrograMMing
Capital letters in string: 7

Example 2: Manual Uppercase Check Without isupper()

This program demonstrates how to count uppercase letters by manually checking ASCII values without using isupper() −

#include <stdio.h>

int main() {
    char string[100];
    int upper = 0;
    int i = 0;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    while (string[i] != '\0') {
        if (string[i] >= 'A' && string[i] <= 'Z') {
            upper++;
        }
        i++;
    }
    
    printf("Uppercase letters: %d<br>", upper);
    return 0;
}

Enter the string: TutOrial
Uppercase letters: 2

Example 3: Character-by-Character Analysis

This program shows individual uppercase characters found in the string −

#include <stdio.h>
#include <ctype.h>

int main() {
    char string[100];
    int count = 0;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    printf("Uppercase letters found: ");
    for (int i = 0; string[i] != '\0'; i++) {
        if (isupper(string[i])) {
            printf("%c ", string[i]);
            count++;
        }
    }
    
    printf("\nTotal uppercase letters: %d<br>", count);
    return 0;
}

Enter the string: Hello World
Uppercase letters found: H W 
Total uppercase letters: 2

Key Points

• The isupper() function only works with ASCII characters A-Z.
• Always include ctype.h header when using isupper().
• Use fgets() instead of gets() for safer string input.
• Manual checking uses ASCII values: uppercase letters range from 65 to 90.

Conclusion

The isupper() function provides a reliable way to identify uppercase characters in C. It simplifies character validation and makes code more readable compared to manual ASCII range checking.