Convert vowels from upper to lower or lower to upper using C program

In C programming, we can convert vowels in a string from uppercase to lowercase or lowercase to uppercase using character manipulation functions. This involves checking each character to identify vowels and then applying the appropriate case conversion.

Syntax

#include <ctype.h>
int toupper(int c);    // Converts to uppercase
int tolower(int c);    // Converts to lowercase

Method 1: Convert Lowercase Vowels to Uppercase

This approach converts all lowercase vowels (a, e, i, o, u) to their uppercase equivalents while keeping other characters unchanged −

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

int main() {
    char string[100];
    int i;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    for(i = 0; string[i] != '\0'; i++) {
        if(string[i] == 'a' || string[i] == 'e' || string[i] == 'i' || 
           string[i] == 'o' || string[i] == 'u') {
            string[i] = toupper(string[i]);
        }
    }
    
    printf("Result: %s", string);
    return 0;
}

Enter the string: hello world
Result: hEllO wOrld

Method 2: Convert Uppercase Vowels to Lowercase

This approach converts all uppercase vowels (A, E, I, O, U) to their lowercase equivalents −

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

int main() {
    char string[100];
    int i;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    for(i = 0; string[i] != '\0'; i++) {
        if(string[i] == 'A' || string[i] == 'E' || string[i] == 'I' || 
           string[i] == 'O' || string[i] == 'U') {
            string[i] = tolower(string[i]);
        }
    }
    
    printf("Result: %s", string);
    return 0;
}

Enter the string: HELLO WORLD
Result: HeLLo WoRLD

Method 3: Toggle Case of All Vowels

This approach toggles the case of vowels − uppercase vowels become lowercase and lowercase vowels become uppercase −

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

int main() {
    char string[100];
    int i;
    
    printf("Enter the string: ");
    fgets(string, sizeof(string), stdin);
    
    for(i = 0; string[i] != '\0'; i++) {
        if(string[i] == 'a' || string[i] == 'e' || string[i] == 'i' || 
           string[i] == 'o' || string[i] == 'u') {
            string[i] = toupper(string[i]);
        }
        else if(string[i] == 'A' || string[i] == 'E' || string[i] == 'I' || 
                string[i] == 'O' || string[i] == 'U') {
            string[i] = tolower(string[i]);
        }
    }
    
    printf("Result: %s", string);
    return 0;
}

Enter the string: Hello World
Result: hEllO wOrld

Key Points

• Use fgets() instead of gets() for safe string input
• Include <ctype.h> header for toupper() and tolower() functions
• Check both uppercase and lowercase vowels for complete conversion
• The string is modified in-place, changing the original content

Conclusion

Vowel case conversion in C can be achieved by iterating through the string and checking each character against vowel patterns. The toupper() and tolower() functions provide safe and standard methods for case conversion.