tolower() Function in C

C tolower() function is part of the C standard Library used to convert the uppercase alphabet to the lowercase alphabet. It does not affect characters other than uppercase characters.

Example

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

int main() {

    // Converting 'A' to 'a'
    printf("%c", tolower('A'));
    return 0;
}

a

Syntax of tolower()

The tolower() function is defined in the <ctype.h> header file.

int tolower(int c);

where,

• c: character that is to be converted. It is generally passed as integer that can represent an unsigned char.

Return Value

• If the passed character is an uppercase character then it returns the ASCII value corresponding to the lowercase of the character passed as the argument.
• If the passed character is already a lowercase or not a letter then it returns the same character.

Examples of tolower() Functions

The below examples demonstrate how to use the tolower() function in C:

Example 1

The below C program demonstrates the tolower() function.

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

int main()
{
    char ch = 'M';
    printf("Original Character: %c\n", ch);
    // convert ch to lowercase
    ch=tolower(ch);
    printf("After using tolower: %c", ch);

    return 0;
}

Original Character: M
After using tolower: m

Example 2

The below code converts all uppercase letters in the string to their lowercase equivalents, while leaving other characters unchanged.

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

int main()
{

    char s[] = "Code_in_C_@0123";
    printf("Original String: %s\n", s);
    // This will just convert
    // uppercase letters in string
    // to lowercase. Other characters
    // will remain unaffected.
    for (int i = 0; i < strlen(s); i++) {
        s[i] = tolower(s[i]);
    }

    printf("Converted to lowercase: %s", s);

    return 0;
}