C Convert String to Lowercase and Uppercase

strlwr() and strupr() are not standard C functions. They work only in some compilers. The portable way is to use tolower() and toupper() from ctype.h on each character.

Convert to uppercase

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

int main(void) {
    char s[] = "Learn C Programming";

    for (int i = 0; s[i] != '\0'; i++) {
        s[i] = toupper((unsigned char)s[i]);
    }
    printf("%s\n", s);
    return 0;
}

Output:

Convert to lowercase

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

int main(void) {
    char s[] = "Learn C Programming";

    for (int i = 0; s[i] != '\0'; i++) {
        s[i] = tolower((unsigned char)s[i]);
    }
    printf("%s\n", s);
    return 0;
}

Output:

Reusable functions

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

void toUpperCase(char *s) {
    for (; *s; s++) *s = toupper((unsigned char)*s);
}

void toLowerCase(char *s) {
    for (; *s; s++) *s = tolower((unsigned char)*s);
}

int main(void) {
    char a[] = "Hello World";

    toUpperCase(a);
    printf("%s\n", a);
    toLowerCase(a);
    printf("%s\n", a);
    return 0;
}

Output:

Other useful ctype functions

Function	Checks
isalpha(c)	Letter
isdigit(c)	Digit 0 to 9
isalnum(c)	Letter or digit
isspace(c)	Space, tab or new line
isupper(c) / islower(c)	Capital or small letter

Next: C Array of Strings.

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