Logic

If the ASCII value of a character lies between 65 (A) to 90 (Z) then it is an uppercase character or if the character’s ASCII value lies between 97 (a) to 122 (z) then it is a lowercase character.

See also: Find ASCII value of a character

See the flow diagram:

Program

LowerUpperCase.CCopy
#include <stdio.h>

int main() {

    char ch;

    printf("Enter any character : ");
    scanf("%c", &ch);

    if(ch >= 'A' && ch <= 'Z') {

        printf("\n%c is UPPERCASE alphabet.", ch);

    } else if(ch >= 'a' && ch <= 'z') {

        printf("\n%c is LOWERCASE alphabet.", ch);

    } else {

        printf("\n%c is not an alphabet.", ch);

    }

    return 0;
}
Try It

Output

Enter any character : S
S is UPPERCASE alphabet.