Alternative way to check whether an ascii character is a letter or not.

isletter2.c

#include <stdio.h>

#define TOLOWER(c) ((c) | 0x20)
#define ISLETTER(c) ((unsigned char)(TOLOWER(c) - 'a') < 26)

int main(int argc, char *argv[])
{
    if (argc != 2 || argv[1][1] != '\0') {
        puts("Usage: isletter <character>");
        return 1;
    }

    if (ISLETTER(argv[1][0])) puts("It is a letter indeed.");
    else puts("Not a letter.");
    
    return 0;
}