Check whether an ascii character is a letter or not.

isletter1.c

/*
 * Thanks to my friend Chrysostomos for the solution.
 */
#include <stdio.h>

int bin_search(int key, int left, int right)
{
    while (left <= right) {
        int middle = (left + right) / 2;

        if (middle < key) left = middle + 1;
        else if (middle > key) right = middle - 1;
        else return middle;
    }
    return -1;
}

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

    int ret = bin_search(argv[1][0], 'A', 'Z')
            + bin_search(argv[1][0], 'a', 'z');

    if (ret < 0) puts("Not a letter.");
    else puts("It is a letter indeed.");
    
    return 0;
}