polybius.c

#include <stdio.h>

const char *square[] = {
    "ABCDE",
    "FGHJK",
    "LMNOP",
    "QRSTU",
    "VWXYZ"
};

/* Assumes arg is NULL or string of the form [1-5][1-5]*/
int
parse(char *arg)
{
    if(arg == NULL)
        return 0;

    return square
        [ arg[0] - '1' ]
        [ arg[1] - '1' ];
}

int
main(int argc, char **argv)
{
    char ch;

    argv++;

    while(ch = parse(*argv++))
        putchar(ch);

    putchar('\n');

    return 0;
}