9.c

/* Implementation of the "9" algorithm
 * The algorithm is simple: take any number > 9, and add its digits together, then subtract the sum from the number.
 * Like the Collatz Conjecture, this will always take you to a certain number (in this case: 9).
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int
get_sum(int number)
{
    int total = 0;

    while(number){
        total += number % 10;
        number /= 10;
    }

    return total;
}

void
transform_loop(char *initial)
{
    for(int number = atoi(initial); number >= 9; number -= get_sum(number))
        printf("%i  (sum = %i)\n", number, get_sum(number));

    printf("0\n----\n");
}

void
parse_args(char **argv)
{
    while(*argv)
        transform_loop(*(argv++));
}

void
usage()
{
    fputs("Usage: 9 n1 [n2 n3 ...]\n", stderr);
    exit(1);
}

int
main(int argc, char **argv)
{
    if(!argv[1])
        usage();

    parse_args(argv + 1);

    return 0;
}