n_choose_k.c

/*
 * Copyright (c) 2015 Kostas Lekkas
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

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

int C(int n, int k) {
  if (k > n)
    return -1;

  if (n == k || k == 0)
    return 1;

  return C(n - 1, k - 1) + C(n - 1, k);
}

int main(int argc, char *argv[]) {
  if (argc != 3) {
    printf("Usage: %s [n] [k]\n", argv[0]);
    return 1;
  }

  int n = atoi(argv[1]);
  int k = atoi(argv[2]);
  int x = C(n,k);
  printf("C(%d, %d) = %d\n", n, k, x);

  return 0;
}