/*----------------------------------------------------------------------
  File    : recsum.c
  Contents: recursive computation of the sum
            of the first n natural numbers
  Author  : Christian Borgelt
  History : 14.05.1998 file created
----------------------------------------------------------------------*/
#include <stdio.h>
#include <stdlib.h>

/*----------------------------------------------------------------------
  Functions
----------------------------------------------------------------------*/

int sum (int n)
{                               /* --- recursive summation function */
  if (n <= 1) return 1;         /* sum_{i=1}^1 = 1 */
  return n + sum(n-1);          /* sum_{i=1}^n = n +sum_{i=1}^{n-1} */
}  /* sum() */

/*--------------------------------------------------------------------*/

int main (int argc, char *argv[])
{                               /* --- main function */
  int n;                        /* upper bound of sum */

  if (argc != 2) {              /* if wrong number of arguments given */
    printf("usage: %s n\n", argv[0]);
    printf("compute the sum of the first n natural numbers\n");
    return 0;                   /* print a usage message */
  }                             /* and abort the function */
  n = atoi(argv[1]);            /* get upper bound of sum */
  if (n <= 0) {                 /* and check it */
    printf("%s: n must be > 0\n", argv[0]); return -1; }
  printf("sum_{i=1}^{%i} i = %i\n", n, sum(n)); 
  return 0;                     /* compute sum and return 'ok' */
}  /* main() */