#include <stdio.h>

void f (double *p, int p_lgt){    /* Double the elements in p  - notice the pointer notation */
  int i;
  double *q;

  for(i = 0, q = p; i < p_lgt; i++, q++)
    *q *= 2; 
}


int main(void) {

  double a[10];
  int i;

  for(i = 0; i < 10; i++)  /* Initialization */
    a[i] = i;

  f(&a[0], 10);    /* Alternative parameter.
                      Maybe less confusing   */

  for(i = 0; i < 10; i++)
    printf("After calling f: a[%d] = %f\n", i, a[i]);

  return 0;
}