void f(double *a,int *qaddress,int p)
{
  a[*qaddress] = a[p];
  *qaddress = p;
}

/* Above this line is the solution. */
/* Below this line is a sample program that checks the solution. */

int main(void)
{
  int p = 0;
  int q = 1;
  double a[] = { 3.14, 2.71 };

  printf("a[0] is %lf, a[1] is %lf, p is %d, q is %d.\n",a[0],a[1],p,q);
  f(a,&q,p);
  printf("a[0] is %lf, a[1] is %lf, p is %d, q is %d.\n",a[0],a[1],p,q);
  return 0;
}