int main(void)
{
  int x[] = { 3, 1, 4, 1, 5, 9, 2, 6 };
  int *p = x;
  int *q = x + 7;
  int i;

  while (p != q)
    if (*p < 4)
      ++p;
    else if (*q >= 4)
      --q;
    else {
      i = *p;
      *p = *q;
      *q = i;
    }

  for (i = 0;i < 8;++i)
    printf("%d\n",x[i]);
}