index-1.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

int a[] =
{
  0,  1,  2,  3,  4,  5,  6,  7,  8,  9,
  10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
  20, 21, 22, 23, 24, 25, 26, 27, 28, 29,
  30, 31, 32, 33, 34, 35, 36, 37, 38, 39
};

int
f (long n)
{
  return a[n - 100000];
}

main ()
{
  if (f (100030L) != 30)
    abort();
  exit (0);
}