if (argc != 4)
  usage ();

{
  long algorithm = stoi (argv[1]) - 1;
  if (algorithm < 0 || algorithm > (long) n_search_func)
    usage ();
  f = &search_func_tab[algorithm];
}

n = stoi (argv[2]);
n_iter = stoi (argv[3]);
if (n < 1 || n_iter < 1)
  usage ();