void mc_sampling(int max_trials, int number_walks, 
                 double move_probability, int *walk_cumulative, 
                 int *walk2_cumulative)
{
  long idum;
  idum=-1;  // initialise random number generator
  for (int trial=1; trial <= max_trials; trial++){
    int position = 0;
    for (int walks = 1; walks <= number_walks; walks++){   
      if (ran0(&idum) <= move_probability) {
	position += 1;
      } 
      else {
	position -= 1;
      }
      walk_cumulative[walks] += position;
      walk2_cumulative[walks] += position*position;
    }  // end of loop over walks
  } // end of loop over trials
}   // end mc_sampling function