Monty Hall problem simulator

monty-hall.c

/*
 * Monty Hall problem.
 * http://en.wikipedia.org/wiki/Monty_Hall_problem
 */

#include <stdio.h>

#ifdef DEBUG
#define verbose printf
#else
#define verbose
#endif

typedef unsigned long ulong;

int main(int argc, char **argv)
{
	int games = 1000000;
	ulong wins_sticker = 0, wins_switcher = 0;

	/* Parse args. */
	if (argc == 2) {
		games = atoi (argv[1]);
	}

	srand (time (0));
	
	int i;
	for (i = 0; i < games; i++) {
		int car, contestant, monty;
		verbose("\n--> GAME NO. %d\n", i);

		/* Place the car behind one door at random. */
		car = rand() % 3;

		/* Contestant picks a door. */
		contestant = rand() % 3;

		/* If contestant has chosen the prize, Monty picks another door at random.
		 * Otherwise, Monty picks the other goat. */
		if (contestant == car) {
			monty = (contestant + (rand() % 2) + 1) % 3;
		} else {
			monty = (((contestant + 1) % 3) == car) ? ((contestant + 2) % 3) : ((contestant + 1) % 3);
		}
		verbose("Car's behind door %d, Contestant opens %d, Monty opens %d\n", car, contestant, monty);

		if (contestant == car) {
			wins_sticker++;
			verbose("Sticker wins.\n");
		} else {
			wins_switcher++;
			verbose("Switcher wins.\n");
		}
	}

	printf("%d games played:\n", games);
	printf("Sticker has won %d times (%0.02f\%)\n", wins_sticker, ((double) wins_sticker / games * 100));
	printf("Switcher has won %d times (%0.02f\%)\n", wins_switcher, ((double) wins_switcher / games * 100));
	
	return 0;
}