A statistical simulation of the Monty Hall problem.

monty-hall.c

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

int rand_lim(int limit);
int try_game();

int main () 
{
    srand(time(NULL));

    int sum = 0;
    int i;
    for (i = 0; i<10000000; i++) {
        int result = try_game();
        sum += result;
    }

    printf("%f\n", sum / (float)i);

    return 0;
}

int try_game()
{
    int car = rand_lim(2);
    int chosen = rand_lim(2);
    int revealed = (rand_lim(1) + chosen + 1) % 3;
    revealed = (revealed == car) ? ((revealed + 1) % 3) : revealed;
    revealed = (revealed == chosen) ? ((revealed + 1) % 3) : revealed;
    int rechose = (revealed + 1) % 3;
    rechose = (rechose == chosen) ? ((chosen + 1) % 3) : rechose;

    if (chosen == revealed || revealed == car || chosen == rechose) {
        printf("%d %d %d %d\n", car, chosen, revealed, rechose);
        exit(1);
    }

    return car == rechose;
}

int rand_lim(int limit) 
{
    int divisor = RAND_MAX/(limit+1);
    int retval;

    do { 
        retval = rand() / divisor;
    } while (retval > limit);

    return retval;
}