Check code for probability.

check_prob.c

//
//  main.c
//  CodeTest
//
//  Created by eien.support@gmail.com on 2013/02/01.
//  Copyright (c) 2013 Eien Factory. All rights reserved.
//

#include <stdio.h>
#include <mach/mach_time.h>
#include <stdlib.h>

int selectOne(int max) {
    long value;
    value = arc4random();
    return value % max;
}

_Bool doTest()
{
    const int max = 3;
    enum {
        kNone = 0,
        kWin = 1,
        kOpen = 2,
    };
    int bascket[max] = {kNone};
    int winIndex = selectOne(max);
    bascket[winIndex] = kWin;
    int chooseIndex = selectOne(max);
//    printf("win = %d : choose = %d\n",winIndex,chooseIndex);
    int openIndex = selectOne(max);
    while (openIndex==chooseIndex||openIndex==winIndex) {
        openIndex = selectOne(max);
    }
    bascket[openIndex] = kOpen;
    int rechooseIndex = selectOne(max);
    while (rechooseIndex==chooseIndex||rechooseIndex==openIndex) {
        rechooseIndex = selectOne(max);
    }
    return (winIndex==rechooseIndex);
}

int main(int argc, const char * argv[])
{
  int winFirst = 0;
  const int repeat = 10000;
        
  for (int i=0; i<repeat; i++) {
      if ( doTest() ) {
          winFirst++;
      }
  }
  printf("win if you changed : %d/%d = %f\n",winFirst,repeat,(double)winFirst/(double)repeat);
  return 0;
}