monty.rb

# http://en.wikipedia.org/wiki/Monty_Hall_problem

def contestant_wins?
  doors = [:car, :goat, :goat].shuffle!
  indexes = 0..doors.size
  first_player_choice = rand(doors.size)
  host_choice = (indexes.select {|i| i != first_player_choice && doors[i] == :goat}).sample
  second_player_choice = indexes.find {|i| i != first_player_choice && i != host_choice}
  doors[second_player_choice] == :car
end

experiments = 1_000_000
wins = 0
experiments.times {wins +=1 if contestant_wins?}

puts "Relative frequency of winning when switching doors: #{wins.to_f/experiments.to_f}"