A demonstration of the Monty Hall Problem (http://en.wikipedia.org/wiki/Monty_Hall_problem).

monty.rb

#!/usr/bin/env ruby
# encoding: utf-8
#
# A silly script to demonstrate the Monty Hall Problem.
# http://en.wikipedia.org/wiki/Monty_Hall_problem
#
# Usage
#   ruby [-v] monty.rb <n>
# where n is the number of times each of the 3 players plays the game.
# 
# From [Wikipedia](http://en.wikipedia.org/wiki/Monty_Hall_problem):
# > Suppose you're on a game show, and you're given the choice of three doors: 
# > Behind one door is a car; behind the others, goats. 
# > You pick a door, say No. 1, and the host, who knows what's behind the doors,
# > opens another door, say No. 3, which has a goat.
# > He then says to you, "Do you want to pick door No. 2?"
# > Is it to your advantage to switch your choice?
#
# Oddly enough, the answer is that you have a 66% chance of winning if you switch to
# the other closed door, 33% if you stay put, and roughly 50% if you randomly
# change (as expected).
#
# The script demonstrates that with Alice (who always switches), Bob (who never switches),
# and Cecil, who does what he likes.
class Game
  class Door
    attr_accessor :open, :prize, :label

    def initialize(game, prize)
      @game = game
      @prize = prize
      @chosen = @open = false
    end
  end

  class Player
    @@players = []

    attr_accessor :name, :plays, :wins
    def initialize(name, switch = nil)
      @name = name
      @plays = 0.0
      @wins = 0.0
      @switch = switch
      @@players << self unless @@players.include?(self)
    end

    def policy
      case @switch
        when nil    then "sometimes switches"
        when true   then "ALWAYS switches"
        when false  then "NEVER switches"
      end
    end

    def to_s
      w = @@players.map { |p| p.name.length }.max
      x = @@players.map { |p| p.policy.length }.max + 2
      y = ($n || 1000000).to_s.length
      success = ((wins / plays) * 100).round(2)
      "%#{w}s %-#{x}s - %#{y}d/%-#{y}d %3.2f%%  " % [name, policy, wins, plays, success]
    end

    def switch?
      if @switch.nil?
        [true, false].shuffle.first
      else
        @switch
      end
    end

    def play
      Game.play(self)
    end
  end

  def initialize
    @doors = [
      Door.new(self, 'Goat'),
      Door.new(self, 'Goat'),
      Door.new(self, 'Car')
    ].shuffle
    @doors[0].label = 'A'
    @doors[1].label = 'B'
    @doors[2].label = 'C'
  end

  def puts(*args)
    STDOUT.puts(*args) if $VERBOSE
  end

  def play(player)
    puts
    puts "Three doors, two goats, one car..."
    @choice = @doors.shuffle.first
    puts "#{player.name} chooses door #{@choice.label}"
    
    door = @doors.reject { |d| d == @choice || d.prize == 'Car' }.shuffle.first
    puts "Monty opens door #{door.label} to reveal a #{door.prize}"
    door.open = true

    if player.switch?
      @choice = @doors.reject { |d| d == @choice }.reject { |d| d.open }.shuffle.first
      puts "#{player.name} changes mind, and chooses door #{@choice.label}"
    else
      puts "#{player.name} stays on door #{@choice.label}"
    end

    puts "Monty opens door #{@choice.label}, to reveal a #{@choice.prize}"

    if @choice.prize == 'Car'
      puts "Congratulations!!"
      player.wins += 1
    else
      puts "Sorry..."
    end
    player.plays += 1
  end

  def self.play(player)
    new.play(player)
  end
end

$n = Integer(ARGV[0] || 100)


alice = Game::Player.new(ARGV[1] || "Alice", true)
bob   = Game::Player.new(ARGV[2] || "Bob",   false)
cecil = Game::Player.new(ARGV[3] || "Cecil", nil)

$n.times { alice.play; print "\r#{alice}" }
puts

$n.times { bob.play; print "\r#{bob}" }
puts

$n.times { cecil.play; print "\r#{cecil}" }
puts