monty-hall.rb

# Basic implementation of the monty hall problem based on
# http://mathforum.org/dr.math/faq/faq.monty.hall.html
#
# Harsh Deep
# 18.7.2016

# This is a acceptable door configuration, :goat means that there is a goat
# behind and :car means that there is a car behind.
doors = [:goat, :car, :goat]

# These two variables hold the count of which choice would count as a win
stay_wins, change_wins = 0, 0

# The number of trials is 10000 per the problem, but the higher number, the
# greater confidence in the results
number_of_trials = 10000

# Here is the loop that runs the evaluation based on the above number
number_of_trials.times do
  # Contestant picks a door
  door_picked = rand(3)

  # If the current door is a car it's stay wins, if not it's change wins
  if doors[door_picked] == :car
    stay_wins += 1
  else
    change_wins += 1
  end
end

# Calculate probability
# The function to_f converts the win counts to floats for percentage calculation
stay_probability = stay_wins.to_f/number_of_trials * 100
change_probability = change_wins.to_f/number_of_trials * 100

# This part is just for the output.
# Note: puts prints string on screen, a puts command without anything after
# prints an empty line
puts "For #{number_of_trials} trails, the results are:"
puts
puts "Stay wins: #{stay_wins}"
puts "Change wins: #{change_wins}"
puts
puts "Stay win probabilty: #{stay_probability}%"
puts "Change win probabilty: #{change_probability}%"