Program to find the probability that n people all have different birthdays

birthday.rb

# Program to find the probability that n people all have different birthdays

class ::Fixnum
  def factorial
    result = self
    self.times do |n|
      result *= (self - n) if not n == 0
    end
    result
  end
end

print 'How many people? '
n = gets.to_i
result = (365.factorial / (365 - n).factorial).to_f / (365 ** n).to_f
puts "Probability their birthdays are unique: #{result}"