simulation of the math problem: "In a country in which people only want boys, every family continues to have children until they have a boy. If they have a girl, they have another child. If they have a boy, they stop. What is the proportion of boys to gir

family-sim.rb

# simulation to calculate an answer to the question:
#
# "In a country in which people only want boys, every family continues to have children until they have a boy. If they
# have a girl, they have another child. If they have a boy, they stop. What is the proportion of boys to girls in the
# country?"
#
# !/usr/bin/env ruby
# usage: ./family-sim.rb [trials]
#
# sample output:
#
# $> ./family-sim.rb 1000000
# Family problem, 1000000 trials:
#
# 999471 girls, 1000000 boys
# 1.999471 children per family
# 0.999471 girls per boy
#
# Ran in 7.445696s

class Family
  def initialize
    @children = {:boy => 0, :girl => 0}
  end

  def boys
    @children[:boy]
  end

  def girls
    @children[:girl]
  end

  def have_child!
    @children[[:boy, :girl].sample] += 1
  end

  def has_boy?
    boys == 1
  end
end

# read from standard input or default to 1000 iterations
iterations = if ARGV.length > 0
               ARGV[0].to_i
             else
               1000
             end

puts "Family problem, #{iterations} trials:"

started = Time.new

results = {:boys => 0, :girls => 0}
iterations.times do
  family = Family.new
  family.have_child! until family.has_boy?
  results[:boys]  += family.boys
  results[:girls] += family.girls
end

family_size = (results[:boys] + results[:girls]).to_f / iterations
ratio       = results[:girls].to_f / results[:boys]

runtime = Time.new - started

puts
puts "#{results[:girls]} girls, #{results[:boys]} boys"
puts "#{family_size} children per family"
puts "#{ratio} girls per boy"
puts
puts "Ran in #{runtime}s"