PLaying with conditional probabilities

gistfile1.rb

# You've been told that the Larsson family has four children
# and that three of them are girls. What's that chance that
# the fourth one is also a girl?
# This is a brute force way to settle that.
sibling_count = 4

1.upto(10) do |n|
  families = []
  # Create a bunch of randomized families
  1.upto(sibling_count * 5000) do 
    family = []
    1.upto(sibling_count) do
      family << (rand(2) == 0 ? :boy : :girl)
    end
    families << family
  end 

  # Remove all families with more than 1 boy
  families = families.select {|family| family.count {|c| c == :boy} <= 1 } 

  # Count the number of families with only girls
  pure_girl_families = families.count {|family| family.all? {|ch| ch == :girl} }

  puts "Study #{n}"
  puts "The #{sibling_count}th child is a girl in #{pure_girl_families} families."
  puts "Total number of families #{families.length}." 
  puts "Chance #{pure_girl_families.to_f / families.length}"
  puts
end