(This is another part of the Fifty Problems series, a set of example applications of Monte Carlo methods. In each post, I present source code which answers a probabilistic question using simulated models of the underlying system.)

Problem 39: a set of glass rods, with one end blue-dotted and the other red-dotted, are dropped. Many break into 3 parts. What is the average length of the blue-dotted pieces of these broken-in-three rods?

#!/usr/bin/env ruby

TRIALS=100000

# Much like 42 and 43, this is pretty trivial 

cum_blue = 0.0

TRIALS.times {
  # Choose two breaking points
  b1 = rand()
  b2 = rand()

  # put them on a stick reasonably...
  if (b1 > b2)
    x = b1
    b1 = b2
    b2 = x
  end

  # and let's say the blue is always on the left
  # that is, the length of the blue segment is b1

  cum_blue += b1
}

puts "After #{TRIALS} trials, average length:"
puts cum_blue/TRIALS

I've been coding my way through Fifty Challenging Problems in Statistics with Solutions. This post is a part of the Fifty Challenging Problems series.

This was brought to you by Josh Myer. He has other fun things at his homepage.