Dice rolling. Compute the probability of rolling n 2's with i dice.

dice_rolling.rb

#!/usr/bin/env ruby

class Die
  def initialize sides=6
    @sides = sides
  end

  def roll
    Random.rand(1..@sides)
  end
end

if ARGV.length != 3
  puts "Usage: dice_rolling.rb target dice_count iterations"
  exit(1)
end

target     = ARGV[0].to_i
dice_count = ARGV[1].to_i
iterations = ARGV[2].to_i

histogram = Hash.new { |h, k| h[k] = 0 }

dice = Array.new dice_count, Die.new

iterations.times do |j|
  count = dice.map{|d| d.roll}
              .select{|r| r == target}
              .length
  histogram[count] += 1
end

(dice_count+1).times {|i| print "(#{i}: #{histogram[i]}) "}
print "\n"