A quick prime number finder

primes.rb

# Prime number finder
# Usage: 20.primes, 100.primes

module GetAllPrimeNumbers
  def primes
    # Get all numbers from 0 to the current number as an array
    nums = (0 .. self).to_a

    # We know 0 and 1 can't be prime
    nums[0] = nums[1] = nil

    # Loop through each number
    nums.each do |n|
      # Ignore the nils
      next if n.nil?

      # Remove any multiples of 2, and bingo
      (n * n).step(self, n) { |m| nums[m] = nil }
    end

    # Finally, get rid of the nils and return
    nums.compact
  end
end

# An example of a great feature of Ruby - Monkey patching. Allows you
# to add/overwrite methods dynamically on any classes; including built-in
# data types (so strings, numbers, whatever).
class Fixnum
  include GetAllPrimeNumbers
end