My solution to the problem described in http://sebastien.saunier.me/blog/2014/01/09/quickly-setup-a-ruby-tdd-throw-away-code.html

integer_formatter.rb

class IntegerFormatter
  def with_commas(integer)
    quotient = integer / 1000
    remainder = integer % 1000

    if quotient == 0
      remainder.to_s
    else
      prepend = with_commas(quotient)
      "#{prepend},#{"%03d" % remainder}"
    end
  end
end

integer_formatter_spec.rb

require "./integer_formatter"

describe IntegerFormatter do
  subject { IntegerFormatter.new }

  describe "#with_commas" do
    it "should handle number until 999 without any commas" do
      test_with_commas(1, "1")
      test_with_commas(10, "10")
      test_with_commas(100, "100")
    end

    it "should handle number between 1000 and 999999" do
      test_with_commas(1000, "1,000")
      test_with_commas(10000, "10,000")
      test_with_commas(100000, "100,000")
    end

    it "should handle number above 10000000" do
      test_with_commas(1000000, "1,000,000")
      test_with_commas(35235235, "35,235,235")
    end
  end

  def test_with_commas(integer, expected_string)
    subject.with_commas(integer).should eq expected_string
  end
end

👍

Being picky but any reason in the spec file for line subject { IntegerFormatter.new }? It would work without this line.