format_number.rb

# Write a method `format` for the class integer that converts the number to a
# string and adds commas at the appropriate places

#To test your solution run `rspec format_number_spec.rb` in your terminal

class Integer

  def format
    #code goes here...
  end

end

format_number_solution.rb

class Integer

  def format
    to_s.reverse.gsub(/(\d{3})(?=\d)/, '\\1,').reverse
  end

end

format_number_spec.rb

require './format_number'

describe Integer, "#format" do

  it "should return a string with a comma every 3 char starting from the right for 4 digit numbers" do
    1000.format.should eq("1,000")
  end

  it "should return a string with a comma every 3 char starting from the right for 4+ digit numbers " do
    1000000.format.should eq("1,000,000")
  end

  it "should return a string with no commas if the number is < 4 char" do
    100.format.should eq("100")
  end
end