ISBN Calculator

isbn_calculator.rb

class ISBNCalculator
  def initialize(barcode)
    @barcode = barcode.to_s
  end

  def ISBN
    "#{@barcode}#{check_digit}"
  end

  private 

  def check_digit
    check_sum = 0
    @barcode.each_char.with_index(1) do |char, index|
      digit = char.to_i
      check_sum += index.odd? ? digit : digit*3
    end

    (10 - check_sum % 10) % 10
  end
end

print "Please enter the barcode: "
barcode = gets.chomp

calculator = ISBNCalculator.new(barcode)
puts "The ISBN for this barcode is #{calculator.ISBN}"

Run this in the command line the next way:
$ ruby isbn_calculator.rb