gistfile1.txt

class InputError < StandardError
end
 
class RPNCalculator
  DIGITS = /\A-?\d+\Z/
  ACCEPTABLE_INPUT = /\d+|[+*-\/]/
  def evaluate(list)
    list_array = list.split
    stack = []
    list_array.each do |element|
      raise InputError if ACCEPTABLE_INPUT !~ element
      if element =~ DIGITS
        stack.push(element)
      elsif stack.length > 1
        last_number = stack.pop
        stack[-1] = (eval(stack[-1] + " #{element} " + last_number)).to_s
      end
    end
    stack[0].to_i
  end
end

calc = RPNCalculator.new
puts "The answer is:"
puts calc.evaluate(ARGV.first)