# Reverse Polish Notation calculator

class RPNCalculator

  def evaluate(string)

  # create single array to push values and results to
  result = []

    # now sort out the string and extract integers and operands
    string.split.each do |x|

      if x == /\d/
          result << x.to_f
      elsif x == /\W/
          if x == "+"
            a = result.pop(2).inject(0) { |sum, e| sum + e }
            result.push(a)
          elsif x == "-"
            a = result.pop(2).inject(0) { |sum, e| e - sum }
            result.push(a)
          else
            a = result.pop(2).inject(0) { |prod, e| prod * e }
            result.push(a)
          end
      else
          result = 0
      end

      return result

    end

  end

end

calc = RPNCalculator.new

p calc.evaluate("1 2 + 1 3 -")