RPN_calculator.rb

class RPNCalculator
  # I'm starting with a simple example of ('1 2 +') to work on this problem 

  # Here is my gameplan: I want to iterate through the array until I hit an operator symbol
  # If its not an operator symbol I want to remove the first item and add it to the back of the array
  # Once I get to an operator symbol I will perform that function on the last two numbers of the array
  # My problem is that it goes through the array once and pushes the 1 to the back, but then when it hits the "2",
  # it triggers the if x == "+" statement
  # Can someone explain to me why it does not push the 2 to the back of the array the same way it does with the 1?
  def evaluate(string)
    start_array = string.split(/ /)
        start_array.each do |x|
          puts start_array.inspect # This statement prints out ["1", "2", "+"] and then ["2", "+", 1]
          if x == "+"
            num1 = start_array.pop
            num2 = start_array.pop
            puts num2 # This returns the '+' which is why it returns the can't convert Fixnum into string error
            start_array << num2 + num1
            start_array.shift
          else
           x = start_array.shift
           start_array << x.to_i
          end
        end
  end
end

my_calc = RPNCalculator.new
my_calc.evaluate('1 2 +')