Reverse Polish Calculator - Version 1 (an example of what not to do)

rp_calc1.rb

# version 1 of 2
# uses while loop and modifies array during looping (an example of what not to do)
# see version 2 here - https://gist.github.com/TGOlson/6218116

class RPNCalculator

  def evaluate(string)
    array = string.split
    i = 0
    while i < array.length
      if array[i] =~ /\D/ #if current item is not a digit
        case array[i]
          when "+" then replace = add(array[i-2].to_i, array[i-1].to_i)
          when "-" then replace = subtract(array[i-2].to_i, array[i-1].to_i)
          when "*" then replace = multiply(array[i-2].to_i, array[i-1].to_i)
          else puts "Invalid Operator."
        end
        array[i] = replace
        array.slice!(i-2,2)
        i -= 2 #rewind index after using slice! on array
      end
      i+=1
    end
    array
  end

  def add(num1, num2) num1 + num2 end
  def subtract(num1, num2) num1 - num2 end
  def multiply(num1, num2) num1 * num2 end
end