calculator.rb

class Calculator
  def evaluate statement
    symbols = statement.split(' ')
    puts calculate(symbols)
  end

  def calculate symbols
    if symbols.length == 1
      return symbols[0]
    else
      queue = []
      symbols.length.times do
        symbol = symbols.shift
        if symbol == '+'
          result = queue.pop.to_i + queue.pop.to_i
          queue << result
          break
        elsif symbol == '-'
          result = queue.pop.to_i - queue.pop.to_i
          queue << result
          break
        elsif symbol == '*'
          result = queue.pop.to_i * queue.pop.to_i
          queue << result
          break
        elsif symbol == '/'
          result = queue.pop.to_i / queue.pop.to_i
          queue << result
          break
        else
          queue << symbol
        end
      end

      queue.length.times do
        symbols.unshift(queue.pop)
      end

      calculate(symbols)
    end
  end
end

c = Calculator.new
c.evaluate("70 10 4 + 5 * -")