test_first.rb

  it "evaluates a string" do
    expect(calculator.evaluate("1 2 3 * +")).to eq(
      ((2 * 3) + 1)
    )

    expect(calculator.evaluate("4 5 -")).to eq(
      (4 - 5)
    )

    expect(calculator.evaluate("2 3 /")).to eq(
      (2.0 / 3.0)
    )

    expect(calculator.evaluate("1 2 3 * + 4 5 - /")).to eq(
      (1.0 + (2 * 3)) / (4 - 5)
    )
  end
end


class RPNCalculator
  def initialize
    @logic = []
  end
  def push(to_push)
    @logic << to_push
  end
  def plus
    if @logic.length < 2
      raise "calculator is empty"
    else
      @logic.push(@logic.pop + @logic.pop)
    end
  end
  def minus
    if @logic.length < 2
      raise "calculator is empty"
    else
      fnum = @logic.pop
      @logic.push(@logic.pop - fnum)
    end
  end
  def value
      @logic.last
  end
  def divide
    if @logic.length < 2
      raise "calculator is empty"
    else
      fnum = @logic.pop
      @logic.push(@logic.pop.to_f / fnum.to_f)
    end
  end
  def times
    if @logic.length < 2
      raise "calculator is empty"
    else
      @logic.push(@logic.pop * @logic.pop)
    end
  end

  def tokens(string)
    val = string.split
    val.map! do |x|
      if x == "-"
        x = :-
      elsif x == "+"
        x = :+
      elsif x == "/"
        x = :/
      elsif x == "*"
        x = :*
      else
        x = x.to_i
      end
    end
    val
  end
#1 2 3 * +
  def evaluate(value)
    tokens = tokens(value)
    symbols = []
    numbers = []
    num = nil
    tokens.each do |x|
      if [:-, :+, :/, :*].include?(x)
        symbols << x
      else
        numbers << x
      end
    end
    while(numbers.length > 0)
      #If this is the first pass, we need to take 2 values instead of 1.
      num = numbers.pop
      if(numbers.length + 1 + symbols.length == tokens.length)
        push(numbers.pop)
        push(num)
      else
        push(num)
      end
      sign = symbols.shift
      if(sign == :+)
        plus
      elsif(sign == :-)
        minus
      elsif(sign == :/)
        divide
      else
        times
      end
    end
    self.value
  end

end