title: Working on a calculator in swift and I'm stuck
body: <p>I'm new to coding and just learning now. swift code and I'm running into a snag and can't seem to figure it out, so I've been flowing along with the Stanford YouTube channel on swift coding and they are doing a calculator. Currently we are trying to get double value. </p>
  
  <pre><code>@IBOutlet weak var display: UILabel!
  
  var userIsInTheMiddleOfTypingANumber = false
  
  @IBAction func appendDigit(sender: UIButton) {
      let digit = sender.currentTitle!
      if userIsInTheMiddleOfTypingANumber {
          display.text = display.text! + digit
      } else {
          display.text = digit
          userIsInTheMiddleOfTypingANumber = true
      }
  
  }
  
  var operandStack: Array&lt;Double&gt; = Array&lt;Double&gt;()
  
  @IBAction func enter() {
      userIsInTheMiddleOfTypingANumber = false
      operandStack.append(displayValue)
      print("operandStack = \(operandStack)")
  
  }
  
  var displayValue: Double {
      get {
          return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
          // This is my problem and comes back with exc_bad_instruction (code=exc_i387_invop  any help would be greatly appreciated on this! thank you for your time!
      }
      set {
          display.text = "\(newValue)"
          userIsInTheMiddleOfTypingANumber = false
  
      }
  
  }
  </code></pre>