class ViewController: UIViewController {

    @IBOutlet weak var display: UILabel!

    var userIsInTheMiddleOfTypingAnNumber: Bool = false;

    @IBAction func numberButtonClick(sender: UIButton) {

        let digit = sender.currentTitle!

        if userIsInTheMiddleOfTypingAnNumber {

            display.text = digit + display.text!

        } else {

            display.text = digit

            userIsInTheMiddleOfTypingAnNumber = true

        }

        println("digit = \(digit)")

    }

}