title: Making a basic tally counter in iOS
body: <p>I am having difficulty in making simple tally counter application. I have put the codes but when i hit "run simulation", it always gives error message. can anyone help me find my mistake? thank you. </p>
  
  <p>my codes are like below:</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  
      var counter: Int!
      @IBOutlet weak var Display: UILabel!
  
      func updateDisplay() {
          Display.text = String(counter)
      }
      override func viewDidLoad() {
          super.viewDidLoad()
          counter = 0
          updateDisplay()
          // Do any additional setup after loading the view, typically from a nib.
      }
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
  
      @IBAction func PlusButton(sender: AnyObject) {
          counter = counter + 1
          updateDisplay()
      }
  
      @IBAction func MinusButton(sender: AnyObject) {
          counter = counter - 1
          updateDisplay()
      }
  
      @IBAction func ClearButton(sender: AnyObject) {
          counter = 0
          updateDisplay()
      }
  }
  </code></pre>
  
  <p>here is the appearance in the Xcode
  <a href="https://www.facebook.com/photo.php?fbid=10204747519271053&amp;set=pcb.10204747520111074&amp;type=1&amp;theater" rel="nofollow">https://www.facebook.com/photo.php?fbid=10204747519271053&amp;set=pcb.10204747520111074&amp;type=1&amp;theater</a></p>
  
  <p>here is the error message when i hit run simulation
  <a href="https://www.facebook.com/photo.php?fbid=10204747519351055&amp;set=pcb.10204747520111074&amp;type=1&amp;theater" rel="nofollow">https://www.facebook.com/photo.php?fbid=10204747519351055&amp;set=pcb.10204747520111074&amp;type=1&amp;theater</a></p>