title: Check if NSUserDefaults key exists
body: <p>In my application, I am saving a key using this code:</p>
  
  <pre><code>func saveKey(){
          var xmineSpillere = mineSpillere
          var defaults = NSUserDefaults.standardUserDefaults()
          defaults.setObject(xmineSpillere, forKey: "YourKey")
      }
  </code></pre>
  
  <p>But how can I check if the key exists? I want the code something like this:</p>
  
  <pre><code>if key("YourKey") exists {
      println("key exists")
  }
  else {
      println("does not exist")
  }
  </code></pre>
  
  <p>How can I do something like this in Swift?</p>