title: can not convert value type "string?" to expected argument type "inout string"
body: <p>this line <code>self.displayResultLable.text +=  (title as! String)</code> error throwing</p>
  
  <blockquote>
    <p>can not convert value type "string?" to expected argument type "inout string"</p>
  </blockquote>
  
  <p>Here is my code :   </p>
  
  <pre><code>   if results.count &gt; 0 {
                      var displayResult : String?
                      for books in results as! [NSManagedObject] {
  
                          if let title = books.valueForKey("title") {
  
                              self.displayResultLable.text +=  (title as! String)
  
                          }
                      }
                  }
  </code></pre>
  
  <p>what is the <code>inout</code> string here ? what is the best practice ?</p>
  
  <p>Note this line  <code>self.displayResultLable.text =  (title as! String)</code> working fine:</p>