title: Cannot change tuple in an array
body: <p>I'm trying to change a tuple in an array , however ,when I try <code>
  emo = (type:emo.type,strength:increaseStrength(emo.strength))</code>
  it gives me error </p>
  
  <p><code>"cannot assign to 'let' value 'emo'</code></p>
  
  <p>here is my code : </p>
  
  <pre><code>var emotions : [(type : String, strength: Int)] = [("happy",0),("scared",0),("tender",0),("excited",0),("sad",0)]
  
  func increaseStrength(i:Int)-&gt;Int {
      switch i {
      case 0: return 1
      case 1: return 2
      case 2: return 3
      case 3: return 0
      default :return 0
      }
  }
  
  @IBAction func HappyBA(sender: AnyObject) {
      for emo in emotions  {
          if (emo.type == "happy" ){
              emo = (type:emo.type,strength:increaseStrength(emo.strength))
  
          }
      }
      println(emotions)
  }
  </code></pre>
  
  <p>If there are better way to do the assignment please tell me I am so appreciated ! Thanks.. </p>