Modifying Array Data

I have to constantly add data to my NSMutableArray to index 0.

  MyVariables.MutableChatUser.insertObject(MyVariables.username, atIndex: 0)

Goal: I'm copying NSArray to NSMutableArray. If the array contains indexes 0,1,2,3. I then write a new object to index 0. I still have index 0,1,2,3 but I now lost the previous value on index 3 because everything got shifted over. Why isn't index 4 being created?

via Chebli Mohamed