title: What structure should I use to store these objects?
body: <p>I am trying to implement something similar to the flight control game.  There will be a set of objects representing planes that get spawned and removed 'randomly'.  Individual planes can then get touched and will respond.  The model should take a plane index as a parameter when something get touched.</p>
  
  <p>My storage requirements are:</p>
  
  <ol>
  <li>Need fast iteration over all elements</li>
  <li>Need fast insertion / deletion</li>
  <li><p>Need to look up and item quickly by index </p>
  
  <ul>
  <li>What should I use? <code>NSMutableArray</code>, <code>NSMutableSet</code>  ?</li>
  <li>Should I store each object in two places? (e.g. Set for fast iteration, Array for fast lookup)?</li>
  </ul></li>
  </ol>