title: How to insert an object at a given index in iphone-sdk
body: <p>I want to insert a object at a given index. and If Is there no object at the index, I want to know that. 
  Is NSDictionary good choice? 
  Is there another good solution?</p>
  
  <pre><code>// Initailization
  NSMutableDictionary *items = [NSMutaleDictionary dictionary];
  ...
  // insert an object.
  [items setObject:item forKey:[NSNumber numberWithInt:3]];
  ...
  // I want to know that there is an object at a given index.
  item = [items objectForKey:[NSNumber numberWithInt:4]];
  if (item)
      // exist
  else
      // non exist
  </code></pre>