title: iPhone - getting unique values from NSArray object
body: <p>I have an <code>NSArray</code> formed with objects of a custom class. The class has 3 (city, state, zip) string properties. I would like to get all unique state values from the <code>array</code>.</p>
  
  <p>I did read through the <code>NSPredicate</code> class but couldn't make much of how to use it in this case. The only examples I could find were for string operations.</p>
  
  <p>Can someone please help me out?</p>