title: NSPredicate to filter array of words to array of letters
body: <p>I am trying to make "weighted" list of letters as words are created. I have a massive list of words in an NSArray. For example I am trying to acquire a new NSArray filled with just the 3rd letters of all the words based off the first two letters entered.</p>
  
  <p>So far I have...</p>
  
  <pre><code>NSArray *filteredArray;
  if (currentWordSize == 0) {
      filteredArray = wordDictionary
  }
  else {
      NSPredicate *filter = [NSPredicate predicateWithFormat:@"SELF beginswith[cd] %@", filterString];
      filteredArray = [wordDictionary filteredArrayUsingPredicate:filter];
  }
  </code></pre>
  
  <p>And that works all good for putting whole words into the filtered array, but that isn't exactly what I need. Can anyone show me a way to just fill the <code>filteredArray</code> with just the 1st, 2nd, or 3rd letters of a random <code>NSString</code> from the <code>wordDictionary</code>?</p>
  
  <p>EDIT: clarified my question.</p>