title: 'advance' keyword in swift?
body: <pre><code>let word = "sample string"
  
  let firstLetter = Character(word.substringToIndex(advance(word.startIndex,1)).uppercaseString)
  </code></pre>
  
  <p>I got the above example from a tutorial. Can anyone know what they mean by "advance" and what is difference between "substringToIndex" and "substringWithRange".</p>