http://api.jquery.com/addClass/

<script type="text/javascript">
$(document).ready(function(){
   $("ul li:last").addClass(function(index) {
      return "item-" + index;
   });
	
   $("ul li").each(function(){
      alert($(this).attr('class'));
   });
});
</script>

<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
</ul>

"Given an unordered list with five <li> elements, this example adds the class "item-4" to the last <li>."

not "item-4" -> "item-0"