Manipulating every Nth item in a <% control %> loop

Often you will want to manipulate every nth item inside a <% control %> loop. For example say you had a gallery and you wanted every 3rd image to start on a new line, you need to be able to test each item from the template to see if it is divisible by 3 and should therefore have the clear CSS class added to it.

Using iteratorPos % we can very easily achieve this. First we need to add the test function to the object we will be iterating through, so for example if you are iterating through a number of ImageItem DataObjects, we would add this function to our ImageItem class:
public function IsThird(){ return ($this->iteratorPos % 3) == 0; }

The % operator calculates the modulous or Remainder after dividing by the number that follows it (in this case 3). If there is a remainder then we know that the item is not divisible by 3 and so should not be cleared, but if the modulous is equal to 0 then we know it is divisible by 3 and should therefore clear a new line. 

We can now use <% if IsThird %> to clear the third image in our control loop like so:
<% control ImageItems %> <li <% if IsThird %>class="clear"<% end_if %> > Image </li> <% end_control %><br>

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