How to change the structure of an array in Codeigniter?

Question

CodeIgniter

I change the structure of the array in the index file something like this

for ($i = 0, $c = count($arr); $i < $c; $i++)
     {
       $new_arr[$arr[$i]['pid']][] = $arr[$i];
     }

Everything is working. But how to do it correctly in relation to the MVC concept?