how to: conditonal config

Hi,

I want to be able to change the cache dir depending whether or not a suer is logged in.

I'm trying to do this from within the head.php view and have tried a bunch of methods including:

 $CI =& get_instance(); 
 if ($cust > 0) { 
 $config['cache_path'] = '/location/to/my/install/application/cache2';
 }


What is the correct syntax please?

Thanks a lot.