Codeigniter - How to get values as array to use in next select with where_not_in

Codeigniter - How to get values as array to use in next select with
where_not_in

I have three tables; user, car and user_x_car. user_x_car holds users who
own car; user_id and car_id are stored. I want to get users who don't own
a car as follows:
$car_owner = $this->db->select()->from('user_x_car')->get()->result();
for ($i = 0; $i < count($car_owners); $i++)
$$car_owner_id[$i] = $car_owner[$i]->user_id;
$non_car_owner = $this->db->select()->from('user')->where_not_in('id',
$car_owner_id)->get()->result();
I get what I want, however, is there any way to bypass the for loop in the
middle which creates and array of id's selected in the first select. Is
there any way to get array of selected user_ids directly?