How do I validate a form field in Codeigniter when using Get parameters?

For CodeIgniter 3, you can pass the GET array into the set_data function. For example:
$this->form_validation->set_data($this->input->get());

Codeigniter has changed since some of these posts. I think gX's answer is correct.

The instructions in the user manual, specifically section Validating an Array (other than $POST), worked great for me (as of today) and it's very simple.

Before your $this->form_validation->set_rules line, you specify the array to be validated:

$data = array(
    'username' => 'johndoe',
    'password' => 'mypassword',
    'passconf' => 'mypassword');

$this->form_validation->set_data($data);

Just add:

$_POST['states'] = $this->input->get('states');

for validate states field in form validation just before

$this->form_validation->set_rules('states', 'states', 'required|trim');

This post on the Codeigniter Forum suggests that form validation does not work with Get Parameters and that is just the way Codeigniter is.