i have a view like this

<p>

   <div id=error_msg class=alert alert-error> //at this div have css .alert{display:none;}

      <a class=close click_close>X</a>

      <strong>oops!</strong> username atau password yang anda masukan salah

   </div>

</p>

and in my controller

public function login_form() {

    $this->form_validation->set_rules('username', 'Username', 'required|trim|xss_clean');

    $this->form_validation->set_rules('password', 'Password', 'required|md5|xss_clean');

    $this->form_validation->set_error_delimiters('<span class=error>', '</span>');



    if($this->form_validation->run()==FALSE) {

        $this->load->view('v_admin_login');

    } else {

        $username = $this->input->post('username');

        $password = $this->input->post('password');



        $cek = $this->m_admin_login->getAdmin($username, $password, 1, 1);



        if($cek <> 0) {

            $this->session->set_userdata('isLogin', TRUE);

            $this->session->set_userdata('username',$username);

            $this->session->set_userdata('level',$level);



            redirect('admin');

        } else {

            //I want to change class alert to .alert{display:block}

            //What should I do?

        }

    }

}

the point is i want to show div with class alert if input username or password was wrong..