Codeigniter:Set validation rules inside the function

I am very new to codeigniter and i am facing a issue with following senario.

i have two forms and validate both by setting the rules inside a function validate_form()

Ex:function validate_form() {
    if (isset ($_POST['next'])) {
       //Set rules for first form. 
        if($this->form_validation->run()===false){ 
            //first form wrong 
        } 
        else { //first form is correct }             
    }
    else if (isset ($_POST['register'])){
        //Set rules for second form. 
        if($this->form_validation->run()===false){ 
            //second form wrong 
        } 
        else { //second form is correct }             
    }
    }


   The problem is i am calling the validate_form() as below 
    public function index()
    {
    $data = array();
    $this->validate_form();
    //other stuff here
}

So my form is only not rendering because both $_post['next'] and $_POST['register'] are not set. how to solve this??