public function f()
    {
        if(is_null($this->arr_f)){
            $arr = $this->frequency();
            array_walk(
                $arr, 
                function(&$v, $k, $n){
                    $v = $v / $n;
                }, 
                count($this)
            );

            $this->arr_f = $arr;
        }

        return $this->arr_f;
    }