public function next(\CArray $x_data_ptr = null, int $x_ind_ptr = null, int $nnz = null, float $y, float $sample_weight = null)
    {
        $current_index = $this->_get_next_index();
        return $this->_sample($x_data_ptr, $x_ind_ptr, $nnz, $y, $sample_weight, $current_index);
    }