Exclude child categories from category archive

function exclude_child_cats($query) {
	if ( is_category() ) :
		$current_cat = single_cat_title('',false);
		$current_cat_ID = get_cat_ID("$current_cat");
		$ex_cats = get_categories("child_of=$current_cat_ID");
 
		foreach ($ex_cats as $ex_cat) :
			$excluded[] = $ex_cat->cat_ID;
		endforeach;
 
		$ex = '-' . implode(' -', $excluded);
      	$query->set('cat', "$current_cat_ID . $ex");
 
		return $query;
	endif;
}
add_filter('pre_get_posts', 'exclude_child_cats');