Exclude categories from the default widget "Categories"

To exclude categories from Wordpress default widget "Categories", add this code in the file functions.php of the using theme:

<?php
function exclude_widget_categories($args){
$exclude = "3,6,18"; // The IDs of the excluding categories
$args["exclude"] = $exclude;
return $args;
}
add_filter("widget_categories_args","exclude_widget_categories");
?>

The method to exclude the categories from the widget is better, as it is in the file of the theme and it will work after the updating Wordpress versions.