如何用Wordpress the_category函数只显示主类？

I am using Wordpress and it's the_category() function to display categories on my site. This works but I'd like to limit the output. I only want to show the main categories.

Do I have to modify this the_category(); or is there another way to achieve this?

UPDATE:

This is my second attempt of understanding your problem:)

I'm now assuming that what you want is for Categories Widget to show only parent (main) categories.

If so you could use this the code:

function include_widget_categories($args)
{

   $cats_id="";
   $parent_args = array(
    'orderby' => 'name',
    'parent' => 0
    );
    $categories = get_categories( $parent_args );

    foreach ( $categories as $category ) 
      $cats_id.=$category->cat_ID.",";

    $cats_id=substr($cats_id,0,-1);

    $args["include"] = $cats_id;// The IDs of the including categories
    return $args;
}
  add_filter("widget_categories_args","include_widget_categories");

Add the above code in functions.php.