Exclude Subcategories in a Loop

functions.php

<?php

function wpse_filter_child_cats( $query ) {

if ( $query->is_category ) {
    $queried_object = get_queried_object();
    $child_cats = (array) get_term_children( $queried_object->term_id, 'category' );

    if ( ! $query->is_admin )
        //exclude the posts in child categories
        $query->set( 'category__not_in', array_merge( $child_cats ) );
    }

    return $query;
}
add_filter( 'pre_get_posts', 'wpse_filter_child_cats' );

Modified from http://wpengineer.com/876/exclude-subcategories-in-a-loop/.