How to write a pre_get_post()

How to write which is recommended. Please let me know.

example 1

use WordPress functions.

add_action( 'pre_get_posts', 'category_display_per_3' );

function category_display_per_3( ) {
 if ( is_main_query() and is_category() ) {
     set_query_var( 'posts_per_page', 3 );
 }
}

example 2

object access.

add_action( 'pre_get_posts', 'foo_modify_query_exclude_category' );

function foo_modify_query_exclude_category( $query ) {
 if ( ! is_admin() && $query->is_main_query() && ! $query->get( 'cat' ) )
 $query->set( 'cat', '-5' );
}

codex@source

I am sorry to be poor at English.

Thank you.