How to set posts per page using WP_Query()

Question

I'm trying to set the number of posts_per_page dynamically using my own variable. This code works fine:

query_posts( $query_string  . "&posts_per_page=" . $myvar )
// then display the loop using get_template_part( 'loop' );

That works fine, but I would like to know how to do it using $wp_query instead of query_posts. However, this does not work:

$wp_query->set('posts_per_page', $myvar)
// then display the loop using get_template_part( 'loop' );

That has no effect on the number of posts displayed. Am I using the set method incorrectly, or is there something I'm missing?

Answer 1

query_posts will do the query again (destroy current wp_query and create a new one and then doing the query with the parameters you pass to it)

To get the same behaviour after setting the new parameter with set_var you need to query the database again using something like this

 $wp_query->set('posts_per_page', 1);
 $wp_query->query($wp_query->query_vars);

Answer 2

Why do you need to use $wp_query->set() at all?

Can't you just use:

<?php
$my_custom_query_args = array(
    'posts_per_page' => $myvar
);
$my_custom_query = new WP_Query( $my_custom_query_args );
?>

Answer 3

when you call query_posts with a modified query in your template, you're actually running another query and discarding the original. trying to set a query var in your template doesn't work because the original query has already happened. to use $wp_query->set() you have to hook in before the database is queried, for example on pre_get_posts.