Pagination not working with Loop - sorting

Question

Page 2 displays the same posts as page 1. What could be the problem?
Below is code from index.php It is used to sort posts by simply clicking on a link such as sort by: "price" or "random".
Pagination
<?php
previous_posts_link();
next_posts_link();
?>
Loop
<?php
$sort= $_GET['sort'];
if($sort == "A")
{
$order= "orderby=rand&posts_per_page =2";
}
if($sort == "B")
{
$order= array (
'meta_key'=>'price',
'orderby'=>'meta_value_num',
'order'=>'DESC',
'posts_per_page' => 2
);
}
?>
random
price
<?php $loop = new WP_Query($order); ?><?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<h2><?php the_title(); ?></h2>
<?php the_excerpt(); the_meta ();?>
<?php endwhile; wp_reset_query();?>

Answer 1

Yourt problem is using a custom query. Why aren't you using the main query. It is totally unnecessary and a waste of resources to run another query here.
As this code is on index.php, which will be your homepage, you can simply make use of the default loop, and use pre_get_posts to alter your main query before it is run. Doing it this way doesn't require extra unnecessary database queries, and it is much cleaner
So, delete the custom query in your index.php, and replace it with the following code. This is all that you are going to need (and don't abuse the php tags :-))
<?php while (have_posts() ) : the_post(); ?>
<h2><?php the_title(); ?></h2>
<?php the_excerpt(); the_meta ();
endwhile;
previous_posts_link();
next_posts_link();
?>
Now, in functions.php, add the following
function my_custom_query($query){
if ( $query->is_home() && $query->is_main_query() ) {
$sort= $_GET['sort'];
if($sort == "A"){
$query->set( 'orderby', 'rand' );
$query->set( 'posts_per_page', '2' );
}
if($sort == "B"){
$query->set( 'meta_key', 'price' );
$query->set( 'orderby', 'meta_value_num' );
$query->set( 'order', 'DESC' );
$query->set( 'posts_per_page', '2' );
}
}
}
add_action( 'pre_get_posts', 'my_custom_query' );
If you, for some reason, need to run a custom query, remember to add the paged variable to your arguments. You can have a look at these parameters in WP_Query
$paged = ( get_query_var('paged') ) ? get_query_var('paged') : 1;
$loop = new WP_Query( array( 'paged' => $paged ) );
This isn't enough though. You need to set the $max_pages parameter as well in next_posts_link( $label , $max_pages ); for pagination to work correctly. You'll need to something like this
next_posts_link( 'Older Entries', $loop->max_num_pages );

Answer 2

If you want to use a custom WP_Query and still want pagination to work, you must pass in the paged argument.
// what page is this? default to 1.
$paged = ( get_query_var('page') ) ? get_query_var('page') : 1;
// "A" args
$order = "orderby=rand&posts_per_page=2&paged={$paged}";
// "B" args
$order = array (
'meta_key'=>'price',
'orderby'=>'meta_value_num',
'order'=>'DESC',
'posts_per_page' => 2,
'paged' => $paged,
);