Advanced Custom Fields (ACF) Checkbox - Promotion of post to 'top story' - wordpress

Question

I am using the ACF plugin which is awesome but am struggling with a feature of it, namely the checkbox.
I am trying to use the checkbox as a means of promoting a blog post to a 'top story'.
So I have set up a ACF checkbox field called 'top_story' and if it is checked it should promote the post and if it is not checked then it won't promote the post.
Now this does work but I keep getting the following error message whenever a blog post does not have that checkbox ticked.
Warning: in_array() [function.in-array]: Wrong datatype for second argument
I have simplified the code so it looks like this:
<?php
if( in_array( 'topstory', get_field('top_story') ) )
{
echo '<h1>This is a top story</h1>';
}
else
{
echo '<h1>This isn't a top story</h1>';
}
?>
So I guess what I want to know is what is going wrong here and how to rectify it? It looks like as there is no value in the array for the posts which aren't 'top stories' then there is no argument passed into the 'get-field' function and it falls over?
I was just going to hide the errors as essentially it still works but that doesn't sit comfortable with me and I'm sure I will need to do this again in the future.
Thanks for all your time and help in advance.

Answer 1

Maybe like this:
<?php
// args to check if "Top Story" os TRUE:
$args = array(
'cat' => '5', // Enter Category for "Topstories"
'posts_per_page' => 3, // How many posts to show if multiple selected "Backend"
'orderby' => 'date', // How to sort posts - date, rand etc...
'order' => 'asc', // How to order posts - ASC, desc etc...
'meta_key' => 'topstory', // Name of ACF field to filter through
'meta_value' => 'yes' // Yes = Show, No = Don't show
);
// The results:
$the_query = new WP_Query( $args );
// The Loop:
<?php if( $the_query->have_posts() ) :?>
<h1>This is a top story</h1>
<?php
while ( $the_query->have_posts() ) : $the_query- >the_post(); ?>
....
// Properties to show you post //
....
endwhile;
endif;
wp_reset_query(); // Reset/kill query
?>

Answer 2

Sounds like there are two things you could be running into here:
If a field isn't set, or doesn't exist, get_field() will return false.
If none of the options in a checkbox field are checked, get_field() will return an empty string.
In either case, you won't have an array to search with in_array, and will get a warning if you try.
I'd try this, following ACF's documentation. You should also consider using ACF's True/False field, which is designed for this sort of thing; the Checkbox field is more for multiple checkboxes, of which more than one can be true.
<?php
$topStory= get_field('top_story');
if($topStory) // Check whether this meta field exists at all
{
if(is_array($topStory) && in_array( 'topstory',$topStory ) {
echo "<h1>This is a top story</h1>";
}
else {
echo "<h1>This isn't a top story</h1>";
}
}
?>
If you had a True/False field, you would make it a bit simpler:
<?php
if(get_field('top_story')) {
echo "<h1>This is a top story</h1>";
} else {
echo "<h1>This isn't a top story</h1>";
}
?>