Adding class to first post not working

Hi there,

I’m trying to add a class to the first post in my loop. Here is my index.php –

<main id="main" class="site-main frame" role="main">

      <?php $c = 0; $class = ''; ?>  

		<?php if ( have_posts() ) : ?>

	        <?php /* Start the Loop */ ?>
		<?php while ( have_posts() ) : the_post(); ?>

                <?php //set count
			   $c++;
			   if ( $c == 1 ) $class .= 'firstItem';
			   if ( $c !== 1 ) $class .= '';
		?>

		<?php get_template_part( 'content', get_post_format() ); ?>

		<?php endwhile; ?>

		<?php byyd_paging_nav(); ?>

		<?php else : ?>

			<?php get_template_part( 'content', 'none' ); ?>

	<?php endif; ?>
</main><!-- #main -->

And here is my content.php –

<article id="post-<?php the_ID(); ?>" <?php post_class( "$class masonryItem" ); ?> onClick="window.open('<?php echo get_permalink(); ?>','_self');">
*MY CONTENT*
</article>

I’ve tried various options but am unable to get this right… Any ideas?

Thanks