how to set a active class on first div in wordpress

<?php $firstMarked = false; ?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<div class="item <?php echo !$firstMarked ? "active":"";?>">
  <?php the_post_thumbnail('full');?>
  <div class="container">
    <div class="caption">
      <h1>
        <?php the_title(); ?>
      </h1>
      <p>
        <?php the_excerpt(); ?>
      </p>
    </div>
  </div>
</div>
<?php $firstMarked = true;?>
<?php endwhile; ?>