How to Add a Class to first post in Wordpress Query?

Question

I"m running multiple loops on a page and on one of the loops I need to add a class to the first post that way I can offset the margin. How can I add a class to the first post from the wp_query?

<?php $my_query = new WP_Query('category_name=news&posts_per_page=3');
while ($my_query->have_posts()) : $my_query->the_post();
$do_not_duplicate[] = $post->ID ?>
    <div class="post-container">
    </div>
<?php endwhile; ?>

I need to add the class to the main div container which has the class "post-container".

Thank you the fix below works perfect but what do I do if I want to use it for a normal loop instead of a wp_query? like this

<?php query_posts(array('showposts' => 3, 'post__not_in'=>$do_not_duplicate));
if (have_posts()) : while (have_posts()) : the_post(); ?> 

Answer 1

Maybe this can help:

$first = false;
if( $my_query->current_post == 0 && !is_paged() ) { $first = true; }

And then your line with you do this:

<div class="post-container <?= ($first ? 'classname' : ''); ?>">

Not tested for syntax errors.

So try this:

<?php 
$first = true;
query_posts(array('showposts' => 3, 'post__not_in'=>$do_not_duplicate));
if (have_posts()) {
    while (have_posts()) {
        the_post();

        echo '<div class="post-container '.($first ? 'classname' : '').'"></div>';

        if ($first) {
            $first = false;
        }
    }
}