How to display an ad after a number of post in homepage?

i got a code to add ads after every 10th post,

please post that code – it might be easier to adapt that code than to suggest a new one.

@alchymyth, here is the code that i currently use (basic outline):

<?php $counter =0; ?>
	<?php if ( have_posts() ) : ?>
		<?php while ( have_posts() ) : the_post(); $count++; ?>
			<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
			<?php the_title(); ?>
			<?php the_excerpt(); ?>
			</div>
			<?php $postnum++; if($postnum%10 == 0) { ?>
			<div>AD CODE HERE</div>
			<?php } ?>
		<?php endwhile; ?>
			<?php wp_pagination(); ?>
		<?php else : ?>
			<?php get_template_part( 'no-results', 'index' ); ?>
		<?php endif; ?>

thanks!

this is the current conditions to show the ad after every 10 posts:

if($postnum%10 == 0)

change it to have just after 10 posts:

if($postnum == 10)

wow! that simple? i wasted couple of days searching the web for that piece of code, i should have asked here first, my bad!

thank you alchymyth for your quick responses.

In Which File should i put the code. Please explain little in details as i am a fresher to WP and PHP.

Thanks In Advacne