ad in random position in loop

<?php $ad_positions = array(1,2,4); // array of preset possible ad positions

do { $rnd_ad = $ad_positions[rand(0,count($ad_positions)-1)]; } while ($rnd_ad >= $wp_query->post_count); ?>

<?php if(have_posts()) : while(have_posts()) : the_post(); // start of the loop // ?>

some possible post output here

<?php if( ($wp_query->current_post+1) == $rnd_ad ) : ?>
adsense code here
<?php endif; ?>

more possible post output here

<?php endwhile; endif; // end of the loop // ?>