PHP: Setting post ID to a variable

I’m trying to create an icon which will change with each post. I’ve got it working right now with a random number but I want to base the icon on the post number so the icons will rotate in a pattern.

In the following code the variable is not being set to the ID because the_ID() prints the post number. Is there a way to assign the post number to the variable?

<?php
          $numa = the_id();
          echo $numa;
        ?>