var cols:int = 2;
height:int = 20;
width:int = 40;

for (var i:int = 0;i<20;i++) {
     var clip:MovieClip = new MovieClip();
     clip.y = (Math.floor(counter/cols) * height);
     clip.x = (counter%cols * width);
}

http://www.ramsesz.be

This is a way to make grids in 2 lines of code

Grids in 2 lines of Code

ActionScript 3