Animate Sprites toward a point - actionscript-3

Question

I'm trying to do something very simple: animate a bunch of sprites so that, no matter where they are on the stage, they all go toward a single point (in this case, the center of the stage).
It only works for some of them. Some of them just sit there. I can't figure out why. I think its got something to do with atan2 and the way I'm using it, but I'm not sure what. Can someone see?
Thanks,
David
package
{
import Ball;
import flash.display.Sprite;
import flash.display.StageAlign;
import flash.display.StageScaleMode;
import flash.events.Event;
public class MoveBalls extends Sprite {
private var balls:Array = new Array();
private var speed:Number = new Number(10);
public function MoveBalls() {
init();
}
private function init():void {
stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;
for(var i:int = 0; i<30; i++)
{
var ball:Ball = new Ball(20);
ball.x = getRandomNumber();
if(ball.x > stage.stageWidth) ball.x = stage.stageWidth -10;
if(ball.x < 0) ball.x = 10;
trace(ball.x);
ball.y = getRandomNumber();
if(ball.y >stage.stageHeight) ball.y = stage.stageHeight - 10;
if(ball.y < 0) ball.y = 10;
addChild(ball);
balls.push(ball);
}
addEventListener(Event.ENTER_FRAME, onEnterFrame);
}
private function getRandomNumber():Number {
var decider:Number = Math.random();
if (decider < 0.5) {
return Math.random()*1000;
} else {
return Math.random()*-1000;
}
}
public function onEnterFrame(event:Event):void {
var targetX:Number = stage.stageWidth/2;
var targetY:Number = stage.stageHeight/2;
for(var i:int=0; i < balls.length; i++)
{
var startX:Number = balls[i].x;
var startY:Number = balls[i].y;
var atanX:Number = (targetX - startX);
var atanY:Number = (targetY - startY);
if(atanX > .1) //just stop when it's close enough to the center
{
var ang:Number = Math.atan2(atanY, atanX);
var xSpeed:Number = Math.cos(ang)*speed;
var ySpeed:Number = Math.sin(ang)*speed;
balls[i].x += xSpeed;
balls[i].y += ySpeed;
}
}
}
}
}
//ball class
package {
import flash.display.Sprite;
public class Ball extends Sprite {
private var radius:Number;
private var color:uint;
public function Ball(radius:Number=40, color:uint=0xff0000) {
this.radius = radius;
this.color = color;
init();
}
public function init():void {
graphics.beginFill(color);
graphics.drawCircle(0, 0, radius);
graphics.endFill();
}
}
}

Answer 1

I don't agree with this line:
if(atanX > .1)
Because if the balls are to go to the CENTER of the stage, like both on X and Y axes, then this condition fails for balls directly under or above the center.
Make it like this
if (atanX > .1 || atanY > .1)
= If either of the differences are bigger than .1, go on with the atan2...
EDIT:
One more obvious error - the atanX or atanY variables can, in 50% be negative. Therefore, use either this:
if (Math.abs(atanX) > .1 || Math.abs(atanY) > .1)
Or this, which is the same but faster (harder to read, though):
if ((atanX > 0 ? atanX : -atanX) > .1 || (atanY > 0 ? atanY : -atanY) > .1)