How can I only drag obj.mc up and down - actionscript-3

Question

Anyone help me please. I'm totally new to AS3.
I have an mc called obj.
How can I only drag it only : up and down?
Here is the code I found somewhere on the internet.
import flash.events.MouseEvent;
import flash.geom.Point;
import flash.events.Event;
var destination:Point=new Point();
var dragging:Boolean=false;
var speed:Number=5;
var offset:Point=new Point(); // our offset
obj.addEventListener(MouseEvent.MOUSE_DOWN,startdrag);
stage.addEventListener(MouseEvent.MOUSE_UP,stopdrag);
obj.addEventListener(Event.ENTER_FRAME,followmouse);
function startdrag(e:MouseEvent):void{
offset.x=obj.mouseX*obj.scaleX; // record offset (pt obj is dragged
offset.y=obj.mouseY*obj.scaleY;
dragging=true;
}
function stopdrag(e:MouseEvent):void{
dragging=false;
}
function followmouse(e:Event):void{
if(dragging){
destination.x=mouseX;
destination.y=mouseY;
}
obj.x-=(obj.x-(destination.x-offset.x))/speed; // apply offset
obj.y-=(obj.y-(destination.y-offset.y))/speed;
}

Answer 1

startDrag() method
public function startDrag(lockCenter:Boolean = false, bounds:Rectangle = null):void
Check the reference before to post a question, sorry but it's clearly explained on the reference guide startDrag
This is the best way to understand the method just set the
bounds:Rectangle = null;
to a value that match the limits.
When bounds are set, you should do what you want.
I most often set the lockCenter to false, but you have to understand the basics.
Sorry I missed something, my English is really poor.
The answer of #Coder3000 is the good one in your case as you want to displace the object smoothly.
Once again, I wanted to answer too fast. :(
So to move your object up and down only, You may delete the following lines in your code :
offset.x=obj.mouseX*obj.scaleX;
destination.x=mouseX;
obj.x-=(obj.x-(destination.x-offset.x))/speed;
Because you don't want to move this object left or right