How to make the new Array start at the same position as the previous and randomise only the newObjec

Question

So i am currently trying to make 5 boxes/ships (3Blue and 2 Green)appear  in a random position(1st-Gree,2nd- Blue,3rd-blue,4th-Green,5th-Green or in some other random way) i the upper corner of the stage.Make them move down.Then make a new 5 boxes and position them in the same starting position as the previous 5 and make only them get randomised (not with the previous 5,what it is doing at the moment).And do this while they are moving down.  so the code so far i have is this :
import flash.events.Event;
var shipCount:Number;
var shipCount2:Number;
var shipArray:Array = new Array();
var shipArrayPosition:Array = new Array();
var counter:int = 0
var positionsX:Array = ["50","100","150","200","250",]
addEventListener(Event.ENTER_FRAME, everyFrame)
function everyFrame(ev:Event):void{
          counter++
          if(counter % 70 == 0){
                         doShips()
          positionShips()
          trace(shipArray.length )
function doShips() {
          shipCount = 3;
          shipCount2 = 2;
          var gap = 10;
          for (var i:int = 0; i < shipCount; i++) {
                    var s = new Ship;
                    shipArray.push(s);
                    //s.x = s.width/2 + (s.width* i) + gap*i
                    //addChild(s)
                    //shipArrayPosition.push(s);
          for (var j:int = 0; j < shipCount2; j++) {
                    s = new Ship2;
                    shipArray.push(s);
          var array:Array=new Array();
          while (shipArray.length>0) {
                    var index:uint = Math.floor(Math.random() * shipArray.length);
                    array.push(shipArray[index]);
                    shipArray.splice(index,1);
          shipArray = array;
          //shipsArray has been randomized
          for (var k:int = shipArray.length - 1; k >= 0; k--) {
                    shipArray[k].x = positionsX[k]
                    addChild(shipArray[k])
function positionShips() {
          for (var i:int = shipArray.length - 1; i >= 0; i--) {
                    shipArray[i].moveDown() //what the code in the Ship and Ship2 class does -> only: this.y += 3
and how to make them stay at one position.Not to move in any X position when the new 5 are added

Answer 1

This one is more efficient yet because it removes ships that moved out of view:
import flash.display.MovieClip;
import flash.display.Sprite;
import flash.events.Event;
var numShips:int = 5;
var shipCount:int = 3;
var shipArray:Vector.<Sprite> = new Vector.<Sprite>();
var counter:int = 0
var positionsX:Array = [];
init();
function init():void
          // populate positions
          for (var i:int = numShips - 1; i >= 0; i--)
                    positionsX.push(50 * i);
          addEventListener(Event.ENTER_FRAME, everyFrame);
function everyFrame(e:Event):void
          counter++
          if (counter % 70 == 0)
                    doShips();
          positionShips();
          trace(shipArray.length);
function doShips():void
          var gap = 10;
          var s:MovieClip;
          var shipsLine:Sprite = new Sprite();
          shipsLine.cacheAsBitmap = true;
          addChild(shipsLine);
          shipArray.push(shipsLine);
          shuffle(positionsX);
          for (var i:int = numShips - 1; i >= 0; i--)
                    s = i < shipCount ? new Ship() : new Ship2();
                    s.x = positionsX[i] + gap;
                    shipsLine.addChild(s);
          shipsLine.y = -shipsLine.height;
function shuffle(a:Array):void
          for (var i:int = a.length - 1; i >= 0; i--)
                    var r:int = Math.floor(Math.random() * (i + 1));
                    var t:Object = a[r];
                    a[r] = a[i];
                    a[i] = t;
function positionShips():void
          for each (var shipLine:Sprite in shipArray)
                    shipLine.y += 3;
                    // remove line that moved out of stage
                    if (shipLine.y > stage.stageHeight + shipLine.height)
                              removeChild(shipLine);
                              shipArray.splice(shipArray.indexOf(shipLine), 1);