帮忙！！！看一下这里“：”什么意思？
//帮忙！！！看一下这里“：”什么意思？解释一下他的用法？
stop();
var i:Number = 1;
var mcName:String = "newMC";
for (i; i<=300; i++) {
ncName = mcName+i;
this.water_mc.duplicateMovieClip(ncName, i);
this[ncName]._x = random(439);
this[ncName]._y = random(436);
this[ncName]._xscale = random(100);
this[ncName]._yscale = random(100);
}
