Millisecond Forums

G'day,
I'm trying to set up a task where participants will see a cluster of dots on the left hand side of screen, and another cluster of dots on the right. The task is to indicate which cluster has more dots in it.
I want each trial to present a random number of dots (between 50 and 100) on one side. The other cluster will have x number of dots more or less than the first cluster (x will vary across trials to manipulate difficulty).
My problem is that I can't figure out how to present a random number of dots. I've tried a few things but always get stuck somewhere. My latest idea was to always present the maximum number of dots (i.e. 100), and then try to make a small subset of them the same colour as the background, essentially making them go away. I've attached the script I tinkered with, which presents 20 dots on the left. The bit of code I tried to use in the trial element to make some of the dots white isn't working because clearly I can't use the <%%> code in this way. I can't figure out another way though.

<trial dottrial>
/ ontrialbegin = [list.dotlist.reset();
  var i = 0;
  while (i < values.selectnumber) {
   shape.<%list.dotlist.nextvalue%>.color = white;
   i += 1;
  };
]
/ stimulusframes = [1 = dot1, dot2, dot3, dot4, dot5, dot6, dot7, dot8, dot9, dot10, dot11, dot12, dot13, dot14, dot15, dot16, dot17, dot18, dot19, dot20]
/ validresponse = (57)
</trial>

Could I get there with this approach? Or is this never going to work?