Expression help

Hello,

I got a issue with using expressions. I've been following this book http://www.amazon.com/After-Effects-Expressions-Marcus-Geduld/dp/02408 0936X and I got to this particular expression:

var Dimensions = wiggle(1,30);

var horizontal = transform.position[0];

var Current_Y = transform.position[1];

var secondWiggle = Dimensions[1];

if(secondWiggle > Current_Y) {

  var vertical = secondWiggle

}

else {

  var vertical = Current_Y;

}

[horizontal, vertical]

Now this should be able to calculate across frames different values and give the appropiate output. But it doesn't. The only thing I get is some fixed random value that doesn't change. For some reason the wiggle is not recalculated across frames. Whatever you get on the very first frame stays that way all the way.

My question is: Is it something wrong in the expressions(noob here) or is it something after effects can't do?

!! Please mind that the expression is the same as in the book and when trying to write my own expressions using if/for/while I get the same issue. The result ain't calculated across multiple frames. It seems stuck and I can't figure out why.

Hope my question makes sense and looking forward for a solution.

Thanks