use the slidervalue to calculate your needed variable.
something like this :

int var = 1;
for(int i = 0;i<sliderVal;i++)var*=2;
println(var);

thx, i will look in it tommorow.

2nd question.

http://www.sojamo.de/libraries/controlP5/reference/index.html

can someone show me a working example of:

onEnter

public void onEnter()

The slider,

i only want to have a shape drawn when i adjust the slider.

(so onLeave meight also be needed)