this is a good example for two nested for loops. one for the x dimension, and one for the y dimension.
first you define the Size of the rects, and the gap between them.
then you create a for loop for the first column of rects.

 for(int y=0; y<height; y+=rectSize+gap) {
  rect(0,y,rectSize,rectSize);
 
 }

you add the Size of the rect and the gap to y as long as it is smaller than the height of the sketch.

now you got a column, around this for loop you put another loop, to multiply these columns

 for(int x=0; x<width; x+=rectSize+gap) {      
 for(int y=0; y<height; y+=rectSize+gap) {
  rect(x,y,rectSize,rectSize);
 
 }
}

and in the end you should get something like this, where you can change the rectSize and the gap and it always fills the screen with rectangles.
you can also define the number of rects and colculate the number of rects needed for a specific size.
maybe try to figure that out as a little test...

1. int rectSize = 20;
   int gap = 10;
   
   
   void setup() {
     size(600,400);
     background(255);
   
   }
   
   void draw() {
   
    for(int x=0; x<width; x+=rectSize+gap) {     
    for(int y=0; y<height; y+=rectSize+gap) {
    
      rect(x,y,rectSize,rectSize);
     
    }
   }
   
   
   }
   
   
    
   

what's up with keyPressed and all the other key related methods in the reference? they are a bit low-level but they work.

you are right, that was wrong what wrote there. it should say. "define the number of rects and calculate the needed size of the rect or screen"

hi sumotku,

Beautiful Andreas, thank you. If I can make it draw (n)rectangles based on textfield input values I'll post it. Still trying to get a working *for* or *while* function to work. So far all I have is a program that can jack up a processor to 100% indefinitely, pretty snazzy, eh?   : P   Long way to go, but it's a good thing this program is addictive.