I commented out the part that makes this an infinite loop and causes processing to hang.

I can see how if I move my mouse to x=0 an infinite loop will be created. But as soon as I move my mouse to the right I think the exit condition will soon be met.

The book I am reading says no.

mouseX and mouseY are updated with new values at the beginning of each cycle through draw().

What don't I understand? Thanks.

int y = 80;        
int x = 0;        
int spacing = 10;  
int len = 20;      
int endLegs = 150;

void setup() {
 size(200,200);
}

void draw() {
 background(0);
 stroke(255);
 x = 0;

//spacing = mouseX / 2;
 while (x <= endLegs) {
   line(x,y,x,y + len);

   x = x + spacing;
 }
}

Use this spacing formula instead.

Code:

spacing = (mouseX / 2)+1; 

Quote:


That's not the point.

The first time draw() is called, at the very beginning of the sketch, x==0 and mouseX==0. As a consequence spacing == mouseX/2 == 0. Moreover (x <= endLegs) is true, and you get stuck in the while loop, because "x = x + spacing" is equivalent to "x = 0 + 0" and (x <= endLegs) will never be false.

Processing won't go further : nothing will be drawn on screen, and draw() will never be called again. You're just stuck in the while loop.

No. Sorry if I haven't explained it well. This is what Processing does :

step 1 : check if some variables have been declared
step 2 : call setup()
step 3 : call draw()
step 4 : wait a bit and go to step 3

In your case, the problem is that you'll never reach step 4, because you're stuck in the while() loop (which is infinite because x cannot get over endLegs). Since you're stuck, you cannot reach the end of the draw() function, and cannot go to step 4.

Lenticular wrote on Aug 18^th, 2009, 3:41pm:


x has to get over endLegs on every frame, otherwise the program will be stuck in the while loop.  When you set up a loop you must ensure that the condition will get triggered at some point.


Lenticular wrote on Aug 18^th, 2009, 3:41pm:


If mouseX only updates once a frame then the draw loop has to complete at least once before mouseX updates from the starting value of 0.  For the draw loop to complete the first frame, the while loop has to complete.  In your original code the while loop would only complete if mouseX was greater than 0, which wouldn't happen because you couldn't get beyond the first frame...  i.e. it would never complete, you wouldn't pass Go and wouldn't collect £200...

By including the "+1" the condition in the while loop will be triggered, since each repetition of the while loop will add 1 to x, meaning it will eventually be greater than endLegs, even if mouseX=0.

And note that the while loop will complete each and every frame - it has to in order for Processing to move on to the next frame...