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Index › Programming Questions & Help › Syntax Questions › Why does this crash
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Why does this crash? (Read 483 times)
movax
Why does this crash?
Apr 6th, 2006, 12:08am  
I want the program to stop and do nothing until a mouse click is received.  Nothing I do seems to work, and my sketches just blow up.  Any ideas?  Thanks guys!

After lots of drawing, this is called from draw():

Code:

void PickBest(){
   choosing=true;	 
   while(choosing==true){
     if(mousePressed==true) {  choosing=false; }
   }
}
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cello
Re: Why does this crash?
Reply #1 - Apr 6th, 2006, 12:44am  
Basically, mousePressed will never become true.  You should not use an infinite loop in your draw() method ever, but instead do something like:

Code:

boolean choosing = true;

void draw() {
  if (!choosing) {
    //do whatever you normally do
  } else {
    if (mousePressed) choosing = false;
  }
}
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movax
Re: Why does this crash?
Reply #2 - Apr 6th, 2006, 1:19am  
>You should not use an infinite loop in your draw() method

I was afraid of that.  Although I don't see why it should be the case.  Especially if having used noLoop().
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seltar
Re: Why does this crash?
Reply #3 - Apr 6th, 2006, 1:36am  
when the applet is run, and it gets to the while loop, it stops, because you can't get two types of userinteraction on one frame.. one frame, the mouse isn't clicked, the next it might be.. This is why your code never gets to the end of the draw() or setup() function, where the screen is drawn
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cello
Re: Why does this crash?
Reply #4 - Apr 6th, 2006, 1:36am  
Well, the program just gets stuck in the loop, and it never gets out of the loop to check if the mouse was pressed.  It may be possible if you use void mousePressed() { choosing = false; }, but it may also have unexpected results (since processing isn't designed as threadsafe, as far as I know).

Marcello
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fry
Re: Why does this crash?
Reply #5 - Apr 6th, 2006, 3:52pm  
the mousePressed var is thread safe, but that means it only gets updated outside of draw(). the mousePressed() method is also thread safe, but may or may not be called in that situation, it sort of depends on the machine: in most cases, the while() loop is so tight that it'll spin out of control and "starve" any other threads because it never takes a breath (which would let the ui thread update itself).

on more than one occasion, i've wanted to remove while() from the syntax for this reason. java's just not very good at dealing with these situations and it means a lot of force-quitting the vm for someone who's not familiar with the issue.
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movax
Re: Why does this crash?
Reply #6 - Apr 6th, 2006, 10:09pm  
Thanks for the good info fry.  Thread starving is exactly what I was witnessing. And it happens QUICKLY!  Now I can see why you want to remove while().

In this particular application, I was not needing (in fact, didn't want) any drawing to take place while waiting for a mouse click.  So I thought I would be safe to just "pause" the draw routine.  My rutted brain wanted to use a while loop so bad.

I rewrote it to use a state variable instead, which works very nicely, but took a little change in my thinking.  A good thing!  

FYI, here's the sketch I was working on, a genetic image mutator:

http://spacefillingcurve.net/programming/processing/mothevolve/

I LOVE PROCESSING!
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