Re: [pygame] more list problems

[Casey Duncan <casey@xxxxxxxxxxx>]

Although I think Noah provided the "correct" solution, which is to  
create a new list by filtering the input list, I figured you also  
might want to know why your solution below doesn't work as you intend.

Iterating over changing mutable sequences is tricky because as the  
list changes, the indexes of elements in the list can also change.  
Imagine the following list of rects:

rect = [Rect(10,10,5,5), Rect(0,0,10,10), Rect(7,7,3,3)]

Now imagine a "click" at position (8,8)

range(0, len(rect)) returns [0, 1, 2] (Note you can omit the first  
argument, but default range() starts at index 0, thus range(len 
(rect)) is equivalent)

The first time though the loop, i=0. The mouse position is not inside  
rect[0], so we move on.
Now i=1, and the mouse is inside rect[1], so we now execute 'del rect 
[1]', thus changing the list to:

[Rect(10,10,5,5), Rect(7,7,3,3)]

The next time through the loop i=2, but now the rect list does not  
have an index 2! So it blows up with an IndexError. Note this is not  
the only possible outcome. Had my list been longer I would have wound  
up skipping the original element at index 2 altogether, meaning it  
would never be checked. Those are the kinds of bugs that can make you  
tear your hair out 8^)

So assuming we needed to mutate the list in this way (and not filter  
it into a new one as Noah suggested), what could we do?

Let me start by saying that anytime I find myself writing a for loop  
over a contrived range(), I pause and think for a second, because in  
Python that is almost never the best way to iterate something. It's a  
bit of a red flag. That's not to say I never do it, it's just not  
typical, even though it's often the only way to do loops in other  
languages.

To fix the above, I would suggest iterating over a copy of the  
original list instead of the range(). That way you can mutate the  
original list without upsetting the loop. you can create a (shallow)  
copy of a list by passing it to the built-in list():

for r in list(rect): # iterate over a copy of rect list
	if r.collidepoint(mousexTEMP, mouseyTEMP):
		rect.remove(r)

Note I omitted the numRect calculation. Can't you just use len(rect)  
at the end of the loop?

Now having "solved" the problem of mutating the rect list in this  
way, I have to say that Noahs solution is more elegant, thus superior  
to the above. It doesn't mutate the list in place at all, thus  
skirting the problem entirely, plus it's much shorter.

The problem you encountered is a very common python coding pitfall.  
The best way to avoid it is to think twice before mutating a sequence  
while iterating it.

hth,

-Casey

On Sep 20, 2007, at 5:18 PM, Eric Hunter wrote:

> so I have been able to write pygame.Rect rectangles into rect-type  
> lists. but now I'm trying to delete unwanted rectangles that are  
> printed to the screen.
>
> when I draw one at a time, I can only delete the last drawn  
> rectangle before deleting the previous ones. I want to be able to  
> delete any rectangle at any time without problems. I've come close  
> but have run into a wall.
>
> below is the "deleting rect" section. if anyone needs more then  
> what's provided i can provide it upon request.
>
> thanks in advance.
>
> [code]
> if event.button == 3:
>                                 mousexTEMP, mouseyTEMP =  
> pygame.mouse.get_pos()
>                                 for i in range(0, len(rect)):
>                                     if rect[i].collidepoint 
> (mousexTEMP, mouseyTEMP):
>                                             del rect[i]
>                                                 numRect -= 1
> [/code]