Re: Having trouble deleting objects from a list...
- From: Mike Meyer <mwm@xxxxxxxxx>
- Date: Wed, 19 Oct 2005 23:23:48 -0400
Jason Stitt <jason@xxxxxxxxxxx> writes:
> On Oct 19, 2005, at 8:16 PM, KraftDiner wrote:
> 'for obj in self.objList' will keep right on iterating through the
> list even if you don't increment i.
And if you modify self.objList while you're iterating over it, the
results are undefined.
> A direct adaptation of your code that should work is:
>
> i = 0
> while i < len(self.objList):
> if self.objList[i].mouseHit:
> self.objList.pop(i)
> flag = True
> else:
> i += 1
You probably want "del self.objList[i]" instead of
"self.objList.pop(i)". That saves a method lookup and the return of a
value you're just going to throw away.
> Or, shorter and a bit more Pythonic, but not in-place:
>
> newObjList = [obj for obj in self.objList if not obj.mouseHit]
> flag = (len(newObjList) != len(self.objList))
> self.objList = newObjList
>
> I don't know how the performance of the two would compare. The second
> involves creating a new list, but Python is supposed to optimize the
> heck out of list comprehension, and removing items from a list in
> place isn't free, either.
Deleting an element from an arbitrary point in a CPython list of
length n is O(n). So deleting m items from an n-element list is
O(n*m). Building the new list will be O(n) no matter how many
elements you delete. Unless space is really tight, you probably want
to build a new list.
If you do have to do the delete in place, you'll improve things by
scanning the list from the end towards the beginnings. That makes the
worst-case behavior go from O(n^2) to O(n), because you no longer have
to move any elements after you delete one.
i = len(self.objList) - 1
while i >= 0:
if self.objList[i].mouseHit:
del self.objList[i]
flag = True
i -= 1
<mike
--
Mike Meyer <mwm@xxxxxxxxx> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
.
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