iterating over collection, deleting entries
 

I want to iterate over a collection and delete all unwanted entries.

for item in collection:
del item

of course doesn´t do anything useful. Two things come to mind:
a) iterating backwards over the collection using indexing, something like:

for i in range(len(collection)-1, -1, -1):
item = collection[i]
if isBad(item) del collection[i]

b) duplicating the collection, iterating over one and deleting from the
other.

Both solutions seem both ugly and expensive to me, solution a)
probably isn´t even O(x) anymore.

Please tell me, what´s the best way to do this?


--
Patrick von Harsdorf
patrick@harsdorf.de

Re: iterating over collection, deleting entries => PLEASE IGNORE
 

Sorry, I missed that SnuSnu just posed a very similar question.
Please ignore my post...

Re: iterating over collection, deleting entries
 

On Sun, 25 Apr 2004 18:25:08 +0200, "Patrick von Harsdorf"
<patrick@harsdorf.de> wrote:

>Please tell me, what´s the best way to do this?

I'm pretty new to python, but normally this is done
by a simple read-write approach. You iterate over
the elements using a "read" pointer, and every time
you find one you want to keep you just write it
and increment the write pointer.
In code...

wrp = 0

for x in container:
if is_good(x):
container[wrp] = x
wrp += 1

del container[wrp:]

This is O(n) and doesn't require any additional memory

I suppose however that it's not so common finding
cases in which this is much better than...

a = [x for x in a if is_good(x)]

The latter will allocate a list for the result
(instead of reusing the same), but can free the
original (and this may be *better* than just
resizing it - it may be better in C++, for example).

HTH
Andrea

Re: iterating over collection, deleting entries
 

I would do:

collection=[i for i in collection if not isBad(i)]

Larry Bates
Syscon, Inc.
"Patrick von Harsdorf" <patrick@harsdorf.de> wrote in message
news:c6gop3$5p7$03$1@news.t-online.com...
> I want to iterate over a collection and delete all unwanted entries.
>
> for item in collection:
> del item
>
> of course doesn´t do anything useful. Two things come to mind:
> a) iterating backwards over the collection using indexing, something like:
>
> for i in range(len(collection)-1, -1, -1):
> item = collection[i]
> if isBad(item) del collection[i]
>
> b) duplicating the collection, iterating over one and deleting from the
> other.
>
> Both solutions seem both ugly and expensive to me, solution a)
> probably isn´t even O(x) anymore.
>
> Please tell me, what´s the best way to do this?
>
>
> --
> Patrick von Harsdorf
> patrick@harsdorf.de
>
>
>
>