iterating over collection, deleting entries

[Patrick von Harsdorf]

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
if isBad(item) del collection

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]

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

 

[Andrea Griffini]

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

 

[Larry Bates]

I would do:

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

Larry Bates
Syscon, Inc.