delete items from list by indices

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blumenkraft

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Sep 23, 2009, 4:25:52 AM9/23/09
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Hi,

I have some list:
x = [8, 9, 1, 7]
and list of indices I want to delete from x:
indices_to_delete = [0, 3], so after deletion x must be equal to [9,
1].

What is the fastest way to do this? Is there any builtin?

Thanks.

Chris Rebert

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Sep 23, 2009, 4:39:39 AM9/23/09
to blumenkraft, pytho...@python.org

#untested & unbenchmarked since it's 1am
offset = 0
for index in indices_to_delete:
del x[index-offset]
offset += 1

Cheers,
Chris
--
http://blog.rebertia.com

Peter Otten

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Sep 23, 2009, 4:48:27 AM9/23/09
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blumenkraft wrote:

Why's that obsession with speed?

>>> items = ["a", "b", "c", "d"]
>>> delenda = [0, 3]
>>> for i in sorted(delenda, reverse=True):
... del items[i]
...
>>> items
['b', 'c']


>>> items = ["a", "b", "c", "d"]
>>> delenda = set([0, 3])
>>> items = [item for index, item in enumerate(items) if index not in
delenda]
>>> items
['b', 'c']

If you really need to modify the list in place change

items = [item for ...]

to

items[:] = [item for ...]

Try these and come back to complain if any of the above slows down your
script significantly...

Peter

Ishwor

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Sep 23, 2009, 5:19:01 AM9/23/09
to blumenkraft, pytho...@python.org
Hi

2009/9/23 blumenkraft <voh...@gmail.com>:

Try this-


>>> x = [8, 9, 1, 7]

>>> [x.pop(i) for i in sorted(indices_to_delete,reverse=True)]
[7, 8]
>>> x
[9, 1]

Built-in used here is `sorted' and method on list used here is `pop'.
With regards to efficiency you may want to use the methods of list
which is more intuitive afaik and useful as its more reflective of
effect on the type list. It's a trivial choice here but later it might
help.
--
Regards,
Ishwor Gurung

blumenkraft

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Sep 23, 2009, 5:30:36 AM9/23/09
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Thanks, it helped (I didn't know about keyword argument "reverse" in
sorted function)

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