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Groups > comp.lang.python > #77849

Re: Iterator, modify data in loop body

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On Sun, Sep 14, 2014 at 1:27 AM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Sat, Sep 13, 2014 at 1:39 AM, Michael Welle <mwe012008@gmx.net> wrote:
>>> In that case, don't iterate over the list at all. Do something like this:
>>>
>>> while lst:
>>>     element = lst.pop(0)
>>>     # work with element
>>>     lst.append(new_element)
>>>
>>> There's no mutation-while-iterating here, and it's clear that you'll
>>> keep going until there's absolutely nothing left.
>> Ah, that looks like a good approach, thank you.
>
> Also note that this approach (appending to the end and popping from
> the front) will be more efficient using a collections.deque than a
> list.

Sure it will - that's kinda the point of a double-ended queue, to
avoid all the inefficient movements :) But the concept is still the
same: do repeated mutations rather than iteration. Either that, or
iterate and build up a new list, either with a comprehension or with
something like this:

newlst = []
for element in lst:
    # work with element
    newlst.append(new_element)

ChrisA

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Iterator, modify data in loop body Michael Welle <mwe012008@gmx.net> - 2014-09-13 08:09 +0200
  Re: Iterator, modify data in loop body Chris Angelico <rosuav@gmail.com> - 2014-09-13 16:22 +1000
    Re: Iterator, modify data in loop body Michael Welle <mwe012008@gmx.net> - 2014-09-13 09:01 +0200
      Re: Iterator, modify data in loop body Chris Angelico <rosuav@gmail.com> - 2014-09-13 17:22 +1000
        Re: Iterator, modify data in loop body Michael Welle <mwe012008@gmx.net> - 2014-09-13 09:39 +0200
          Re: Iterator, modify data in loop body Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-13 09:27 -0600
            Re: Iterator, modify data in loop body Michael Welle <mwe012008@gmx.net> - 2014-09-13 18:58 +0200
          Re: Iterator, modify data in loop body Chris Angelico <rosuav@gmail.com> - 2014-09-14 01:32 +1000
        Re: Iterator, modify data in loop body Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2014-09-16 10:49 +0200
          Re: Iterator, modify data in loop body Chris Angelico <rosuav@gmail.com> - 2014-09-17 01:19 +1000
  Re: Iterator, modify data in loop body Peter Otten <__peter__@web.de> - 2014-09-13 09:34 +0200
    Re: Iterator, modify data in loop body Michael Welle <mwe012008@gmx.net> - 2014-09-13 09:55 +0200

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