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Groups > comp.lang.python > #102289
| From | Christian Gollwitzer <auriocus@gmx.de> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: Mimick tac with python. |
| Date | 2016-01-30 10:23 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <n8hv9c$g2s$1@dont-email.me> (permalink) |
| References | <n8hf7c$lqe$1@aspen.stu.neva.ru> <mailman.111.1454129921.2338.python-list@python.org> <n8hjis$gte$1@dont-email.me> <lf5vb6blcu8.fsf@ling.helsinki.fi> |
Am 30.01.16 um 08:56 schrieb Jussi Piitulainen:
> Christian Gollwitzer writes:
>
>> Am 30.01.16 um 05:58 schrieb Random832:
>>> On Fri, Jan 29, 2016, at 23:46, Hongyi Zhao wrote:
>>>> awk '{a[NR]=$0} END {while (NR) print a[NR--]}' input_file
>>>> perl -e 'print reverse<>' input_file
>>>
>>> Well, both of those read the whole file into memory - tac is sometimes
>>> smarter than that, but that makes for a more complex program.
>>
>> Now I'm curious. How is it possible to output the first line as last
>> again if not by remembering it from the every beginning? How could tac
>> be implemented other than sucking up everything into memory?
>
> It may be possible to map the data into virtual memory so that the
> program sees it as an array of bytes. The data is paged in when
> accessed. The program just scans the array backwards, looking for
> end-of-line characters. I believe they can be identified reliably, as
> bytes, even in a backward scan of UTF-8-encoded data.
>
> The data needs to be in a file.
If it's in a file, then I agree. I was thinking about the case where tac
is used in a pipe - obviously here you can't reverse the file in
constant memory.
Christian
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Mimick tac with python. Hongyi Zhao <hongyi.zhao@gmail.com> - 2016-01-30 04:46 +0000
Re: Mimick tac with python. Random832 <random832@fastmail.com> - 2016-01-29 23:58 -0500
Re: Mimick tac with python. Christian Gollwitzer <auriocus@gmx.de> - 2016-01-30 07:03 +0100
Re: Mimick tac with python. Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-01-30 09:56 +0200
Re: Mimick tac with python. Christian Gollwitzer <auriocus@gmx.de> - 2016-01-30 10:23 +0100
Re: Mimick tac with python. Peter Otten <__peter__@web.de> - 2016-01-30 09:21 +0100
Re: Mimick tac with python. Terry Reedy <tjreedy@udel.edu> - 2016-01-30 04:38 -0500
Re: Mimick tac with python. Hongyi Zhao <hongyi.zhao@gmail.com> - 2016-01-30 06:18 +0000
Re: Mimick tac with python. Chris Angelico <rosuav@gmail.com> - 2016-01-30 15:56 +1100
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