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From: Terry Reedy <tjreedy@udel.edu>
Newsgroups: comp.lang.python
Subject: Re: Mimick tac with python.
Date: Sat, 30 Jan 2016 04:38:54 -0500
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On 1/30/2016 1:03 AM, Christian Gollwitzer wrote:
> 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?

One could read the file by lines and make a list of start-of-line 
positions.  Reverse the list. Read each line.  Details omitted.

-- 
Terry Jan Reedy