GNU bug report logs - #19896
23.1; reverse-region is slow compared to sort-columns

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: asparagus <at> comcast.net
Subject: bug#19896: closed (Re: bug#19896: Please modify documentation to
 close this bug)
Date: Sun, 19 Apr 2015 00:50:03 +0000

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Your bug report

#19896: 23.1; reverse-region is slow compared to sort-columns

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 19896 <at> debbugs.gnu.org.

-- 
19896: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19896
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Glenn Morris <rgm <at> gnu.org>
To: 19896-done <at> debbugs.gnu.org
Subject: Re: bug#19896: Please modify documentation to close this bug
Date: Sat, 18 Apr 2015 20:49:10 -0400

Version: 25.1

asparagus <at> comcast.net wrote:

> Add the following to the description of M-x sort-columns:
> "A numeric argument means sort into descending order."
>
> Remove "or columns" from the description of M-x reverse-region.

Thanks; done in the source. (The versions on www.gnu.org will probably
not be updated until the next Emacs release.)

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From: asparagus <at> comcast.net
To: bug-gnu-emacs <at> gnu.org
Subject: 23.1; reverse-region is slow compared to sort-columns
Date: Wed, 18 Feb 2015 17:35:42 +0000 (UTC)

The emacs manual recommends 'M-x reverse-region' as a companion to 'M-x sort-columns' to achieve a descending-order column sort.  However, for large regions (>100k lines), 'M-x reverse-region' is noticeably slower than 'M-x sort-columns', making descending sort much slower than ascending sort.

A workaround on linux: use 'C-u M-x shell-command-on-region' with tac (the opposite of cat) instead of using 'M-x reverse-region'.

FROM THE MANUAL:
https://www.gnu.org/software/emacs/manual/html_node/emacs/Sorting.html
M-x reverse-region
    Reverse the order of the lines in the region. This is useful for sorting into descending order by fields or columns, since those sort commands do not have a feature for doing that.

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