Is there a convenient way to reverse lines in a region (like tac
command does)?
reddit / emacs

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brightlystar · 2022-09-09 10:32

Is there an easy way to reverse lines in a region? Like if I have a
buffer with this text

    a
    b
    c
    d
    e

and I select a region consisting of the middle 3 lines and do the
reverse lines thing, I should get a buffer like this

    a
    d
    c
    b
    e

Possible?

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[deleted] · 2022-09-09 10:45

[deleted]

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lindyhopdreams · 2022-09-09 10:51

M-x reverse-region

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[deleted] · 2022-09-09 10:51

[deleted]

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redguardtoo · 2022-09-09 13:25

See `sort-lines` doc,

```
sort-lines is an autoloaded, interactive and natively compiled
function defined in sort.el.

Signature
(sort-lines REVERSE BEG END)

Documentation
Sort lines in region alphabetically; REVERSE non-nil means descending
order.

Interactively, REVERSE is the prefix argument, and BEG and END are the
region.
Called from a program, there are three arguments:
REVERSE (non-nil means reverse order), BEG and END (region to sort).
The variable sort-fold-case determines whether alphabetic case affects
the sort order.
```

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pwnedary · 2022-09-09 14:31

If using Evil: `'<,'>g/^/m'<-`

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jplindstrom · 2022-09-09 15:36

Relatedly, the other day I had to sort the contents of a single line
string that had comma separated values, like so:

```
jkl,ghi,abc,def
```

And I had to muck about recording a macro to replace `,` with newlines
etc. etc. so I could use `sort-lines`.

Is there perchance a function for what I needed?

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Illiamen · 2022-09-09 15:55

Try `query-replace` (`M-%`) and `query-replace-regexp` (`C-M-%`). Use
`C-q C-j` to insert the newline in the minibuffer. You can higlight a
region and use `!` to replace all matches in the region. Otherwise, it
starts replacing at point.

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jplindstrom · 2022-09-09 16:00

Yeah I already did it that way, I'm asking about an existing Emacs
function to sort columns of a row or similar.

(which I did search for at some length, but didn't find anything)

Shout-out to the tip about `C-q C-j` though, that's a very unintuitive
way to search-and-replace newlines, so good to highlight.

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jplindstrom · 2022-09-09 16:22

Should have Ducked it. Just found `sort-symbols` which seems to do the
trick!

https://www.emacswiki.org/emacs/SortWords

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Kwisacks · 2022-09-09 18:07

or just `'<,'>sort!`

Evil's fidelity is impressive.  I guess Emacs already had all the
infrastructure in place (look at the `reverse-region` and `sort-lines`
commandsin the other comments.

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harizvi · 2022-09-09 22:12

For similar scenarios I've found shell-command-on-region by itself,
and with prefix arg very handy.

In this case, it will be a simple C-u M-| tac <enter>

By itself, the output goes to a temp buffer, so you can build the
command that does what you need. You can also use a series of pipes
(just like you would in a shell). For example, you can use the
following to add a line number:    tac | awk '{print NR, $0}'

Once your command is final, you can use the prefix arg to replace the
region with the output.

Similarly, the question above about sorting comma-separated fields can
be done using a pipeline of:   tr ',' '\\n'| sort | tr '\\n' ','

It was the first Emacs magic I learnt those decades ago, and have used
it hundreds of times over the years :-)

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Patrick-Poitras · 2022-09-10 02:31

M-x butterfly moment