[9fans] text utility

[9fans] text utility

[pac]

Hi, friends,

I've put a small text utility "tac" (meaning "reverse cat", i.e., write out
lines with reverted order of chars) onto the neXt Archive
ftp://ftp.gli.cas.cz/gli/cejchan/neXt/text/

It may look like toy but I use it when genes are transcribed from the
complementary DNA string. If somebody would like to teach it runes,
(s)he is welcome!

Enjoy, if you can!

++pac.



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Re: [9fans] text utility

[boyd, rounin]

err, what happened to rev(1)?

Re: [9fans] text utility

[rog]

> err, what happened to rev(1)?

different thing.
line vs. file

has anyone actually used rev(1) in seriousness before?

Re: [9fans] text utility

[boyd, rounin]

> different thing.
> line vs. file

    i.e., write out lines with reverted order of chars) onto the neXt Archive

i read that as 'reverse the order of the chars on each line'.

    http://www.gsp.com/cgi-bin/man.cgi?section=1&topic=rev

    The rev utility copies the specified files to the standard output,
    reversing the order of characters in every line

> has anyone actually used rev(1) in seriousness before?

yes, iirc, someone at the labs used it to generate a dictionary
for anagrams or crosswords with some:

    rev | ... | rev

construction.

Re: [9fans] text utility

[rog]

> i read that as 'reverse the order of the chars on each line'.

looking at the original... yes. so not really "reverse cat" at all.

BTW pac, i can't access your ftp site;
connection works ok, but (passive-mode) directory
listing just hangs under all the clients i've tried.

Re: [9fans] text utility

[Geoff Collyer]

I'm pretty sure it was Doug McIlroy who used

	rev <dict | sort >rhymdict

to produce a rhyming dictionary.  To find rhyming words, you'd reverse
the suffix of a word, look that up and reverse the result:

	look `{echo suffix | rev} rhymdict | rev

This is described in a memo called `Webster's 2nd on the head of a
pin' and dates from the mid-1970s.  (I have a copy somewhere.)

As for tac,

	tail -1000000000r

does the job for most files of interest (those under a billion lines).
I just tried it on a file of ~250,000 lines; it's quite quick.
Alternatively,

	awk '{print NR, $0}' | sort -nr | sed 's/^[0-9]+ //'

does the job more slowly.

Re: [9fans] text utility

[rog]

> As for tac,
>
> 	tail -1000000000r

interesting... i'd imagined that tac would send all
the *bytes* (or maybe characters) in a file to stdout,
in reverse.

that's three (or four) possible definitions for tac so far.
i wonder how many others there might be.

Re: [9fans] text utility

[Dan Cross]

Why not use the definition of the tac utility that used to be
distributed with BSD Unix?  I think it printed out lines in reverse
order (similar to tail -r).  Something to print out all the bytes in a
file in reverse order might be useful, though.  Currently, I can't
think why.

	- Dan C.

Re: [9fans] text utility

[matt]

Dan Cross wrote:

>Why not use the definition of the tac utility that used to be
>distributed with BSD Unix?  I think it printed out lines in reverse
>order (similar to tail -r).  Something to print out all the bytes in a
>file in reverse order might be useful, though.  Currently, I can't
>think why.
>
>	- Dan C.
>

to see what interesting UTF-8 comes out of course

Re: [9fans] text utility

[Ralph Corderoy]

Hi Geoff,

>	awk '{print NR, $0}' | sort -nr | sed 's/^[0-9]+ //'

That's what I use, except `cat -n' instead of having awk deal with each
line.

Cheers,

--
Ralph Corderoy.      http://inputplus.co.uk/ralph/     http://troff.org/

Re: [9fans] text utility

[Geoff Collyer]

Of course "cat -n" is almost as evil as "cat -v" (-v has a
sublanguage) and doesn't work on Plan 9:

	cpu% cat -n
	cat: can't open -n: '-n' directory entry not found

One could use "pr -tn" instead of the little awk script, but the
difference in speed really isn't that great.  Reading a cached ~2.5MB
file of ~250K lines,

cpu% time awk '{print NR, $0}' test >/dev/null
2.85u 0.13s 3.53r 	 awk {print NR, $0} test
cpu% time pr -tn test >/dev/null
1.75u 0.13s 2.18r 	 pr -tn test

Re: [9fans] text utility

[Ralph Corderoy]

Hi Geoff,

> One could use "pr -tn" instead of the little awk script, but the
> difference in speed really isn't that great.  Reading a cached ~2.5MB
> file of ~250K lines,
>
> cpu% time awk '{print NR, $0}' test >/dev/null
> 2.85u 0.13s 3.53r 	 awk {print NR, $0} test
> cpu% time pr -tn test >/dev/null
> 1.75u 0.13s 2.18r 	 pr -tn test

Fair enough.  Under AIX and Linux cat's a win.  Using yes(1) so awk's
little work to do:

    $ yes y | head -n 250000 | time awk '{print NR, $0}' >/dev/null
    real 1.13
    user 1.06
    sys 0.01
    $ yes y | head -n 250000 | time cat -n >/dev/null
    real 0.08
    user 0.05
    sys 0.00

I used it heavily when doing ad-hoc queries on files with millions of
lines.  But, like you say, if it's not present, it's no use.

Cheers,

--
Ralph Corderoy.      http://inputplus.co.uk/ralph/     http://troff.org/