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Full Discussion: Reversing numbers in a datafile of rows and columns
Top Forums Shell Programming and Scripting Reversing numbers in a datafile of rows and columns Post 302305290 by drl on Wednesday 8th of April 2009 12:23:56 PM
Old 04-08-2009
Hi.

The original tac reverses tokens separated by spaces. The last token on each line is "number-followed-by-newline". When the tokens are reversed, the last token is "24-newline", and so on.

The solutions posted are fine. Here is a compact perl solution:
Code:
#!/usr/bin/env bash

# @(#) s1       Demonstrate reversal of symbols on lines (perl).

echo
set +o nounset
LC_ALL=C ; LANG=C ; export LC_ALL LANG
echo "Environment: LC_ALL = $LC_ALL, LANG = $LANG"
echo "(Versions displayed with local utility \"version\")"
version >/dev/null 2>&1 && version "=o" $(_eat $0 $1) tac perl
set -o nounset
echo

FILE=${1-data1}

echo " Data file $FILE:"
cat $FILE

echo
echo " Original results (trailing newline added):"
tac --separator=" " $FILE
echo

echo
echo " Re-written results:"
tac $FILE |
perl -n -e 'chomp;print join(" ",reverse(split(/ /))),"\n";'

exit 0

producing:
Code:
% ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.11-x1, i686
Distribution        : Xandros Desktop 3.0.3 Business
GNU bash 2.05b.0
tac (coreutils) 5.2.1
perl 5.8.4

 Data file data1:
01 02 03 04 05 06
07 08 09 10 11 12
13 14 15 16 17 18
19 20 21 22 23 24

 Original results (trailing newline added):
24
23 22 21 20 18
19 17 16 15 14 12
13 11 10 09 08 06
07 05 04 03 02 01

 Re-written results:
24 23 22 21 20 19
18 17 16 15 14 13
12 11 10 09 08 07
06 05 04 03 02 01

The heart of the operation: after the lines are reversed by tac, process them with a perl script. The easy-to-read-but-not-so-easy-to-understand perl code stacks a number of functions: split the line into an array of tokens, reverse the content of the array (and thus the tokens), join the array with a space as separator into a string, print the string with a newline at the end. The "-n" says to do that series of operations for each line ... cheers, drl
 

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LEARN ABOUT PLAN9

tac

TAC(1)                                                             User Commands                                                            TAC(1)

NAME

       tac - concatenate and print files in reverse

SYNOPSIS

       tac [OPTION]... [FILE]...

DESCRIPTION

       Write each FILE to standard output, last line first.

       With no FILE, or when FILE is -, read standard input.

       Mandatory arguments to long options are mandatory for short options too.

       -b, --before
              attach the separator before instead of after

       -r, --regex
              interpret the separator as a regular expression

       -s, --separator=STRING
              use STRING as the separator instead of newline

       --help display this help and exit

       --version
              output version information and exit

AUTHOR

       Written by Jay Lepreau and David MacKenzie.

REPORTING BUGS

       GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
       Report tac translation bugs to <http://translationproject.org/team/>

COPYRIGHT

       Copyright (C) 2017 Free Software Foundation, Inc.  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
       This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to the extent permitted by law.

SEE ALSO

       rev(1)

       Full documentation at: <http://www.gnu.org/software/coreutils/tac>
       or available locally via: info '(coreutils) tac invocation'

GNU coreutils 8.28                                                 January 2018                                                             TAC(1)
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