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Top Forums Shell Programming and Scripting Avoiding For Loop in Shell Script Post 302548589 by hydrabane on Thursday 18th of August 2011 08:45:17 PM
Old 08-18-2011
Avoiding For Loop in Shell Script

I am looking to a solution to the following problem. I have a very large file that looks something like this:

Quote:
0.111 0.111 0.788 0.101 0.800 0.099 0.500 0.255 0.245
0.234 0.675 0.091 0.100 0.088 0.812 0 0 1
Each group of three numbers on each line are three probabilities that sum to one.

I want to output the maximum for each group of three. So desired output would be:

Quote:
0.788 0.800 0.500
0.675 0.812 1
or

Quote:
0.788
0.800
0.500
0.675
0.812
1
I've written the following kludge which works just fine on a small subset of this data but does not scale up well (because it involves an ugly triple nested for loop):

Code:
#!/bin/sh
length=$(awk '{n++}END{print n}' file )
width=$(awk 'NR == 1 { print NF }' file )
width=$( expr $width / 3 )
 
for ((i=1; i <= length; i++))
do
for ((j=1; j <= width; j++))
do
a=$(expr 1 + \( \( $j - 1 \) \* 3 \) )
b=$(expr $a + 2)
awk -v i=$i -v a=$a -v b=$b 'NR == i { for (k = a; k <= b; k++ )if ( $k > max) max = $k } END { print max}' file >> out
done
done

Does anyone know of a solution that will scale up well? Can it be accomplished with basic unix utilities/shell scripting?

Thanks in advance!
 

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SHELL-QUOTE(1p) 					User Contributed Perl Documentation					   SHELL-QUOTE(1p)

NAME
shell-quote - quote arguments for safe use, unmodified in a shell command SYNOPSIS
shell-quote [switch]... arg... DESCRIPTION
shell-quote lets you pass arbitrary strings through the shell so that they won't be changed by the shell. This lets you process commands or files with embedded white space or shell globbing characters safely. Here are a few examples. EXAMPLES
ssh preserving args When running a remote command with ssh, ssh doesn't preserve the separate arguments it receives. It just joins them with spaces and passes them to "$SHELL -c". This doesn't work as intended: ssh host touch 'hi there' # fails It creates 2 files, hi and there. Instead, do this: cmd=`shell-quote touch 'hi there'` ssh host "$cmd" This gives you just 1 file, hi there. process find output It's not ordinarily possible to process an arbitrary list of files output by find with a shell script. Anything you put in $IFS to split up the output could legitimately be in a file's name. Here's how you can do it using shell-quote: eval set -- `find -type f -print0 | xargs -0 shell-quote --` debug shell scripts shell-quote is better than echo for debugging shell scripts. debug() { [ -z "$debug" ] || shell-quote "debug:" "$@" } With echo you can't tell the difference between "debug 'foo bar'" and "debug foo bar", but with shell-quote you can. save a command for later shell-quote can be used to build up a shell command to run later. Say you want the user to be able to give you switches for a command you're going to run. If you don't want the switches to be re-evaluated by the shell (which is usually a good idea, else there are things the user can't pass through), you can do something like this: user_switches= while [ $# != 0 ] do case x$1 in x--pass-through) [ $# -gt 1 ] || die "need an argument for $1" user_switches="$user_switches "`shell-quote -- "$2"` shift;; # process other switches esac shift done # later eval "shell-quote some-command $user_switches my args" OPTIONS
--debug Turn debugging on. --help Show the usage message and die. --version Show the version number and exit. AVAILABILITY
The code is licensed under the GNU GPL. Check http://www.argon.org/~roderick/ or CPAN for updated versions. AUTHOR
Roderick Schertler <roderick@argon.org> perl v5.8.4 2005-05-03 SHELL-QUOTE(1p)
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