how to loop

rsiedl has asked for the wisdom of the Perl Monks concerning the following question:

Hi monks,

I've written a script which contains a loop, and i was wondering if i am doing it the right way, or if there could be a better (more efficient) way?

Here's the loop code:

loop:

# Get a random background from the array
my $new = $backs[int(rand(@backs))];

# Set the background to something new
system($command . ' --type=string -s '. $gconf_item . ' \'' . $new . '
+\'');

if ($mode eq "running") {
  sleep($interval*60);
  goto("loop");
} # end-if
[download]

Here's the script for anyone who's interested:

#!/usr/bin/perl

use strict;
use warnings;

use XML::Simple;

srand;

# Mode to run
#my $mode = "startup";
my $mode = "running";

# Interval to change at if $mode eq "running"
my $interval = 60;

# gconf vars
my $command = "gconftool-2";
my $gconf_item = "/desktop/gnome/background/picture_filename";

# The location of the background xml file
my $xmlfile = $ENV{HOME} . "/.gnome2/backgrounds.xml";

# Find the current background so we dont change to this
my $current = `$command -g $gconf_item`;

# Read in the xml file contents to get our list of backgrounds
my $config = XMLin($xmlfile);

# Something to store our filenames in
my @backs;
foreach my $name (keys %{$config->{wallpaper}}) {

  # Skip any that have a deleted flag in the XML file
  next if ($config->{wallpaper}->{$name}->{deleted} eq "true");

  # Set the filename
  my $file = $config->{wallpaper}->{$name}->{filename};

  # Ignore the current background and the (none) background
  next if (($file eq $current) || ($file eq "(none)"));

  # Store the name in our array
  push(@backs, $file);

} # end-foreach

loop:

# Get a random background from the array
my $new = $backs[int(rand(@backs))];

# Set the background to something new
system($command . ' --type=string -s '. $gconf_item . ' \'' . $new . '
+\'');

if ($mode eq "running") {
  sleep($interval*60);
  goto("loop");
} # end-if

exit;
[download]

Thanks for your help.

Cheers,
Reagen

Comment on how to loop Select or Download Code

Replies are listed 'Best First'.
Re: how to loop by Zaxo (Archbishop) on Feb 21, 2007 at 02:14 UTC
How about this? `while ($mode eq 'running') { my $new = $backs[int(rand(@backs))]; system($command . ' --type=string -s '. $gconf_item . ' \'' . $new + . '\''); sleep($interval*60); }` [download] That's not exactly equivalent to your code, but all sorts of adjustments are possible After Compline, Zaxo	[reply] [d/l]
Re^2: how to loop by rsiedl (Friar) on Feb 21, 2007 at 02:20 UTC
thanks. i guess this'll work: `while (1) { # Get a random background from the array my $new = $backs[int(rand(@backs))]; # Set the background to something new system($command . ' --type=string -s '. $gconf_item . ' \'' . $new . + '\''); last if ($mode ne "running"); sleep($interval*60); } # end-while` [download]	[reply] [d/l]
Re^3: how to loop by cbrandtbuffalo (Deacon) on Feb 21, 2007 at 15:00 UTC
From a maintenance perspective, I like the previous example a bit more because you can understand the exit condition by reading the while line. Even a non-coder could understand it: while ($mode eq 'running'); keep going while the mode equals 'running'. In your example, I have to search through the entire block and find the 'last if' line to figure out how you break out of the loop. They are basically equal, but I find the first example easier to read and understand.	[reply]
Re: how to loop by muba (Priest) on Feb 21, 2007 at 04:37 UTC
... (insert thoughtful face here) ... is it just me, or does $mode never change value inside the loop in any of the snippets posted so far?	[reply]
Re^2: how to loop by rsiedl (Friar) on Feb 21, 2007 at 05:27 UTC
you're right. $mode never changes value during the running of the script. it's just set manually before running.	[reply]
Re^3: how to loop by graff (Chancellor) on Feb 21, 2007 at 07:42 UTC
Um... so if you edit the script so that $mode is set to something other than "running", the loop will execute just once, and the script exits; but if you edit the script so that $mode is set to "running", the script loops indefinitely until you kill it (^C or whatever). And that's the intended design? I would have expected that either (a) the value assigned to $mode would depend on @ARGV (or %ENV, or some other external input), or (b) there would be something inside the loop that could change the value of $mode, or else (c) each time you edit the script to change its behavior, rather than editing a variable assignment, you would just edit the loop condition -- e.g. `do {...} while(0);` vs. `do {...} while(1);`, or something to that effect. (Note that the former "do" block will execute exactly once, whereas the block in `while(0){...}` never executes.) As it is, I tend to share muba's puzzlement. (updated to fix spelling)	[reply] [d/l] [select]
Re^4: how to loop by rsiedl (Friar) on Feb 21, 2007 at 07:57 UTC
Re^5: how to loop by graff (Chancellor) on Feb 21, 2007 at 08:13 UTC
Re: how to loop by andye (Curate) on Feb 21, 2007 at 18:52 UTC
Hi Reagen, You might want to use a 'do while', which works like this: `$a=0; do { print $a++, "\n" } while ($a < 10)` [download] Because it checks the condition at the end, it'll run the loop once through even if the condition is false to begin with (try changing the condition to $a < -10 and you'll see what I mean). You can find out more in the docs under perlsyn. HTH! Best wishes, a.	[reply] [d/l]
Re^2: how to loop by geekphilosopher (Friar) on Feb 21, 2007 at 19:57 UTC
Be careful though - you can't use loop directives (next, last, redo, etc.) in a do while.	[reply]