Dear Co-Monks,

I did hit a brick wall recently when I was working through Learning Perl Objects, References & Modules and wanted to use sort in a sub.

In chapter 4 the exercise was to sort a hash with network traffic data twice... which shouldn't be a big problem.

I tried to write the code with a sort sub and failed miserably... The code:

        #!/usr/local/bin/perl
        
        use warnings;
        use diagnostics;
        
        use strict;
        use Data::Dumper;
        
        my %data;
        my $data_file = '/users/oliversti/documents/development/perl/a
+lpaca book/alpaca_files/ch04/coconet.dat';
        my $all = '** all machines combined **';
        
        open FH, "$data_file" or
            die "Couldn't open $data_file: $!\n";
        
        while (<FH>)
        {
            next if (/^#/);
            my ($source, $destination, $bytes) = split;
            $data{$source}{$destination} += $bytes;
            $data{$source}{$all} += $bytes;
        }
        
        close FH or
            die "Couldn't close $data_file: $!\n";
        
        sub traffic_combined
        {
            $data{$b}{$all} <=> $data{$a}{$all}
        }
        
        for my $source (sort traffic_combined keys %data)
        {    
            print "$source data transfer out: $data{$source}{$all}.\n"
+;
            sub traffic_machine
            {
                $data{$source}{$b} <=> $data{$source}{$a}
            }
            
            for my $destination (sort traffic_machine keys %{$data{$so
+urce}})
            {
                next if ($destination eq $all);
                print "$source -> $destination: $data{$source}{$destin
+ation}.\n";
            }
        }
[download]

I couldn't understand why the second sort ('traffic_machine') didn't work. I debugged the script and could print the values of $a and $b. So everything seemed fine... until it hit me: By using a sub the %data and $source variables were out of scope.

After trying to pass the hash and scalar into the sub, which failed miserably I came up with two solutions. Or better said a hack and a solution.

The hack: Make the variables that I want to access global by using OUR.

The solution: Using the code block of the subs in-line as shown below:

        for my $source (sort {$data{$b}{$all} <=> $data{$a}{$all}} key
+s %data)
        {    
            print "$source data transfer out: $data{$source}{$all}.\n"
+;
        
            for my $destination (sort {$data{$source}{$b} <=> $data{$s
+ource}{$a}} keys %{$data{$source}})
            {
                next if ($destination eq $all);
                print "$source -> $destination: $data{$source}{$destin
+ation}.\n";
            }
        }
[download]

My final questions to the monks is: Is there a way that I overlooked or don't know that allows me to pass variables (next to $a and $b) into a sort sub?

thanks
/oliver/

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