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When extracting configuration data from managed network gear such as Juniper or Cisco switches, consecutive vlans are combined into ranges "from-to". Given a $list of vlans such as "2-4,10-12" I needed an @array of every single vlan. An easy way to accomplish this is to make use of Perl's ".." range operator like so:
my $list = "2-4,10-12"; $list =~ s/\-/../g; my @array = eval( $list ); # @array is now (2,3,4,10,11,12)

So far so good. What I'm looking for is an equally simple way to reverse the process. I mashed together the following code to get the job done but it's just painful to look at:

my @array = (2,3,4,10,11,12); my @vlans = (); my $span = undef; my $last = undef; foreach my $vlan (sort @array) { if (defined $last && $vlan == $last+1) { unless (defined $span) { $span = $last; } $vlans[$#vlans] = $span.'-'.$vlan; } else { push @vlans, $vlan; $span = undef; } $last = $vlan; } my $list = join(',', @vlans); # $list is now "2-4,10-12" again

Surely there must be a better way?!

Edit: Single elements should appear like "2-4,7,10-12,20". Sorry for not pointing this out.

-- Time flies when you don't know what you're doing

In reply to [Solved] Converting a list of numbers to use a range operator by FloydATC

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