Re: [FWP] Formatting integer sets with ranges

[Ofer Inbar <cos@polyamory.org>]

Rick <rklement@pacbell.net> wrote:
> 
> Peter Scott wrote:
> > 
> > $_ = '1,2,3,5,6,7,8,9,10,11,12,13,14,16,17,19,20,21';
> > 
> > s/(\d+),\d+,(\d+)/$2==$1+2?"$1-$2":$&/eg;
> > s/-(\d+),(\d+)/$2==$1+1?"-$2":$&/eg;
> > 1 while s/-\d+(-\d)/$1/g;
> > 
> > print "$_\n";
> > 
> > 1-3,5-14,16,17,19-21
> > 
> > Has the property of not hyphenating itmes differing only by 1, a pet peeve
> > of mine.
> 
> 1,2,3,4,5,8,9,10 => 1-4,5,8,9,10   ????
> 
> That three-at-a-time probably needs a "1 while ..."

I think all you need to do is run the second substitution twice.  The
problem is that putting a /g on that substitution isn't good enough
because wherever it does succeed, the next number it needs to start
matching at on the next iteration, to be correct, is the thing it just
substituted in, and /g (for obvious reasons) will skip that and start
one element later in the list.  But at most it will skip one step of
"dashifying" at a time, and correctly "dashify" the next pair.  So if
you run it a second time, you catch everything it missed the first run.

Putting a "1 while" on either of the /eg substitutions won't work,
because part of the logic of whether or not to alter the string is in
the substitution part.  The match part of these s///eg's will *always*
match something in the list, as long as the list has more than one
number in it, even if there's nothing left to alter.  So a while will
never terminate.

sub hyphenatedlist
{ local($_) = join(',', sort @_);

  s/(\d+),\d+,(\d+)/$2==$1+2?"$1-$2":$&/eg;
  s/-(\d+),(\d+)/$2==$1+1?"-$2":$&/eg;
  s/-(\d+),(\d+)/$2==$1+1?"-$2":$&/eg;
  1 while s/-\d+(-\d)/$1/g;

  return $_;
}

  --  Cos (Ofer Inbar)           --  cos@polyamory.org  cos@wbrs.org
  --  Exodus Professional Services           --  cos@ne.cohesive.com
  "OSI is a beautiful dream, and TCP/IP is living it!"
   -- Einar Stefferud <Stef@nma.com>, IETF mailing list, 12 May 1992

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