in reply to [perlre/perlgolf] Golf this: return -1 or return +1 on regexp subroutine please

I'm not sure what algorithm sort uses, but in general, it's a bad idea not to implement identity whenever you write a comparision function.

In your example, the function will always return -1 for the three sets of identical pairs - something that will result in interesting sort orders, depending on the algorithm used.

You should at least rewrite the routine like this :

sub by_priority {
  # Return 0 on string identity
  return 0 if $a eq $b;

  return -1 if $a =~ /^Organization/ ;
  return  1 if $b =~ /^Organization/ ;
  return -1 if $a =~ /^Service/ ;
  return  1 if $b =~ /^Service/ ;
  return -1 if $a =~ /^FAQ/ ;
  return  1 if $b =~ /^FAQ/ ;

  return  0 ;

}

[download]

My personal favourite when rewriting that code would be via grep :

my @priorities = qw(Organization FAQ Service);
my $prio_top = join "|", map { "^$_" } @priorities;
sub by_priority {
  # Return 0 on string identity
  return 0 if $a eq $b;

  return -1 if $a =~ /$prio_top/o;
  return  1 if $b =~ /$prio_top/o;

  # Alphabetical sort for the rest
  return $a cmp $b;
}

[download]

If you're concerned with raw sort speed (for many values), a Guttman-Rosler-Transform would be the best thing - you simply encode the priorities together with each item in a string, let sort rip through it and unpack it afterwards.

If you're not tied to your current algorithm, it might be a good idea to first partition your dataset into the four categories, indivitually sort them and afterwards simply concatenate them together.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web

[download]

Replies are listed 'Best First'.
Re: Re: [perlre/perlgolf] Golf this: return -1 or return +1 on regexp subroutine please
by boo_radley (Parson) on Oct 31, 2002 at 21:43 UTC

    Corion,
    the algorithm in your sort sub doesn't do what princepawn wants : put 'Organization' before 'Services' before 'FAQ' (and not care about anything else...) .
    In fact, if you're sorting a list based only on these, the function will always return 1, because the first match will always be true.
    I think the best you could reduce this to (excluding the transform you mentioned, which I'm unfamiliar with) would be a foreach loop, like so (shows pp's implementation, yours and mine) : 

    #! perl -l
my @foo =( "Service", "Organization", "FAQ");

my @priorities = qw(Organization FAQ Service);
my $prio_top = join "|", map { "^$_" } @priorities;

print join " ", sort by_priority @foo;
print join " ", sort boo @foo;
print join " ", sort corion @foo;
sub by_priority {

  return -1 if $a =~ /^Organization/ ;
  return  1 if $b =~ /^Organization/ ;
  return -1 if $a =~ /^Service/ ;
  return  1 if $b =~ /^Service/ ;
  return -1 if $a =~ /^FAQ/ ;
  return  1 if $b =~ /^FAQ/ ;

  return  0 ;

}

sub boo {
    foreach ("Organization", "Service", "FAQ") {
        return 1 if $b=~/^\Q$_\E/;
        return -1 if $a=~/^\Q$_\E/;
    }
    return 0;
}

sub corion {
  # Return 0 on string identity
  return 0 if $a eq $b;

  return -1 if $a =~ /$prio_top/o;
  return  1 if $b =~ /$prio_top/o;

  # Alphabetical sort for the rest
  return $a cmp $b;
}

    [download]

[reply]
[d/l]

      Ooops - yes, my code does an unstable sort on the categories, because the order of the bail-out is different from what princepawn wanted. I wanted to introduce independence from the order of the input values for the normal strings, and lost it on the categories ...

      perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web

      [download]
[reply]
[d/l]

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