Re: sort direction

[radiantmatrix]

Accoring to the documentation for sort, it expects a function that returns -1, 0, or 1 for (a<b),(a==b),(a>b), respectively. So, that's all your sub needs to do.

# note passing your sub $a and $b
foreach my $id (sort { mysort($direction,$a,$b) } keys %hash) {
  print "$id - $hash{$id}{$orderby}\n";
} # end-foreach

sub mysort {
    my ($direction, $_a, $_b) = @_;
    my $result =  
        $hash{$_a}{$orderby} <=> $hash{$_b}{$orderby}
        || $hash{$_a}{$orderby} cmp $hash{$_b}{$orderby};
    
    # default to ascending, but if the dir is 'DESC', we
    # return the *opposite* answer
    return ($direction eq 'DESC') ? -($result) : $result;
}

Untested, of course, but the idea should be basically sound. If I understand what you're trying to accomplish, though, you might take a different approach.

# we pass the sub the actual values to compare
foreach my $id (
  sort { mysort($direction,$hash{$a}{$orderby},$hash{$b}{$orderby}) } 
+keys %hash
) {
    print "$id - $hash{$id}{$orderby}\n";
} # end-foreach

sub mysort {
    my ($direction, $_a, $_b) = @_;
    # generalized, simplified comparison
    my $result = $_a <=> $_b || $_a cmp $_b;
    
    # default to ascending, but if the dir is 'DESC', we
    # return the *opposite* answer
    return ($direction eq 'DESC') ? -($result) : $result;
}

[Roger_B]

Accoring to the documentation for sort, it expects a function that returns -1, 0, or 1 for (a<b),(a==b),(a>b), respectively. So, that's all your sub needs to do.

Actually, according to your link:

SUBNAME ... gives the name of a subroutine that returns an integer less than, equal to, or greater than 0

which is rather less stringent than requiring -1, 0 or 1.

Roger

[radiantmatrix]

You are correct, if pedantic. :-) Just out of curiosity, under what circumstances would it be easier or faster to return something other than -1,0,1 ?

[BrowserUk]

Well, using sort{$b-$a} @numbers rather than sort{$b<=>$a} @numbers seems to run quite a bit more quickly and produce the desired result:

perl> cmpthese -1, {
    '<=>' => q[@s1=sort{$b<=>$a} 1..10000], 
    '-'   => q[@s2=sort{$b - $a} 1..10000] 
};;
       Rate  <=>    -
 <=> 33.8/s   -- -82%
-     183/s 441%   --

There are probably good reasons why this shoudln't be done outside of golf though.