No, you are right and I am wrong!

If the data you need for sorting is there in front of you with no need for any "mangling", the "straightforward sort block" is simpler to write than the Schwartzian Transform. I was confusing the tranforming done as a performance measure with other transformations to the data that are necessary because it is not yet in a sortable form.

If you do need to change the data in some way before the sort can take place then the Schwartzian Transform starts to gain in the straighforwardness factor, I think. The sort block method gains in readability because you can use meaningful variable names rather than $_; the Schwartzian Transform gains because the flow seems to me to be more obvious, albeit up the page which is counter-intuitive if you are used to piping commands in the shell. I tend to comment what is going on in the transform for those that are not familiar although I haven't below.

The examples I give here sort the output of df -k by controller, target, device and slice. First, the "straightforward sort block". 

open DF, "df -k |" or die "$!\n";
print scalar <DF>;

while(<DF>)
{
    next unless m{^/dev/dsk/c(\d+)t(\d+)d(\d+)s(\d+)};
    push @lines, [$_, $1, $2, $3, $4];
}

close DF;

@sorted = sort {
    $a->[1] <=> $b->[1] ||
    $a->[2] <=> $b->[2] ||
    $a->[3] <=> $b->[3] ||
    $a->[4] <=> $b->[4]}
    @lines;

print $_->[0] for @sorted;

[download]
Now the Schwartzian Transform. 
open DF, "df -k |" or die "$!\n";
print scalar <DF>;

print
    map {$_->[0]}
    sort {
       $a->[1] <=> $b->[1] ||
       $a->[2] <=> $b->[2] ||
       $a->[3] <=> $b->[3] ||
       $a->[4] <=> $b->[4]
    }
    grep {defined $_->[1]}
    map {[
       $_,
       m{^/dev/dsk/c(\d+)t(\d+)d(\d+)s(\d+)}
    ]}
    <DF>;

close DF;

[download]
I think there are arguments in favour of either method. I have got used to Schwartian Transforms so I automatically reach for those when sorting problems come up.

Cheers,

JohnGG

In reply to Re^8: top ten things every Perl hacker should know by johngg
in thread top ten things every Perl hacker should know by apotheon

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