Re: Sort directories by date

[Laurent_R]

Although it may not matter that much (depending on how many subdirectories you have in your root directory), sorting the whole list just to get the most recent item is somewhat inefficient, even with a fast sorting algorithm and using Schwartzian transform or Guttman-Rosler transform, because it requires the computer to do much more work than what is actually needed.

I usually would not care that much about that with a short list of subdirectories, but it sometimes matter that there are more efficient algorithms to pick up the latest (or largest, or smallest, whatever) element in a list.

For example, something like this at the command line (quick test):

$ perl -e '
> my @list = qw/12112014
> 01052015
> 02202015
> 03102015
> 01012011
> 10102014
> 04092015
> 09092015
> 09092013/;
> chomp @list;
> my $max_y = "0000";
> my $max_d = "0000";
> for my $dir  (@list) {
>      my ($d, $y) = $dir =~ /(\d{4})(\d{4})/;
>      if ($y > $max_y) {
>          $max_y = $y;
>      } elsif ($y == $max_y) {
>          $max_d = $d if $d > $max_d;
>     }
> }
> print "$max_d$max_y\n";
> '
09092015

This may look slightly more complex, but it is more efficient for a long list of directories. Which is why I would care only if the list is long.

[AnomalousMonk]

It just occurred to me that a GRTish, guaranteed-single-pass solution is possible:

c:\@Work\Perl\monks>perl -wMstrict -le
"use List::Util qw(maxstr);
 ;;
 my @dates = qw(
   12112014
   12012014
   01052015
   12202014
   12022014
   02202015
   03102015
   01012011
   09092015
   04092015
   );
 ;;
 my $most_recent =
   unpack 'x4a*',
   maxstr
   map pack('a4a*', unpack('x4a4', $_), $_),
   @dates
   ;
 ;;
 print $most_recent;
"
09092015

Use  minstr for least-recent date. See the core module List::Util. No efficiency/performance testing done nor claims made. (Update: Actually, I'd be surprised if there's any advantage unless you're dealing with really large lists; default sort (with no subroutine block) is pretty fast!)

Update: See also pack, unpack, perlpacktut.

[Laurent_R]

Yes, it is a fairly nice way of doing it.++

I had also been thinking about some similar form of GR-like transform (though I had not thought about using the maxstr function of List::Until), but I finally preferred to make my algorithmic point with a simple basic straight-forward and manual search of the maximum date.

I also agree that the various ways of doing that have little consequence on performance unless the list is really very long.