RE: [Omaha.pm] Sort quickie

["Miller, Scott L (Omaha Networks)" <scott.l.miller@hp.com>]

Very cool! This thread has taught me about the map function.  I'd pretty much either ignored or glossed over ever reading about them before.

And as a bonus, I found out about the ST, GRT topics!  mmm, mmm, good...

Thanks!

-Scott

-----Original Message-----
From: omaha-pm-bounces@mail.pm.org
[mailto:omaha-pm-bounces@mail.pm.org]On Behalf Of Hugh Jarce
Sent: Thursday, August 05, 2004 6:22 PM
To: Perl Mongers of Omaha, Nebraska USA; Perl Mongers of Omaha, Nebraska
USA
Subject: Re: [Omaha.pm] Sort quickie


Jay Hannah <jay@jays.net>:
> -ponder-  I need to study Benchmark. Looks cool. What's the point of
> @x? Can
> 
>     sub j9 { my @x = sort by_date @dates }
> 
> be written as
> 
>     sub j9 { sort by_date @dates }
> ?

No. Benchmark calls the subs in scalar context; the my @x is used to
force sort to be called in list context. As for what sort *should*
do in scalar context, noone is really sure -- the latest consensus
is that it should play nethack:

<http://www.nntp.perl.org/group/perl.perl5.porters/92250>
<http://www.nntp.perl.org/group/perl.perl5.porters/92477>
 
>> sub j1 {
>>     my @x = map { substr($_,4) } sort map { substr($_,4).$_ } @dates
>> }
> 
> -laugh-  Took me 3 minutes to figure out what that does. map tends to 
> confuse my limited synapses.
> 
>> Which surprised me. I expected j1 to be much faster.
> 
> Usually its faster for me to re-write a use of map than it is for me to 
> figure out what I was doing when I wrote it. -grin- Typically I can't 
> grok map at a glance, which slows me down. Can't argue w/ fast 
> benchmarking though!
> 
> Thanks for the tips!

You're welcome. Actually, I wasn't trying to be silly on purpoose
(for once), just following the standard GRT sort hack:

<http://www.perlmonks.org/index.pl?node_id=145659>

The GRT and its cousin the Schwartzian Transform are usually a lot
faster whenever the comparison function is expensive because they
pre-compute the sort keys just n times (rather than n log n when
called by sort). The GRT is usually faster than the Schwartzian
Transform because sort without a sort block or function is done
in C and so is faster than interpreting the Perl sort function.

Hugh



		
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