[Omaha.pm] Sort quickie

[Hugh Jarce]

Jay Hannah wrote:
> sub sort_by_cap_date ($$) {
>     # We have to throw some mojo here since capdate is MMDDYYYY and obviously
>     # we can't sort until we turn it into YYYYMMDD... -jhannah 6/14/04
>     my ($a, $b) = @_;
>     for ($a, $b) {
>         s/(\d\d)(\d\d)(\d\d\d\d)/$3$1$2/;
>     }
>     $a <=> $b;
> }
 
This sub can be written more efficiently as:
  sub by_date2 { substr($a,4) cmp substr($b,4) || $a cmp $b }
or:
  sub by_date3 { substr($a,4).$a cmp substr($b,4).$b }

Out of curiosity, I benchmarked 4 different ways to do it:
 
use strict;
use Benchmark;
my @dates = ( '05012003', '02012004', '11012002',
              '05012002', '02012003', '11012001',
              '06012002', '05022002', '05022004' ) x 9999;
sub by_date ($$) {
    my ($a, $b) = @_;
    for ($a, $b) {
       s/(\d\d)(\d\d)(\d\d\d\d)/$3$1$2/;
    }
    $a <=> $b;
}
sub by_date2 { substr($a,4) cmp substr($b,4) || $a cmp $b }
sub by_date3 { substr($a,4).$a cmp substr($b,4).$b }
sub j9 { my @x = sort by_date @dates }
sub j2 { my @x = sort by_date2 @dates }
sub j3 { my @x = sort by_date3 @dates }
sub j1 {
    my @x = map { substr($_,4) } sort map { substr($_,4).$_ } @dates
}
timethese(10, {
    'j1' => \&j1,
    'j2' => \&j2,
    'j3' => \&j3,
    'j9' => \&j9 });

Results on Linux were:
 
perl 5.8.4:
j1:  7 wallclock secs ( 7.54 usr +  0.03 sys =  7.57 CPU) @  1.32/s (n=10)
j2: 10 wallclock secs (10.04 usr +  0.01 sys = 10.05 CPU) @  1.00/s (n=10)
j3: 13 wallclock secs (13.45 usr +  0.00 sys = 13.45 CPU) @  0.74/s (n=10)
j9: 169 wallclock secs (168.82 usr +  0.00 sys = 168.82 CPU) @  0.06/s (n=10)
 
perl 5.6.1:
j1:  6 wallclock secs ( 6.07 usr +  0.02 sys =  6.09 CPU) @  1.64/s (n=10)
j2:  5 wallclock secs ( 5.34 usr +  0.00 sys =  5.34 CPU) @  1.87/s (n=10)
j3:  7 wallclock secs ( 7.00 usr +  0.00 sys =  7.00 CPU) @  1.43/s (n=10)
j9: 75 wallclock secs (74.57 usr +  0.00 sys = 74.57 CPU) @  0.13/s (n=10)
 
Which surprised me. I expected j1 to be much faster.


Hugh
 

		
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