local vs. my (was Re: code line of the day)

On Sep 8, 2006, at 6:48 PM, A. Pagaltzis wrote:

> * Chris Dolan <[email protected]> [2006-09-08 17:10]:
>> Why did you use "local"?  Shouldn't the following work?
>>
>>     sub flatten_copy {
>>         my $s = shift;
>>         ref $s eq 'SCALAR' ? "$$s" : "$s";
>>     }
>
> Works the same. I often use `local $_` in tiny functions that
> mangle just a single value. Matter of taste/style.

Ahh, I see -- cargo cult.  ;-)

Changing the topic a little bit...  I've always suspected there was a  
speed difference between local and my.  So, I just benchmarked the  
two versions, and "my" wins by a wide margin:

% perl test.pl
           Rate local    my
local 467290/s    --  -33%
my    699301/s   50%    --

% perl -v

This is perl, v5.8.6 built for darwin-thread-multi-2level
(with 2 registered patches, see perl -V for more detail)


=============  test.pl ==============
#!/usr/bin/perl -w

use strict;
use Benchmark qw(cmpthese);

my $s = 'foo';
$_ = 'foo';

cmpthese(1_000_000, {
    'local' => sub {
       flatten_copy_local('bar');
    },
    'my' => sub {
       flatten_copy_my('bar');
    },
});

sub flatten_copy_local {
    local $_ = shift;
    ref $_ eq 'SCALAR' ? "$$_" : "$_";
}
sub flatten_copy_my {
    my $s = shift;
    ref $s eq 'SCALAR' ? "$$s" : "$s";
}
========================================

-- 
Chris Dolan, Software Developer, Clotho Advanced Media Inc.
608-294-7900, fax 294-7025, 1435 E Main St, Madison WI 53703
vCard: http://www.chrisdolan.net/ChrisDolan.vcf

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