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explain code section please... onlineviewer 08-28-2006
Posted by onlineviewer on August 28, 2006, 4:41 pm
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Hello All,

Can someone please explain this code section to me. This is from the
O'reilly book. learning objects,references. I see the end result, but i
am not sure how and in what order it runs. I see that the $callback
variable is a reference to the subroutine,
'create_find_callback_that_sums_the_size' then the find method is
called with the $callback reference and the bin directory. Then the
subroutine executes on each of the contents of the bin directory. Is
that right so far ?? Thanks...

use File::Find;

sub create_find_callback_that_sums_the_size {
my $total_size = 0;

return sub {
if (@_) {
return $total_size;
} else {
$total_size += -s if -f;
}
};

}

my $callback = create_find_callback_that_sums_the_size( );
find($callback, "bin");
my $total_size = $callback->("dummy");
print "total size of bin is $total_size\n";


Posted by Mumia W. on August 28, 2006, 5:49 pm
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On 08/28/2006 03:41 PM, onlineviewer wrote:
> Hello All,
>
> Can someone please explain this code section to me. This is from the
> O'reilly book. learning objects,references. I see the end result, but i
> am not sure how and in what order it runs. I see that the $callback
> variable is a reference to the subroutine,
> 'create_find_callback_that_sums_the_size' then the find method is
> called with the $callback reference and the bin directory. Then the
> subroutine executes on each of the contents of the bin directory. Is
> that right so far ??

Yes

> Thanks...
>
> use File::Find;
>
> sub create_find_callback_that_sums_the_size {
> my $total_size = 0;
>
> return sub {
> if (@_) {
> return $total_size;
> } else {
> $total_size += -s if -f;
> }
> };
>
> }
>
> my $callback = create_find_callback_that_sums_the_size( );
> find($callback, "bin");
> my $total_size = $callback->("dummy");
> print "total size of bin is $total_size\n";
>

"$Total_size" is only available within the anonymous
subroutine, and the sub, when called without parameters, adds
the size of the current file to $total_size and, when called
with parameters, outputs $total_size.

The code that returns the anonymous sub might also be written
like so:

return sub {
if (@_) {
return $total_size;
} else {
my $current_size = -s $_;
if (-f $_) {
$total_size += $current_size
}
}
};

WARNING: UNTESTED CODE


Posted by John W. Krahn on August 28, 2006, 6:35 pm
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onlineviewer wrote:
>
> Can someone please explain this code section to me. This is from the
> O'reilly book. learning objects,references. I see the end result, but i
> am not sure how and in what order it runs. I see that the $callback
> variable is a reference to the subroutine,
> 'create_find_callback_that_sums_the_size'

Wrong. $callback is a reference to an anonymous sub that is CREATED by
'create_find_callback_that_sums_the_size'.

perldoc -q closure


> then the find method is
> called with the $callback reference and the bin directory.

Correct.


> Then the
> subroutine executes on each of the contents of the bin directory. Is
> that right so far ?? Thanks...

File::Find::find traverses the directory tree starting at 'bin' and for each
entry it calls the subroutine supplied by you and puts the name of that entry
in the $_ variable.


> use File::Find;
>
> sub create_find_callback_that_sums_the_size {
> my $total_size = 0;
>
> return sub {
> if (@_) {
> return $total_size;
> } else {
> $total_size += -s if -f;
> }
> };
>
> }
>
> my $callback = create_find_callback_that_sums_the_size( );
> find($callback, "bin");
> my $total_size = $callback->("dummy");
> print "total size of bin is $total_size\n";


John
--
use Perl;
program
fulfillment

Posted by onlineviewer on August 28, 2006, 8:26 pm
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> Peter Billam wrote:
>> Greetings xs folk. Suppose I have a bug-fix in a .xs file looking like
>> v0 &= 0xffffffff;
>> which is only necessary on 64-bit architectures... For speed reasons
>> I'd like to not include that line on 32-bit machines. Is there some
>> convenient
>> #ifdef 64whatever
>> v0 &= 0xffffffff;
>> #endif
>> defined by the xs that I can use ?
>
> Why? Just add -DMY_64bit_FIX_NEEDED in Makefile.PL
>
OK, I think I get it...
You mean in Makefile.PL have, if ($Config) then
DEFINE => '-DMY_64bit_FIX_NEEDED',
and then put
#ifdef MY_64bit_FIX_NEEDED
v0 &= 0xffffffff;
#endif
in the .xs ?

> after inspecting %Config *for the conditions you need*,
> not some "random thing" being 64-bit...
>
Presumably in this case I'd be looking for $Config ?

Thanks for your help, Peter

--
AUS/TAS/DPIW/CIT/Servers hbt/lnd/l8 6233 3061 http://www.pjb.com.au
Pasaré, pasarémos dice el agua y canta la verdad contra la piedra
-- Pablo Neruda

Posted by John W. Krahn on August 28, 2006, 9:00 pm
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onlineviewer wrote:
>
> John W. Krahn wrote:
>>
>>onlineviewer wrote:
>>>Can someone please explain this code section to me. This is from the
>>>O'reilly book. learning objects,references. I see the end result, but i
>>>am not sure how and in what order it runs. I see that the $callback
>>>variable is a reference to the subroutine,
>>>'create_find_callback_that_sums_the_size'
>>
>>Wrong. $callback is a reference to an anonymous sub that is CREATED by
>>'create_find_callback_that_sums_the_size'.
>>
>>perldoc -q closure
>>
>>>then the find method is
>>>called with the $callback reference and the bin directory.
>>
>>Correct.
>>
>>>Then the
>>>subroutine executes on each of the contents of the bin directory. Is
>>>that right so far ?? Thanks...
>>
>>File::Find::find traverses the directory tree starting at 'bin' and for each
>>entry it calls the subroutine supplied by you and puts the name of that entry
>>in the $_ variable.
>
> Thanks for the replies, it is very helpful. One more question, is the
> subroutine called for as many elements that there are in the bin
> directory,

Yes.

> or is the subroutine being invoked only once and the
> contents of bin are being copied into the subroutine to be worked on

No.


John
--
use Perl;
program
fulfillment

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