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Hello All,
Could someone explain what Perl does behind here?
Assign to Substring..
substr($string, 0 , 5) = 'Greetings';
Regards,
Emeka
--
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*
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On 11-09-06 08:04 AM, Emeka wrote:
> Hello All,
>
> Could someone explain what Perl does behind here?
>
> Assign to Substring..
> substr($string, 0 , 5) = 'Greetings';
>
>
> Regards,
> Emeka
It replaces the first 5 characters in $string with 'Greetings'.
#!/usr/bin/env perl
use strict;
use warnings;
my $string = 'Hello, world';
print "$string\n";
substr($string, 0 , 5) = 'Greetings';
print "$string\n";
__END__
Can also be written as:
#!/usr/bin/env perl
use strict;
use warnings;
my $string = 'Hello, world';
print "$string\n";
substr($string, 0 , 5, 'Greetings' );
print "$string\n";
__END__
--
Just my 0.00000002 million dollars worth,
Shawn
Confusion is the first step of understanding.
Programming is as much about organization and communication
as it is about coding.
The secret to great software: Fail early & often.
Eliminate software piracy: use only FLOSS.
"Make something worthwhile." -- Dear Hunter
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On Tue, Sep 6, 2011 at 1:19 PM, Shawn H Corey
> On 11-09-06 08:04 AM, Emeka wrote:
>
>> Hello All,
>>
>> Could someone explain what Perl does behind here?
>>
>> Assign to Substring..
>> substr($string, 0 , 5) = 'Greetings';
>>
>>
>> Regards,
>> Emeka
>>
>
> It replaces the first 5 characters in $string with 'Greetings'.
>
> #!/usr/bin/env perl
>
> use strict;
> use warnings;
>
> my $string = 'Hello, world';
> print "$string\n";
>
>
> substr($string, 0 , 5) = 'Greetings';
> print "$string\n"
>
I asked what Perl does behind.. Is it an alias or what? I want internal
details....or something technical.
Emeka
> __END__
>
>
>
> Can also be written as:
>
> #!/usr/bin/env perl
>
> use strict;
> use warnings;
>
> my $string = 'Hello, world';
> print "$string\n";
>
> substr($string, 0 , 5, 'Greetings' );
> print "$string\n";
>
> __END__
>
>
>
> --
> Just my 0.00000002 million dollars worth,
> Shawn
>
> Confusion is the first step of understanding.
>
> Programming is as much about organization and communication
> as it is about coding.
>
> The secret to great software: Fail early & often.
>
> Eliminate software piracy: use only FLOSS.
>
> "Make something worthwhile." -- Dear Hunter
>
> --
> To unsubscribe, e-mail: beginners-unsubscribe@perl.org
> For additional commands, e-mail: beginners-help@perl.org
> http://learn.perl.org/
>
>
>
--
*Satajanus Nig. Ltd
*
--001517402bfa870c5d04ac4505dc--
On 11-09-06 08:32 AM, Emeka wrote:
> I asked what Perl does behind.. Is it an alias or what? I want internal
> details....or something technical.
See `perldoc -f substr` and search for /lvalue/.
--
Just my 0.00000002 million dollars worth,
Shawn
Confusion is the first step of understanding.
Programming is as much about organization and communication
as it is about coding.
The secret to great software: Fail early & often.
Eliminate software piracy: use only FLOSS.
"Make something worthwhile." -- Dear Hunter
--
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http://learn.perl.org/
On 06/09/2011 13:04, Emeka wrote:
>
> Could someone explain what Perl does behind here?
>
> Assign to Substring..
> substr($string, 0 , 5) = 'Greetings';
What do you want to know Emeka?
There is no copy of the 'old' string, and substr behaves as documented.
I have the C code of substr in front of me, but there is little else to
say.
Rob
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Rob,
I want to check out the C code. Kindly mail it to my box.
Have a great day!
Emeka
On Wed, Sep 7, 2011 at 8:00 PM, Rob Dixon
> On 06/09/2011 13:04, Emeka wrote:
>
>>
>> Could someone explain what Perl does behind here?
>>
>> Assign to Substring..
>> substr($string, 0 , 5) = 'Greetings';
>>
>
> What do you want to know Emeka?
>
> There is no copy of the 'old' string, and substr behaves as documented.
>
> I have the C code of substr in front of me, but there is little else to
> say.
>
> Rob
>
--
*Satajanus Nig. Ltd
*
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Rob,
Which C file should check out?
Emeka
On Wed, Sep 7, 2011 at 8:08 PM, Emeka
> Rob,
>
> I want to check out the C code. Kindly mail it to my box.
> Have a great day!
>
> Emeka
>
>
> On Wed, Sep 7, 2011 at 8:00 PM, Rob Dixon
>
>> On 06/09/2011 13:04, Emeka wrote:
>>
>>>
>>> Could someone explain what Perl does behind here?
>>>
>>> Assign to Substring..
>>> substr($string, 0 , 5) = 'Greetings';
>>>
>>
>> What do you want to know Emeka?
>>
>> There is no copy of the 'old' string, and substr behaves as documented.
>>
>> I have the C code of substr in front of me, but there is little else to
>> say.
>>
>> Rob
>>
>
>
>
> --
> *Satajanus Nig. Ltd
>
>
> *
>
--
*Satajanus Nig. Ltd
*
--001517447a2ef41d1a04ac5f3e5c--
Hi Emeka,
On Wed, 7 Sep 2011 20:49:44 +0100
Emeka
> Rob,
>=20
> Which C file should check out?
>=20
> Emeka
>=20
Please look at the function definition starting from «PP(pp_substr)=C2=
=BB in pp.c in
the Perl 5 distribution:
https://github.com/mirrors/perl/blob/blead/pp.c
It seems pretty complicated, and I did not try to understand it yet.
Regards,
Shlomi Fish
--=20
------------------------------------------------------------ -----
Shlomi Fish http://www.shlomifish.org/
My Favourite FOSS - http://www.shlomifish.org/open-source/favourite/
Do not meddle in the affairs of Dragons, for you are crunchy and taste good
with ketchup. â=94 Unknown sourc=
e.
Please reply to list if it's a mailing list post - http://shlom.in/reply .
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On 07/09/2011 20:49, Emeka wrote:
> On Wed, Sep 7, 2011 at 8:00 PM, Rob Dixon
>> On Wed, Sep 7, 2011 at 8:08 PM, Emeka
>>> On 06/09/2011 13:04, Emeka wrote:
>>>
>>>>
>>>> Could someone explain what Perl does behind here?
>>>>
>>>> Assign to Substring..
>>>> substr($string, 0 , 5) = 'Greetings';
>>>>
>>>
>>> What do you want to know Emeka?
>>>
>>> There is no copy of the 'old' string, and substr behaves as documented.
>>>
>>> I have the C code of substr in front of me, but there is little else to
>>> say.
>>>
>>
>> I want to check out the C code. Kindly mail it to my box.
>> Have a great day!
>>
>
> Which C file should check out?
Hi Emeka.
It would help a lot if you would start putting your responses at the end
of the post you are replying to. That way extended threads are made much
more readable.
As Shlomi said, the code for the substr op is "PP(pp_substr)" pp.c. Most
of the code there is for validating the parameters and deciding which of
several different things should be done. It doesn't actually do any
substring extraction, it just sets up the return value TARG and pushes
it onto the stack.
The code that deals specifically with lvalues is the block starting
else if (lvalue) { /* it's an lvalue! */
at line 3253 in the version I'm looking at, 5.12.4. In particular you
will see that it assigns
LvTARGOFF(TARG) = pos;
LvTARGLEN(TARG) = len;
to indicate the position and length of the substring that TARG specifies.
The assignment to the substring is done by function
'Perl_magic_setsubstr' in mg.c. Which ultimately calls 'sv_insert',
#defined in sv.h as a call to Perl_sv_insert_flags' which is in sv.c.
I hope that helps. The internals of Perl are complex and not for the
feint-hearted.
Rob
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