Rob Hanson wrote:
>> $string =~ s/^.(.*).$/$1/;
>
> It's prettier, but it's very inefficient. It's probably 20+ times slower
> then doing it in two steps.
>
> It has to do with how Perl internally handled string data. When you s///
> or substr() all it does is move a pointer.
>
this is an over generalization and is only true for a small set of
operation. it only moves pointer when you are deleting stuff from the
beginning:
[panda]$ perl -MDevel::Peek -e '$_=1234; s/.//; Dump $_'
SV = PVIV(0x804c700) at 0x804bfb8
REFCNT = 1
FLAGS = (POK,OOK,pPOK)
IV = 1 (OFFSET)
PV = 0x805c599 ( "1" . ) "234"\0
CUR = 3
LEN = 4
[panda]$
without going into too much detail (because this is a beginner list). notice
the OFFSET flag is set for $_. $_ is essentially called an offset string
because Perl "moves" the "start" of the string 1 byte forward without
rewriting anything. This happens transparently to the user and it's very
efficient. not every s/.// will result in an offset string though:
[panda]$ perl -MDevel::Peek -e '$_=1234; s/34//; Dump $_'
SV = PVIV(0x804c700) at 0x804bfb8
REFCNT = 1
FLAGS = (POK,pPOK)
IV = 1234
PV = 0x805c598 "12"\0
CUR = 2
LEN = 5
so is substr:
[panda]$ perl -MDevel::Peek -e '$_=1234; substr($_,2,2,''); Dump $_'
SV = PVIV(0x804c700) at 0x804bfb8
REFCNT = 1
FLAGS = (IOK,POK,pIOK,pPOK)
IV = 1234
PV = 0x805c5a8 "1234"\0
CUR = 4
LEN = 5
in both cases, there are no offset string because you are not removing stuff
from the beginning.
david
--
$_=q,015001450154015401570040016701570162015401440041,,*,=*|=*_,split+local$";
map{~$_&1&&{$,<<=1,[EMAIL PROTECTED]||3])=>~}}0..s~.~~g-1;*_=*#,
goto=>print+eval
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
