On Apr 11, 2005 4:20 AM, Brent Clark <[EMAIL PROTECTED]> wrote:
> Hi all
>
> If anyone has the time and / or the will to help me understand.
>
> I know how to create / use references for perl. But would why would you
> use it.
> And I think more importantly when.
>
> Im busy reading / learning the Oreilly Advanced Perl Programming book.
> But for the likes of me I cant undertand when or why I would use it.
>
> Just something I was thinking.
>
> Kind Regards and thanks in advance
> Brent Clark
Brent,
In my mind, there are basically four general cases where you want
references. The first is the one Ankur mentioned: you want to pass
multiple lists to or from a subroutine. The second John and Johnathan
covered: creating complext data structures. You can use references to
create "deep" hashes or hashes of arrays, etc. You can also use them
for things like circular linked lists (think of a website that cycles
ads endlessly), or more complex thnks like binary trees.
Two others:
1) You want a variable that is scoped to a subroutine (i.e. with
"my"), but you don't want it destroyed when the subroutine
exits--maybe it's a counting routine, and you want it to remember
where ti left off counting. Or you want to keep a hash of e-mail
adresses you've already mailed, or something like that. If you return
a reference, the variable isn't destroyed. But it is still scoped to
the subroutine, so if you've used a generic name like $email you can
reuse that in another block without clobbering the value where you
want it.
2) You want to pass a very large variable to or from a subroutine.
Consider the following:
my @newarray = foo(@bigarray) ;
sub foo {
my @array = @_ ;
return @array ;
}
let's say that that @bigarray has a million email addresses. At the
end of the subroutine, you'll have 2 million addresses eating up
memory: @bigarray and @newarray. (You may briefly have something on
the order of 3 million as @array gets copied out). If you use
references, however, there's only one copy of the data in memory, a
50% savings. It's also a tremenous time savings for large data
structures: copying an array requires at least one operation on every
item in the array, which can really add up with large data sets.
Creating a reference only requires a few pointer to be rearranged.
HTH,
--jay
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