difference between @_ and shift

[Graeme McLaren]

Hi all, I'm having a look in to object oriented perl.  Can anyone tell me 
what the difference between @_ and shift is?  As far as I know there is no 
difference except shift removes the parameter from the @_ array so if you 
were to shift all parameters passed to a function nothing would be 
containted in @_   is this correct?

I'm asking because I'm a little confused about using it.  Why can't I do 
this:

###
sub nickname {
   my $self = shift;
   return $self-{NICK};
}
###
But I can do this:
###
sub nickname {
   my $self = shift;
if (@_) { $self-{NICK} = shift
}
   return $self-{NICK};
   }
###
_
Express yourself with cool new emoticons http://www.msn.co.uk/specials/myemo
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Wiggins d Anconia]

 Hi all, I'm having a look in to object oriented perl.  Can anyone tell me 
 what the difference between @_ and shift is?  

One is a variable, one is function/operator ;-)

As far as I know there is no 
 difference except shift removes the parameter from the @_ array so
if you 
 were to shift all parameters passed to a function nothing would be 
 containted in @_   is this correct?
 

@_ is just a special Perl variable, it so happens that it is what is
used to store the arguments to a subroutine temporarily (it has other
uses, see perldoc perlvar).  It also happens that the first argument to
a subroutine acting like a method is always the object/class that the
method is invoked on. Cshift is just a function that removes the
first/top element of an array, any array, in OOP or not. 

This happens to make things very convenient, aka in most methods you
will want to remove the instance/class from the argument list to get the
 actual argument list back, and shift by itself will default to use @_.
So essentially you are left with some syntactic sugar to make life easier.

So,

my $self = shift;

Just says to grab the instance from @_ (by default), store it in $self,
and restore the arg list to what the user actually passed.  You don't
*have* to do it, but it is very convenient.

 I'm asking because I'm a little confused about using it.  Why can't I do 
 this:
 
 ###
 sub nickname {
 my $self = shift;
 return $self-{NICK};
 }

No reason you can't.  Provided you have already set $self-{NICK} or
don't mind getting an undefined value. 


 ###
 
 But I can do this:
 
 ###
 sub nickname {
 my $self = shift;
   if (@_) { $self-{NICK} = shift
   }
 return $self-{NICK};
 }
 ###
 

Only difference here is that you set $self-{NICK} in the event that an
argument was passed, aka after shift'ing there is still an argument in
@_ (namely the value you want to set NICK to).

Additional suggested reading:

perldoc perlboot
perldoc perltoot
perldoc perltooc

http://danconia.org

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Errin Larsen]

I am (admitedly) unfamiliar with OO Perl.  I understand enough to grok
what you are saying, Wiggins, but I have a question.

Does a sub (like the one above) have a problem with being called with
 as opposed to not being called with an  with OO Perl?  That
questions was worded weird.  Let me try again.  As I understand it, if
you call a sub with 'subname', the sub's @_ variable will share the
calling scope's @_ variable, BUT, if you call the sub with 'subname()'
it will get it's own, fresh @_.  is that true?  And if it is, does
this affect the subs being used with OO Perl?

--Errin

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Chris Devers]

On Thu, 30 Sep 2004, Graeme McLaren wrote:

  Can anyone tell me what the difference between @_ and shift is?

The former is a variable; the latter is a function. 

@_ is one of Perl's pronoun variables: just as $_ refers to that 
which you were most recently working with, @_ refers to those which 
you were most recently working with. $_ is a scalar -- it or that; a 
single thing. @_ is a list -- those or these; a plural thing.

shift is a function for pulling off the first element from a list. If 
you've ever had to take a data structures class, you may remember that 
two of the main ways to deal with collections of things as as stacks 
(where you only deal with the top, and you can push onto it or pop off 
of it) and queues (where everything moves in a line, and you enqueue 
onto the end and dequeue from the front). Perl has functions that let 
you work with arrays in these ways. You can push to and pop from the end 
of lists to treat them as stacks; you can shift from and unshift to the 
front of lists to, well, also treat them as stacks; and you can push  
shift or pop  unshift to treat it as forward or reverse queues. 

So. In the context of subroutine arguments, you're generally passing in 
one or more arguments. If you're only passing one, then you're right --

   my $arg = @_;
   my $arg = shift;
   my $arg = $_;

-- are all equivalent. If, on the other hand, you have multiple args, 
then these will all do different things. 

   my @args = @_;

   my $arg[0] = shift;
   my $arg[0] = $_;

If you pass the wrong thing and aren't ready for it, you can throw away 
incoming data and possibly break things. 

 As far as I know there is no difference except shift removes the 
 parameter from the @_ array so if you were to shift all parameters 
 passed to a function nothing would be containted in @_ is this 
 correct?

Well, yes, in that this will deplete @_, but that doesn't mean that 
there is no difference between shift and @_ -- they're different things.
 


-- 
Chris Devers

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Sano Babu]

On Thu, 30 Sep 2004 20:19:11 +0100, Graeme McLaren
[EMAIL PROTECTED] wrote:
 Hi all, I'm having a look in to object oriented perl.  Can anyone tell me
 what the difference between @_ and shift is?  As far as I know there is no

Straight from the Llama :
@array = qw /dino fred barney/;
$a = $shift @array; # $a gets dino, @array reduced to (fred , barney);

 difference except shift removes the parameter from the @_ array so if you
 were to shift all parameters passed to a function nothing would be
 containted in @_   is this correct?

@_ variable is local to the subroutine. It stores argument to a
subroutine. If u keep on doing shift inside a subroutine then @_
will be empty  will become uninitialized again.
  sub test {
  #$a = shift ;
  my $a = shift @_ ;
  print $a;
  my $b = shift @_;
  print $b;
  }
test(1);
-perl -w test.pl
-Use of uninitialized value in print at test.pl line 9.
1

But this doesn't in anyway affect the @_ variable outside the
subroutine if there happens to be any.

 I'm asking because I'm a little confused about using it.  Why can't I do
 this:

 ###
 sub nickname {
my $self = shift;
return $self-{NICK};
 }
 ###

[snip]

$self looks alot like this in Java.. no? :)

-- 
Cheers,
SanoBabu

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Wiggins d Anconia]

 I am (admitedly) unfamiliar with OO Perl.  I understand enough to grok
 what you are saying, Wiggins, but I have a question.
 
 Does a sub (like the one above) have a problem with being called with
  as opposed to not being called with an  with OO Perl?  That
 questions was worded weird.  Let me try again.  As I understand it, if
 you call a sub with 'subname', the sub's @_ variable will share the
 calling scope's @_ variable, BUT, if you call the sub with 'subname()'
 it will get it's own, fresh @_.  is that true?  And if it is, does
 this affect the subs being used with OO Perl?
 
 --Errin

Solid question, which took some hunting in the docs but they are
solid docs, from perldoc perltoot, 

From the C++ perspective, all methods in Perl are virtual.  This, by
the way, is why they are never checked for function prototypes in the
argument list as regular builtin and user-defined functions can be.

This confirmed my hunch.  The C has a number of purposes beyond just
passing the current @_ in recursive like functions, such as
dereferencing subroutines, etc.   See perldoc perlsub for lots about
subroutines, prototypes, and their behaviours.  My hunch was that since
you invoke a method on an object/class, such as

My::Class-method();
or 
$object-method();

You would have to figure out where to put the sigil, 

$object-method();

Would either cause confusion wrt trying to dereference a subroutine, or
trying to chain a method on the return value of a dereferenced
subroutine, which would be pretty cool, though not terribly readable. Or,

$object-method();

Which I suspected could work, but had never seen it before (not that I
am an expert) but it strikes me as something difficult to parse.

Now to the more important part, and the more literal meaning of your
question, in Perl it doesn't affect anything!  Why? because the method,
when invoked as a regular sub call will run just fine!  The danger is
that you (may) have broken the interface, so if the sub is really a
method *expecting* the first argument to be an object/class, then it may
fail to function as documented. This is where the beauty of Perl comes
in though, why can't you have both!  For instance many subs can check
their first argument to see if it is a reference, in that case it treats
the call like a method, if the first arg isn't, then it treats it like a
regular sub call.  This is similar to other areas of Perl that function
based on the *context*.  Many modules that provide a functional and OOP
interface are operating under this principal.  So whether or not Perl
will let you, it will, whether or not you can/should depends on the
underlying implementation and whether it is documented.  Calling what is
normally a method with C as a regular sub call will pass @_
unmangled, the question becomes does @_ already have the object/class
attached, or is the sub smart enough to recognize that it doesn't.

http://danconia.org

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Wiggins d Anconia]

 On Thu, 30 Sep 2004, Graeme McLaren wrote:
 
   Can anyone tell me what the difference between @_ and shift is?
 

snip

 So. In the context of subroutine arguments, you're generally passing in 
 one or more arguments. If you're only passing one, then you're right --
 
my $arg = @_;

DANGER, DANGER the above takes a list in scalar context, $arg is now
'1', not the value of the first argument.  Throw in some parens to fix
that guy

my ($arg) = @_;

my $arg = shift;
my $arg = $_;
 
 -- are all equivalent. If, on the other hand, you have multiple args, 
 then these will all do different things. 
 

snip

 
 -- 
 Chris Devers
 

http://danconia.org

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Sano Babu]

  So. In the context of subroutine arguments, you're generally passing in
  one or more arguments. If you're only passing one, then you're right --

  my $arg = @_;
So what wiggins saying, here $arg has now 1 ? 

  my $arg = shift;
here $arg has now the value of the first element(argument) ? 

  my $arg = $_;
here $arg gets default value?

   -- are all equivalent.

I agree. :)

[snip]
-- 
Cheers,
SanoBabu

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Wiggins d Anconia]

   So. In the context of subroutine arguments, you're generally passing in
   one or more arguments. If you're only passing one, then you're right --
 
   my $arg = @_;
 So what wiggins saying, here $arg has now 1 ? 


Yes. It is the length of the array, or a list taken in scalar context.
 
   my $arg = shift;
 here $arg has now the value of the first element(argument) ? 
 
   my $arg = $_;
 here $arg gets default value?

Ha, didn't even notice that one, yep $_ is definitely different than
$_[0], which is what the others would have. 

 
-- are all equivalent.
 
 I agree. :)
 

?? You are agreeing that they are, or aren't?  They definitely aren't.

 [snip]
 -- 
 Cheers,
 SanoBabu
 

http://danconia.org


-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Sano Babu]

 Ha, didn't even notice that one, yep $_ is definitely different than
 $_[0], which is what the others would have.
 
 
 -- are all equivalent.
 
  I agree. :)
 
 
 ?? You are agreeing that they are, or aren't?  They definitely aren't.

hahah..That was a bad post..I mean I agree with what u said. :-) 

 
[snip]

-- 
Cheers,
SanoBabu

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[John W. Krahn]

Chris Devers wrote:
The former is a variable; the latter is a function. 

@_ is one of Perl's pronoun variables: just as $_ refers to that 
which you were most recently working with, @_ refers to those which 
you were most recently working with. $_ is a scalar -- it or that; a 
single thing. @_ is a list -- those or these; a plural thing.

shift is a function for pulling off the first element from a list.
Wrong!  If you try to use shift() with a list it will print out a nice error 
message Type of arg 1 to shift must be array.

perldoc -q What is the difference between a list and an array
John
--
use Perl;
program
fulfillment
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response

Re: difference between @_ and shift

[Chris Devers]

On Thu, 30 Sep 2004, John W. Krahn wrote:

 Chris Devers wrote:
  
  shift is a function for pulling off the first element from a list.
 
 Wrong!  If you try to use shift() with a list it will print out a nice 
 error message Type of arg 1 to shift must be array.
 
 perldoc -q What is the difference between a list and an array

Whoops! I stand corrected. Sorry about that.

In any case, the bigger point is that @_ and shift are much different 
things -- they may be used in similar places, but that doesn't make them 
into synonyms.


 
-- 
Chris Devers

-- 
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
http://learn.perl.org/ http://learn.perl.org/first-response