PERL Programming - Re: Exported constants in module (HELP WITH)

Author

Re: Exported constants in module (HELP WITH)

Paul Lalli

2006-01-30, 6:56 pm

Gregory Layman wrote:
> In article <1138630603.478498.318420@o13g2000cwo.googlegroups.com>,
> mritty@gmail.com says...
> Can you explain or direct me towards a link or book that would explain
> constants and how they are used?

perldoc constant

Basically, Perl constants are implemented as subroutines that do
nothing but return a value. That way, they can be "read", but not
assigned to. When I type "READONLY" up there, I'm actually calling a
subroutine READONLY(), and using the return value of that subroutine
(which in this case is the number 1. HIDDEN() returns 2, COMPRESSED()
returns 2048, etc).

> What does the "&" symbol do?

perldoc perlop

& is the bit-wise 'AND' operator. It takes both of its arguments and
does a bit-wise AND'ing of those arguments. For example, let's say
$attr came out to be 2050. In binary, 2050 is represented as
0000 1000 0000 0010
2048 in binary is:
0000 1000 0000 0000
We "and" together each of those bits, meaning that the result of `2050
& 2048` is the number comprised of 1s where both arguments have a 1,
and 0s where either argument has a 0:
0000 1000 0000 0000
That is the number 2048. 2048 is a true value, and so `2050 & 2048` is
true, meaning that our example is a COMPRESSED() file.

Alternatively, the READONLY() check determines if `2050 & 1` is true.
Now we are AND'ing:
0000 1000 0000 0010
0000 0000 0000 0001
Which results in:
0000 0000 0000 0000
Which is the number 0, and is false. So READONLY is not set on this
file.

> I have tried to look this stuff up, but it seems I am lacking some basics on
> constants and modules (so I am not getting very far).

Out of curiousity, where are you looking? Allow me to recommend you
use the built-in perldoc program at the command line, or visit
http://perldoc.perl.org

Paul Lalli

P.S. If you don't like Perl's default implementation of constants,
you're not alone. There is a nicer alternative, known as Readonly,
available on CPAN:
http://search.cpan.org/~roode/Readonly-1.03/Readonly.pm

Gregory Layman

2006-01-30, 6:56 pm

In article <1138645647.206904.86250@g49g2000cwa.googlegroups.com>,
mritty@gmail.com says...
> Gregory Layman wrote:
>
> perldoc constant
>
> Basically, Perl constants are implemented as subroutines that do
> nothing but return a value. That way, they can be "read", but not
> assigned to. When I type "READONLY" up there, I'm actually calling a
> subroutine READONLY(), and using the return value of that subroutine
> (which in this case is the number 1. HIDDEN() returns 2, COMPRESSED()
> returns 2048, etc).
>
>
> perldoc perlop
>
> & is the bit-wise 'AND' operator. It takes both of its arguments and
> does a bit-wise AND'ing of those arguments. For example, let's say
> $attr came out to be 2050. In binary, 2050 is represented as
> 0000 1000 0000 0010
> 2048 in binary is:
> 0000 1000 0000 0000
> We "and" together each of those bits, meaning that the result of `2050
> & 2048` is the number comprised of 1s where both arguments have a 1,
> and 0s where either argument has a 0:
> 0000 1000 0000 0000
> That is the number 2048. 2048 is a true value, and so `2050 & 2048` is
> true, meaning that our example is a COMPRESSED() file.
>
> Alternatively, the READONLY() check determines if `2050 & 1` is true.
> Now we are AND'ing:
> 0000 1000 0000 0010
> 0000 0000 0000 0001
> Which results in:
> 0000 0000 0000 0000
> Which is the number 0, and is false. So READONLY is not set on this
> file.

Paul,

Thanks for this info. It is making sense now. I am basically stuck
to my two O'Reilly books on Perl and scouring the Internet. I have
heard of perldoc before, but never looked into using it as reference.
That will now change.

Again thanks for all your help.

Greg

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