Quoting "$vars" and open()

[Chris Marshall]

I know its wrong to quote "$vars" - but is there an easy way around it
when using open() and your filename is in a $var ?

Thanks
Chris

[Gunnar Hjalmarsson]

Chris Marshall wrote:

> I know its wrong to quote "$vars" - but is there an easy way around
> it when using open() and your filename is in a $var ?

Who says it's wrong to quote a variable? I suppose you are referring
to statements claiming that it's bad style to quote variables *when
there is no reason for doing so*. But, for instance, if you have a
directory path in the variable $dir and a file name in the variable
$file, there is absolutely nothing wrong with doing:

open FH, "$dir/$file" or die $!;

--
Gunnar Hjalmarsson
Email: [url]http://www.gunnar.cc/cgi-bin/contact.pl[/url]

[Andreas Kahari]

In article <cb9c7b76.0309170556.5bac0a80@posting.google.com >,
Chris Marshall wrote:

> I know its wrong to quote "$vars" - but is there an easy way around it
> when using open() and your filename is in a $var ?

I wouldn't say it's "wrong", but it's easy to avoid:

open(DATA, $var) or die; # Reading

open(DATA, ">" . $var) or die; # Writing

--
Andreas Kähäri

[Gunnar Hjalmarsson]

Andreas Kahari wrote:

> In article <cb9c7b76.0309170556.5bac0a80@posting.google.com >,
> Chris Marshall wrote:

>>I know its wrong to quote "$vars" - but is there an easy way around it
>>when using open() and your filename is in a $var ?

>
> I wouldn't say it's "wrong", but it's easy to avoid:
>
> open(DATA, $var) or die; # Reading
>
> open(DATA, ">" . $var) or die; # Writing

If you are writing portable code (for Perl < 5.6.0), you can't avoid
it when including MODE, so the latter needs to be written:

open(DATA, "> $var") or die;

--
Gunnar Hjalmarsson
Email: [url]http://www.gunnar.cc/cgi-bin/contact.pl[/url]

[Tad McClellan]

Chris Marshall <c_j_marshall@hotmail.com> wrote:

> I know its wrong to quote "$vars" - but is there an easy way around it

Yes:

Delete the double quote characters!


--
Tad McClellan SGML consulting
[email]tadmc@augustmail.com[/email] Perl programming
Fort Worth, Texas

[Tad McClellan]

Andreas Kahari <ak+usenet@freeshell.org> wrote:

> In article <cb9c7b76.0309170556.5bac0a80@posting.google.com >,
> Chris Marshall wrote:

>> I know its wrong to quote "$vars" - but is there an easy way around it
>> when using open() and your filename is in a $var ?

>
> I wouldn't say it's "wrong",

If you are going to contradict a Perl FAQ, a little bit of your
rationale would be appreciated.

It may aid in improving the answer given in the FAQ.

How can stringifying a reference be "not wrong"?

> open(DATA, ">" . $var) or die; # Writing

This is not related to the current discussion.

We are not saying that

">$var"

is wrong, we are saying that

"$var"

is wrong.


--
Tad McClellan SGML consulting
[email]tadmc@augustmail.com[/email] Perl programming
Fort Worth, Texas

[Andreas Kahari]

In article <slrnbmgsr9.7o0.tadmc@magna.augustmail.com>, Tad McClellan wrote:

> Andreas Kahari <ak+usenet@freeshell.org> wrote:

>> In article <cb9c7b76.0309170556.5bac0a80@posting.google.com >,
>> Chris Marshall wrote:

>>> I know its wrong to quote "$vars" - but is there an easy way around it
>>> when using open() and your filename is in a $var ?

>>
>> I wouldn't say it's "wrong",

>
> If you are going to contradict a Perl FAQ, a little bit of your
> rationale would be appreciated.

Good evening to you too.

> It may aid in improving the answer given in the FAQ.
>
> How can stringifying a reference be "not wrong"?

A reference? I tought we were talking about a scalar containing
a filename?

>> open(DATA, ">" . $var) or die; # Writing

>
> This is not related to the current discussion.

Hmm... I must have misread the question. How do you interpret
the question?

> We are not saying that
> ">$var"
> is wrong, we are saying that
> "$var"
> is wrong.

Which is correct in a general way, but we're discussing it in
the context of the open() command.


Cheers,
Andreas

--
Andreas Kähäri

[Tad McClellan]

Andreas Kahari <ak+usenet@freeshell.org> wrote:

> In article <slrnbmgsr9.7o0.tadmc@magna.augustmail.com>, Tad McClellan wrote:

>> If you are going to contradict a Perl FAQ, a little bit of your
>> rationale would be appreciated.

>> How can stringifying a reference be "not wrong"?

>
> A reference?

We are discussing this Perl FAQ:

perldoc -q vars

What's wrong with always quoting "$vars"?


"What's wrong" according the FAQ answer is the danger of
stringifying a reference.

> I tought we were talking about a scalar containing
> a filename?

I thought we were talking about useless uses of double quotes,
without regard to what the variable's value is, nor where
the variable is used.

You _never_ need

"$vars"

because

$vars

will always work in its place.

>>> open(DATA, ">" . $var) or die; # Writing

>>
>> This is not related to the current discussion.

>
> Hmm... I must have misread the question. How do you interpret
> the question?

I thought he was asking about the Perl FAQ above.


--
Tad McClellan SGML consulting
[email]tadmc@augustmail.com[/email] Perl programming
Fort Worth, Texas

[Tassilo v. Parseval]

Also sprach Tad McClellan:

> I thought we were talking about useless uses of double quotes,
> without regard to what the variable's value is, nor where
> the variable is used.
>
> You _never_ need
>
> "$vars"
>
> because
>
> $vars
>
> will always work in its place.

It should, and actually will for all Perl-builtins. Yet there is a
difference on the inside between these two (which makes your points even
more valid):

ethan@ethan:~$ perl -MDevel::Peek
$a = 1;
Dump($a);
Dump("$a");
__END__
SV = IV(0x812b084) at 0x8128ab0
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 1
SV = PV(0x811e638) at 0x8128ac8
REFCNT = 1
FLAGS = (PADTMP,POK,pPOK)
PV = 0x8123078 "1"\0
CUR = 1
LEN = 2

The stringification will upgrade the scalar value (well, not really
upgrading - the IV part is lost; there are also cases possible where
both PV and IV exist). So

$a = 1;
$b = "$a";

is the equivalent to the other direction as in

$a = "1";
$b = $a + 0;

To make people stop using these casts in cases where they are pointless,
it might be worth to remark that a stringification can make a scalar use
more memory and thus reduce the performance slightly:

ethan@ethan:~$ perl -MDevel::Size=size
print size(1), "\n";
print size("1"), "\n";
__END__
16
26

When doing these conversions in a very unclever way, as in

$a = $b = $c = 1;
@a = ("$a", "$b", "$c");
my $sum;
$sum += $_ for @a;

which means: IV -> PV -> IV, this has runtime-costs since these casts
naturally come at some computational costs that could easily be avoided.

Tassilo
--
$_=q#",}])!JAPH!qq(tsuJ[{@"tnirp}3..0}_$;//::niam/s~=)]3[))_$-3(rellac(=_$({
pam{rekcahbus})(rekcah{lrePbus})(lreP{rehtonabus}) !JAPH!qq(rehtona{tsuJbus#;
$_=reverse,s+(?<=sub).+q#q!'"qq.\t$&."'!#+sexisexi ixesixeseg;y~\n~~dddd;eval

[Gunnar Hjalmarsson]

Gunnar Hjalmarsson wrote:

> Andreas Kahari wrote:
>

>> open(DATA, ">" . $var) or die; # Writing

>
> If you are writing portable code (for Perl < 5.6.0), you can't
> avoid it when including MODE, so the latter needs to be written:
>
> open(DATA, "> $var") or die;

Hmm.. I'd better correct myself before somebody else does. ;-)

Both those open() statements specify just two arguments, so they work
in Perl versions before 5.6.0. What require Perl 5.6.0 (or later)
are open() statements with three or more arguments, such as:

open(DATA, ">", $var) or die;

--
Gunnar Hjalmarsson
Email: [url]http://www.gunnar.cc/cgi-bin/contact.pl[/url]

[Chris Marshall]

Andreas> open(DATA, ">" . $var) or die; # Writing

Tad> This is not related to the current discussion.
Andreas> Hmm... I must have misread the question. How do you
interpret
Andreas> the question?

Tad> I thought he was asking about the Perl FAQ above.


I guess that Andreas was "right" in interpreting my question - I was
hoping to come away with what he gave me:

open(DATA, $var) or die; # Reading

open(DATA, ">" . $var) or die; # Writing

[Chris Marshall]

Andreas Kahari <ak+usenet@freeshell.org> wrote in message news:<slrnbmgqce.cru.ak+usenet@vinland.freeshell.o rg>...

> In article <cb9c7b76.0309170556.5bac0a80@posting.google.com >,
> Chris Marshall wrote:

> > I know its wrong to quote "$vars" - but is there an easy way around it
> > when using open() and your filename is in a $var ?

>
> I wouldn't say it's "wrong", but it's easy to avoid:
>
> open(DATA, $var) or die; # Reading
>
> open(DATA, ">" . $var) or die; # Writing

Just what I was looking for - thanks.

[Chris Marshall]

[email]tadmc@augustmail.com[/email] (Tad McClellan) wrote in message news:<slrnbmgsjg.7o0.tadmc@magna.augustmail.com>.. .

> Chris Marshall <c_j_marshall@hotmail.com> wrote:
>

> > I know its wrong to quote "$vars" - but is there an easy way around it

>
>
> Yes:
>
> Delete the double quote characters!

How about when using open() to write to a file. e.g.

#!/usr/bin/perl
use warnings;
use strict;

my $var = "file.txt";

open(DATA, > $var) or die "$!"; # Writing
print DATA "hello world\n";


../test.pl
syntax error at test.pl line 7, near ", >"
Execution of test.pl aborted due to compilation errors.

[Sam Holden]

On 17 Sep 2003 23:54:18 -0700, Chris Marshall <c_j_marshall@hotmail.com> wrote:

> [email]tadmc@augustmail.com[/email] (Tad McClellan) wrote in message news:<slrnbmgsjg.7o0.tadmc@magna.augustmail.com>.. .

>> Chris Marshall <c_j_marshall@hotmail.com> wrote:
>>

>> > I know its wrong to quote "$vars" - but is there an easy way around it

>>
>>
>> Yes:
>>
>> Delete the double quote characters!

>
>
> How about when using open() to write to a file. e.g.

Then you use quotes obviously, since it is a completely different case.

">$vars" is completely different than "$vars".

[snip silly example]

--
Sam Holden

[Anno Siegel]

Tassilo v. Parseval <tassilo.parseval@post.rwth-aachen.de> wrote in comp.lang.perl.misc:

> Also sprach Tad McClellan:
>

> > I thought we were talking about useless uses of double quotes,
> > without regard to what the variable's value is, nor where
> > the variable is used.
> >
> > You _never_ need
> >
> > "$vars"
> >
> > because
> >
> > $vars
> >
> > will always work in its place.

>
> It should, and actually will for all Perl-builtins. Yet there is a
> difference on the inside between these two (which makes your points even
> more valid):

[internal workings snipped]

There is another case where "$vars" and $vars are different, quite on
the surface. It happens when $vars is an object whose stringification
is overloaded. Obviously, $var is the object, but "$var" can be anything.

In the presence of overloading a few basic rules in Perl don't apply
anymore. Variable stringification is one example, and "\ @$x", "\ %$x"
and "\ $$x" aren't necessarily no-ops for overloaded $x.

Anno

[Chris Marshall]

[email]sholden@flexal.cs.usyd.edu.au[/email] (Sam Holden) wrote in message

> > How about when using open() to write to a file. e.g.

>
> Then you use quotes obviously, since it is a completely different case.
>
> ">$vars" is completely different than "$vars".
>
> [snip silly example]

Well calling it silly is a bit harsh.

The whole basis for my question was:
1) the faq says not to enclose a $var in quotes
2) when you open a file for writing you are tempted to do just that.

Anyway - the 3 parameter form of open() is just what I was looking for
and if ">$vars" is perfectly safe then even better.

Cheers,
Chris

[Tad McClellan]

Chris Marshall <c_j_marshall@hotmail.com> wrote:

> [email]sholden@flexal.cs.usyd.edu.au[/email] (Sam Holden) wrote in message

>> > How about when using open() to write to a file. e.g.

>>
>> Then you use quotes obviously, since it is a completely different case.
>>
>> ">$vars" is completely different than "$vars".
>>
>> [snip silly example]

>
>
> Well calling it silly is a bit harsh.
>
> The whole basis for my question was:
> 1) the faq says not to enclose a $var in quotes

It was "silly" because it did not illustrate the situation that
the FAQ is talking about.

If your question was about a Perl FAQ, it would have been helpful
to point that out said FAQ, it would have avoided some of the
resulting misunderstandings...

> 2) when you open a file for writing you are tempted to do just that.

No you're not.

You are tempted to do ">$var", which is *not* what the FAQ is talking about.

You have misunderstood the FAQ.

What is "bad" is quoting when the *only thing* quoted is a
scalar variable.

The FAQ says that

"$var"

is wrong. The FAQ does not say anything about

">$var"

> Anyway - the 3 parameter form of open() is just what I was looking for

"$var" being "wrong" has nothing to do with open().

print "$var";

is just as wrong.


--
Tad McClellan SGML consulting
[email]tadmc@augustmail.com[/email] Perl programming
Fort Worth, Texas

[Chris Marshall]

[email]tadmc@augustmail.com[/email] (Tad McClellan) wrote in message

>
> What is "bad" is quoting when the *only thing* quoted is a
> scalar variable.
>
> The FAQ says that
>
> "$var"
>
> is wrong. The FAQ does not say anything about
>
> ">$var"
>

ah - ok I get it now.

Thanks for the advice and apologies if the question was badly worded.