PERL Miscellaneous - file glob in scalar context

Author

file glob in scalar context

nan li

2006-03-29, 7:00 pm

Hello,

I have a question about glob in scalar context.

The perldoc says,
' In scalar context, glob iterates through such
filename expansions, returning undef when
the list is exhausted.'

So I wrote the following and it returns all pl files.

while ( my $f = glob "*.pl" ) {
print $f, "\n";
}

But without a loop, glob seems to give me just the first
file all the time.

my $f = glob "*.pl";
print $f, "\n"; # "first.pl"

$f = glob "*.pl";
print $f, "\n"; # "first.pl" again

Does any one have an explanation. I am

.

Thanks,
Nan

xhoster@gmail.com

2006-03-29, 7:00 pm

"nan li" <nan.li.g@gmail.com> wrote:
> Hello,
>
> I have a question about glob in scalar context.
>
> The perldoc says,
> ' In scalar context, glob iterates through such
> filename expansions, returning undef when
> the list is exhausted.'
>
> So I wrote the following and it returns all pl files.
>
> while ( my $f = glob "*.pl" ) {
> print $f, "\n";
> }
>
> But without a loop, glob seems to give me just the first
> file all the time.
>
> my $f = glob "*.pl";
> print $f, "\n"; # "first.pl"
>
> $f = glob "*.pl";
> print $f, "\n"; # "first.pl" again
>
> Does any one have an explanation. I am

.

You have two separate globs. Each maintains separate, independent,
state.

Xho

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Kraven

2006-03-29, 7:00 pm

nan li wrote:
> Hello,
>
> I have a question about glob in scalar context.
>
> The perldoc says,
> ' In scalar context, glob iterates through such
> filename expansions, returning undef when
> the list is exhausted.'
>
> So I wrote the following and it returns all pl files.

#get a file name with an extension called .pl
> while ( my $f = glob "*.pl" ) {
#print that file
> print $f, "\n";
#don't exit the program yet we haven't seen undef!! Do it all again...
> }
>
> But without a loop, glob seems to give me just the first
> file all the time.
>
>
#let's make $f equal the file name that glob returns
> my $f = glob "*.pl";
#print that file name
> print $f, "\n"; # "first.pl"
#looks like we're done here, exit the program
>
>
#same as your second example
> $f = glob "*.pl";
> print $f, "\n"; # "first.pl" again
>
> Does any one have an explanation. I am

.
>
> Thanks,
> Nan
>

Kraven

2006-03-29, 7:00 pm

Kraven wrote:[color=darkred]
> nan li wrote:
>
>
>
> #get a file name with an extension called .pl
>
>
> #print that file
>
>
> #don't exit the program yet we haven't seen undef!! Do it all again...
>
> #let's make $f equal the file name that glob returns
>
>
> #print that file name
>
>
> #looks like we're done here, exit the program
>
> #same as your second example
>
ps... maybe you want something like this:

use strict;
use warnings;

my @offiles = glob "*.pl";
foreach (@offiles) {
print "$_\n";
}

nan li

2006-03-29, 7:00 pm

xhoster@gmail.com wrote:
> "nan li" <nan.li.g@gmail.com> wrote:
>
> You have two separate globs. Each maintains separate, independent,
> state.
>
> Xho

Thanks. It seems in a loop, the glob is executed only once, and returns
something
like a stream handle. I'd really like to know how it is implemented.
But anyway,
I would say it is very counterintuitive because the following 2 are
not the same.

___________________________
for ( 1 .. 2 ) {
$f = glob "*.pl";
print $f, "\n";
}
___________________________

$f = glob "*.pl";
print $f, "\n";
$f = glob "*.pl";
print $f, "\n";
__________________________

George

2006-03-29, 7:00 pm

> > nan li wrote:

Kraven wrote:[color=darkred]
> ps... maybe you want something like this:
>
> use strict;
> use warnings;
>
> my @offiles = glob "*.pl";
> foreach (@offiles) {
> print "$_\n";
> }

....or even just use a simple wrapper subroutine, like 'glob1' for
example:

use strict;
use warnings;

sub glob1 { glob shift }

my $f;
$f = glob1 "*.pl"; print $f, "\n"; # "first.pl"
$f = glob1 "*.pl"; print $f, "\n"; # "second.pl"

xhoster@gmail.com

2006-03-29, 7:00 pm

"nan li" <nan.li.g@gmail.com> wrote:
> xhoster@gmail.com wrote:
....[color=darkred]
>
> Thanks. It seems in a loop, the glob is executed only once, and returns
> something like a stream handle.

Kind of like that. It doesn't return the handle, it just stores it
internally and serves files out of it.

> I'd really like to know how it is implemented.

Did you every hear the joke of the two economists who see a sports car
drive by? One of them says "I always wanted a car like that." The other
one says "Obviously not."

Perl is open source. If you *really* would like to know how it is
implemented, you can go look and see. :)

> But anyway,
> I would say it is very counterintuitive because the following 2 are
> not the same.
>
> ___________________________
> for ( 1 .. 2 ) {
> $f = glob "*.pl";
> print $f, "\n";
> }
> ___________________________
>
> $f = glob "*.pl";
> print $f, "\n";
> $f = glob "*.pl";
> print $f, "\n";

Your intuition can be trained. To me, it is intuitive because I
intuit it like the /g on a scalar-context regex:

$ perl -lw
for (1..2) {
"foobar" =~ /(...)/g;
print $1;
};
__END__
foo
bar

$ perl -lw
"foobar" =~ /(...)/g;
print $1;
"foobar" =~ /(...)/g;
print $1;
__END__
foo
foo

But I pretty much never use glob in a scalar context, anyway.

Xho

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