On Tue, 2004-07-27 at 12:54, Daniel Allen wrote:
> LINE: while (defined($_ = <ARGV>)) {
> /(\d+)/;
> print "$1 ";
> }
> continue {
> # print $_;
> }
The continue interposes another block into the loop. According to
perlre as quoted by Steve below, $1 et al. are scoped until the end of
the enclosing block. So when execution enters the continue block each
time through the loop, $1 is wiped clean. Removal of the continue block
permits the previous value of $1 to persist into the next execution of
the while block.
However, if you remove the continue block and rework the loop to use
redo/last, $1 is still wiped on each loop:
$ cat test-redo.pl
while (1) {
defined ($_ = <ARGV>) or last;
/(\d+)/;
print "\$1: $1\n";
redo;
}
$ echo -e "9 \n \n 2" | perl test-redo.pl
$1: 9
$1:
$1: 2
This I cannot explain. :)
-- Jeremy
> Coincidentally, my local 'mongers group is discussing a very similar
> situation. So far, I'm not satisfied with perlre's explanation of how
> numbered variables are scoped, and the real situation seems to be
> confusing, too.
>
> I started by forgetting an if (/whatever/) around my print, and wrote this:
>
> $ echo -e "9 \n \n 2" | perl -ne'/(\d+)/; print "$1 ";'
>
> This has the following result:
>
> 9 9 2
>
> ...instead of "9 2" as I'd originally expected. OK, $1 is set to the
> last successful value. I can deal with that. The weird part is what
> happens under -p instead, and when you look at both after deparsing.
> Deparsing this gives:
>
> LINE: while (defined($_ = <ARGV>)) {
> /(\d+)/;
> print "$1 ";
> }
>
> Changing my code from -n to -p :
>
> $ echo -e "9 \n \n 2" | perl -pe'/(\d+)/; print "$1 ";'
>
> prints as if $1 were reset to undef on non-match:
>
> 9 9
>
> 2 2
>
> This code deparses as:
>
> LINE: while (defined($_ = <ARGV>)) {
> /(\d+)/;
> print "$1 ";
> }
>
> continue {
> print $_;
> }
>
> If I use this code, commenting out the 'print $_;', I get the answer
> I'd originally expected:
>
> $ cat testperl.pl
> LINE: while (defined($_ = <ARGV>)) {
> /(\d+)/;
> print "$1 ";
> }
> continue {
> # print $_;
> }
>
> $ echo -e "9 \n \n 2"| perl testperl.pl
> 9 2
>
> Can anybody tell me why?
>
> -Daniel
> --
> http://kw.pm.org/ - Kitchener-Waterloo Perl Mongers - [EMAIL PROTECTED]
> http://coder.com/ - Prescient Code Solutions - (519) 575-3733 [EMAIL PROTECTED]
>
>
> On Wed, 21 Jul 2004 13:04:25 -0400, Tolkin, Steve <[EMAIL PROTECTED]> wrote:
> > OK, here is the answer:
> > http://www.perldoc.com/perl5.6.1/pod/perlre.html says:
> > The numbered variables ($1, $2, $3, etc.) and the related punctuation
> > set ($+, $&, $`, and $') are all dynamically scoped until the end of the
> > enclosing block or until the next successful match, whichever comes
> > first.
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