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search position is getting reset after 'local'

by jesuashok (Curate)
on Jun 12, 2006 at 11:31 UTC ( #554784=perlquestion: print w/ replies, xml ) Need Help??
jesuashok has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

When I executed the below code, I found that the variable which is being used in a separate block with the scope 'local' is not being able to use outside the block.

Please check how the '$x' is being used in the below code.

#!/usr/bin/perl -w $x = "123 56"; $x =~ / /g; print "$x, ", pos($x), "\n"; { local $x } print "$x, ", pos($x), "\n";
Output :-
123 56, 4 Use of uninitialized value in print at l.pl line 5. 123 56,
what does the 'pos' do with '$x' ?

"Keep pouring your ideas"

2006-10-08 Unapproved by planetscape once evidence of habitual plagiarism uncovered.

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Re: search position is getting reset after 'local'
by japhy (Canon) on Jun 12, 2006 at 11:50 UTC
    It's a known bug in Perl -- nearly four years old. But your post makes no sense to me. The variable CAN be used outside the block; what can't be used (or more specifically, what has lost its value) is the internal position in the string held by the variable.

    Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
    How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
Re: search position is getting reset after 'local'
by Joost (Canon) on Jun 12, 2006 at 11:51 UTC
      No, local is more close to this:
      { $orig_value = \$x; *x = \$tmp_new_value; ....; *x = $orig_value; }

      and the OP has reported a genuine bug

      Dave.

Re: search position is getting reset after 'local'
by reasonablekeith (Deacon) on Jun 12, 2006 at 13:34 UTC
    so another reason to "use strict;" then?

    I can't see how you'd reproduce this bug with it switched on.

    Update: I just _knew_ someone would see this post as a challenge. I meant that, if one were using strict and declaring lexically scoped varaible, you probably wouldn't hit this, as you well know Corion.

    Anyway, I thought "use vars;" was obsolete, so I'm not sure it counts? Any other offers? :)

    ---
    my name's not Keith, and I'm not reasonable.
      #!/usr/bin/perl -w use strict; use vars qw($x); $x = "123 56"; $x =~ / /g; print "$x, ", pos($x), "\n"; { local $x } print "$x, ", pos($x), "\n";

      Re your update, here's a version that doesn't use use vars:

      #!/usr/bin/perl -w use strict; our $x = "123 56"; $x =~ / /g; print "$x, ", pos($x), "\n"; { local $x } print "$x, ", pos($x), "\n";

      I think you meant, "another reason to avoid using package variables".

      By the way, I don't see how you got it to work with strict on. It doesn't work if you keep $x as a package variable, and local $x doesn't work if you changed $x into a lexical (i.e. used my $x).

Re: search position is getting reset after 'local'
by Errto (Vicar) on Jun 12, 2006 at 18:24 UTC
    Based on the discussion here this appears to be a genuine bug, but I have a question: where in actual code would you bump into this scenario?
      I don't remember what I was doing, but I encountered this bug before. It was possibly in some kind of lexer or parser.

      Not an actual usage, but I could easily see something which did a match, called a sub which localized $_, and then tried to use pos() after that sub returned. Granted it'd be kinda sloppy code (like depending on $1 and friends to remain set rather than saving values off if you need to use them), but again I could see it happening.

      I think the main potential problem would be when you're using localized $_ combined with m//g matches.

      let's say you convert this (working) code:

      $_ = "abcdefg"; my @some_array = (1 ..2); while (m/./g) { for (@some_array) { # implicitly localized - works # do something; } print $&; }
      to this (stupid but more or less equivalent) code:
      $_ = "abcdefg"; while (m/./g) { my @some_array = (1 ..2); while (local $_ = shift @some_array) { # manually localized - break +s # do something; } print $&; }

      Some of the parsing code i've written makes heavy use of $_ and then you can run into this problem when you're refactoring.

      update: the above code breaks, because it's not the pos() function that's broken, but the match position field itself gets reset by localizing a variable - in other words, the while (m/./g) statement will loop forever.

Re: search position is getting reset after 'local' (Workaround)
by ikegami (Pope) on Jun 12, 2006 at 22:27 UTC

    There is a workaround:

    #!/usr/bin/perl -w $x = "123 56"; $x =~ / /g; print "$x, ", pos($x), "\n"; { local *x } # <--- print "$x, ", pos($x), "\n";

    The catch: @x, %x, &x, etc will get localized too. In other words, if you use local *_ to protect the pos of $_, you will lose access to the current @_ for the remainder of block.

Re: search position is getting reset after 'local'
by shmem (Canon) on Oct 06, 2006 at 14:42 UTC
    Yet another plagiarized OP.

    The code you present has a striking similarity with the TODO block of this test.

    I wonder what's your intention asking that here? What answer other than "yeah, that's on the TODO list for the pos operator" do you expect?

    --shmem

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                                  /\_¯/(q    /
    ----------------------------  \__(m.====·.(_("always off the crowd"))."·
    ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
      Actually, the link you have is not the original source of the code. I added the test to the core as a TODO. The original code is actually from here.

      That's a good question. From what I've read in his other threads he's not likely to answer such a direct question though.

      Unfortunately, from having read all of jesuashok's nodes now, I have learned (thanks to the good detective work of monks such as liverpole and yourself) that a high percentage of them, like this one, are plagerised.

      It is apparent that a fair number of them (despite having had lots of monks -- them) still have a net positive reputation.

      For example, his (currently, as of 18 Nov 2006) highest scoring node (which also happens to be plagerised) at the moment has a rep of 80 even though 45 people have downvoted it.

      It seems to me that he wants to increase his XP, (like money, it's a lot more important to those who don't have much of it) and he has found that by copying interesting (and as obscure as possible) questions off the Interwebs and posting them here... enough people will come along and see them and think "that's a good question" and ++ it - not knowing (or perhaps even caring? - I hope they would!) at the time that they vote on it, that it is a plagerised question.

      The net result of this, even though the finding of the node may cause outrage in the monastary and score him lots of --, is that he gains XP.

      Even though plagerism goes against what the monastary believes in, even though there's no personal pride in posting it (unless he has managed to sneak in some plagerised questions which he has changed enough for the originals to not be Googleable from them, so they're below the radar and haven't been picked up by us yet - perhaps he gets a kick out of that), he has learned through experience that posting other people's questions helps him get the XP he wants.

      From the range of his questions, some incredibly technical and in-depth (sometimes right over my head) and some that seem so basic that I'm shocked that someone who could understand enough to post the in-depth questions would need to ask such a thing, I become more and more suspicious that there are more of his posts that have not been brought to light, but like the others are cut and pasted from other places.

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