bugsbunny has asked for the wisdom of the Perl Monks concerning the following question:

hi,
I have very weird problem, or i'm too stupid..so, 
diff myprog myprog.new
276a266
>       my $xx = $db[$vars{_recNo}]{'name'};

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Ok the variable $xx is not used in any place of the program, just this assignment.. but the program yeld different result if I comment it ?! It is not nececery to make assignment, just using it changes the result.. in my case it was into one reg-ex.. but assignment is easer do debug...so lets continue..
do u have any explanation on this...
some examples, lets call result sets - rs1 and rs2 : 
my $xx = $db[$vars{_recNo}]{'name'};

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yield rs1 
my $xx = $db[$vars{_recNo}]{'name'};
$xx = 0;

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yield rs1 
#my $xx = $db[$vars{_recNo}]{'name'};

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yield rs2 
my $xx = $db[5]{'name'};

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yield rs2, never mind what number I use.. 
my $xx = $db[$vars{_recNo}];

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yield rs2

I'm again mentioning this virable is not used in any part of the script....
Tell me other testing scenarios ot better some solution :") I'm with Gentoo - perl-5.8.0-r12
It seems to me like some problem with hashes ! or me just doing stupid things !!

Replies are listed 'Best First'.
Re: weird problem or perl-bug?
by PodMaster (Abbot) on Dec 20, 2003 at 12:31 UTC
    Heard of autovivification? (btw, its documented in perlref...) 
    use strict;
use warnings;
use Data::Dumper;
my @bar;
my $foo = $bar[1]{bar};
die Dumper( \@bar );
__END__
$VAR1 = [
          undef,
          {}
        ];

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    MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
    I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
    ** The third rule of perl club is a statement of fact: pod is sexy.

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      yep i've heard about it... forgot about it.. have to test... thanx alot
      Does : 
      print 'a' if defined $vars[xxx]{zzz};

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      brings the element into existense.. i.e. how to check some element w/o turning it into existense if it doesnt exists already ?!
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        print 'a' if exists $vars[xxx]{zzz};

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        will check whether the hash contains an element with the key zzz, but not autovivify it. 

        perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web

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Re: weird problem or perl-bug?
by bart (Canon) on Dec 20, 2003 at 12:38 UTC
    Just mentioning $db[$vars{_recNo}]{'name'} might make it spring to life. It's a process known as "autovivification" ("automatic bringing to life", or somesuch). Note this little testscript: 
    use Data::Dumper;
my %vars = ( _recNo => 3 );
my @db;
(undef) = $db[$vars{_recNo}]{'name'};
print Dumper \@db;

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    Resulting in: 
    $VAR1 = [
          undef,
          ${\$VAR1->[0]},
          ${\$VAR1->[0]},
          {}
        ];

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    Never mind the ${\$VAR1->[0]}, it's just Data::Dumper's silly way of saying it's uninitialized (the "same" undef value as the first array item), but trying to check a (nonexistent) field in what isn't even a hash, makes the hash spring to life. That's why you see an empty anonymous hash there, as a fourth array element.

    Searching through Google for perl autovivify OR autovivification brings up this article as a first hit: Autovivification : What Is It and Why Do I Care?.

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Re: weird problem or perl-bug?
by Corion (Patriarch) on Dec 20, 2003 at 12:39 UTC

    My guess is, that each element of @db needs to contain a name entry in its keys. Maybe try:

    #my $xx = $db[$vars{_recNo}]{'name'};
$db[$vars{_recNo}]{'name'} = 'exists';

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    or 
    #my $xx = $db[$vars{_recNo}]{'name'};
$db[$vars{_recNo}]{'name'} = undef;

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    and see whether this gives you the same results. This is the "autovivication" PodMaster talks about. 
    perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web

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Re: weird problem or perl-bug?
by pg (Canon) on Dec 20, 2003 at 17:01 UTC

    In Perl, in case of handling a multi-tier collection (hash or array), to read (including test) an element, will automatically bring its ancestors (in the tree-like structure) into existance.

    I don't like this "feature" at all, as it largely reduces your application's maintainability. But we have to live with it (at least for now).

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Re: weird problem or perl-bug?
by Anonymous Monk on Dec 20, 2003 at 12:17 UTC
    Is this a joke?
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      I want it to be a joke, but no it is not a joke...:"(
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      Result set 1 :
78a
7a4
7ec
7ee
7f0
820
85e
866
868
890
896
8aa
8c2
8d6
8f4
90c
910


Result set 2 :
788
7a2
7ea
7ec
7ee
81e
85c
864
866
88e
894
8a8
8c0
8d4
8f2
90a
90e

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