$h4X4_|=73}{ has asked for the wisdom of the Perl Monks concerning the following question:

I have some thing happening in a package that seems strange to me. Lets say I have a blank array @array = (); when I check if the array{hash} is defined that should not be. Then push to that array and check the number again the array shifts to the next number.
I hope the code explains it better... 

#!/usr/bin/perl
use warnings;
use strict;
# add to @Foo::THING
push(@Foo::THING, { 'check' => '1',} );
# Should say "Some Thing"
Foo::check_THING(0)
? print "Some Thing\n"
: print "No Thing\n";

# Should say "No Thing"
Foo::check_THING(1)
? print "Some Thing\n"
: print "No Thing\n";

# add to @Foo::THING
push(@Foo::THING, { 'check' => '1',} );
# Check if it was added?
Foo::check_THING(1)
? print "Some Thing\n" # I think it should be this
: print "No Thing\n"; # But we get this

# Why is it 2?
Foo::check_THING(2)
? print "Some Thing\n" # WTF?
: print "No Thing\n";

package Foo;

our @THING = ();

sub check_THING {
my $id = shift;
defined $THING[$id]{check}
 ? return 1
 : return 0;
}

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Re: Array skips number if checked for defined.
by Athanasius (Archbishop) on Jun 26, 2016 at 14:35 UTC

    Hello $h4X4_|=73}{,

    To expand a little on hippo’s answer: If you print out the contents of @Foo::THING immediately after the statement beginning Foo::check_THING(1), you’ll see an additional, empty, anonymous hash has been autovivified by the action of referring to $THING[1]{check}1:

     0:20 >perl 1665_SoPW.pl
Some Thing
No Thing
[{ check => 1 }, {}]
No Thing
Some Thing

 0:20 >

    [download]

    (I’m using Data::Dump.) And if you add this line:

    no autovivification;

    [download]

    immediately before the line our @THING = ();, you’ll get the behaviour you expect:

     0:20 >perl 1665_SoPW.pl
Some Thing
No Thing
[{ check => 1 }]
Some Thing
No Thing

 0:24 >

    [download]

    (See the autovivification pragma.)

    1Note that you can replace check with any key name here and get the same result.

    Hope that helps,

    Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: Array skips number if checked for defined.
by hippo (Archbishop) on Jun 26, 2016 at 14:06 UTC
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Re: Array skips number if checked for defined.
by FreeBeerReekingMonk (Deacon) on Jun 26, 2016 at 14:59 UTC
    Alternatively, if you change your check function to:

    sub check_THING {
  my $id = shift;
  return 0 unless defined $THING[$id];
  defined $THING[$id]{check}
    ? return 1
    : return 0;
}

    [download]

    you get: 

    Some Thing
No Thing
Some Thing
No Thing

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