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Re: Autovivification sucking the life out of me

by Skeeve (Parson)
on Oct 14, 2005 at 07:37 UTC ( #500149=note: print w/replies, xml ) Need Help??

in reply to Autovivification sucking the life out of me

Your autovivification happens here:
see_args( @ENV{qw(TMPDIR TEMP TMP)} );
If you explicitly tell perl to vivify $ENV{TMPDIR} it will do so. It's not the call doing it, but the parameters you pass in your call.


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Re^2: Autovivification sucking the life out of me
by Perl Mouse (Chaplain) on Oct 14, 2005 at 12:34 UTC
    Whenever a non-existing hash or array element is used in anything but a simple context, it's autovivificated. The *default* is to autovivificate - it's just that the exceptions happen to be the most often used cases, so it's a surprise when perl autovivifies.
    #!/usr/bin/perl use strict; use warnings; my %hash; sub X {} my $x; $x = exists $hash{key1}; printf "key1 %sautovivified\n", exists $hash{key1} ? "" : "not "; $x = $hash{key2}; printf "key2 %sautovivified\n", exists $hash{key2} ? "" : "not "; $x = exists $hash{key4}{key3}; printf "key3 %sautovivified\n", exists $hash{key4}{key3} ? "" : "not +"; printf "key4 %sautovivified\n", exists $hash{key4} ? "" : "not "; ($x) = @hash{key5}; printf "key5 %sautovivified\n", exists $hash{key5} ? "" : "not "; ($x) = @hash{"key6", "key7"}; printf "key6 %sautovivified\n", exists $hash{key6} ? "" : "not "; printf "key7 %sautovivified\n", exists $hash{key7} ? "" : "not "; X($hash{key8}); printf "key8 %sautovivified\n", exists $hash{key8} ? "" : "not "; X(@hash{key9}); printf "key9 %sautovivified\n", exists $hash{key9} ? "" : "not "; X(@hash{"keyA","keyB"}); printf "keyA %sautovivified\n", exists $hash{keyA} ? "" : "not "; printf "keyB %sautovivified\n", exists $hash{keyB} ? "" : "not "; __END__ key1 not autovivified key2 not autovivified key3 not autovivified key4 autovivified key5 not autovivified key6 not autovivified key7 not autovivified key8 not autovivified key9 autovivified keyA autovivified keyB autovivified
    Perl --((8:>*
      Wow, thanks for the examples. It would seem "simple usage" is not what people would assume it is. I will go file a bug report against PathTools, but it may be instructive to see how easy it is to fall into this trap. Here is the line of code in that caused me all the grief:
      $tmpdir = $_[0]->_tmpdir( @ENV{qw(TMPDIR TEMP TMP)}, 'SYS:/temp', 'C:\system\temp', 'C:/temp', '/tmp', '/' );
      Sure looks innocent, huh? Of course the 'fix' will be to change the references to 'simple' ones,
      $tmpdir = $_[0]->_tmpdir( $ENV{TMPDIR}, $ENV{TEMP}, $ENV{TMP},

      And in answer to Skeeve, yes, I am a "bug-magnet"

Re^2: Autovivification sucking the life out of me
by shenme (Priest) on Oct 14, 2005 at 07:51 UTC
    But I am not explicitly telling Perl to autovivify, rather I am (intending to) just reference the values, in this case using a slice.

    Refer to my simpler example above. Using see_args( @foo{qw(BAR BAZ)} ); causes result:

    at start: BAR '<absent>' at end: BAR '<undef>'
    But changing that to see_args( $foo{BAR}, $foo{BAZ} ); results in:
    at start: BAR '<absent>' at end: BAR '<absent>'
    This is most perplexing to me.

      Tell you what: It's perplexing to me the other way around!

      Until 2 days ago I thought something like
      if ($foo{'BAR'}) ...
      would already vivify that hash entry.

      But then I learned in the Chatter Box that it doesn't in perl 5.8 (or maybe a bit earlier).

      And now I did a test. Compare the output

      use Data::Dumper; %foo=(BAZ=>1); print Dumper \%foo if $foo{BAR} || 1; print Dumper \%foo if @foo{BAR,BAZ} || 1;

      $VAR1 = {
                'BAZ' => 1
      $VAR1 = {
                'BAZ' => 1

      use Data::Dumper; %foo=(BAZ=>1); print Dumper \%foo if call($foo{BAR}); print Dumper \%foo if call(@foo{BAR,BAZ}); sub call { return 1 }

      $VAR1 = {
                'BAZ' => 1
      $VAR1 = {
                'BAR' => undef,
                'BAZ' => 1

      So it's really the fact that the slice is used in a subroutine call. When you replace call with print, autovivification does not happen

      So now I ask myself: Is there a bug and you are certified bugfinder!? ;-)


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