in reply to Re: Erroneous defined detection of array elements
in thread Erroneous defined detection of array elements

and will not auto-vivify them

defined will not autovivify either: 

my @array;
if (defined $array[20]) { }                           # no autovivific
+ation, thus
if (defined $array[20]) { warn "[20] does exist!" }   # no warning

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Typically, it's a dereferencing operation that unexpededly autovivifies.  exists shares the same problem: 

my @array;
if (exists $array[20]->[42]) { }
if (exists $array[20]) { warn "[20] does exist!" }

__END__
[20] does exist! at ./808911.pl line 5.

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Re^3: Erroneous defined detection of array elements
by keszler (Priest) on Nov 23, 2009 at 20:32 UTC
    I should have been more clear - defined does not distinguish between "never existed" and "is undef" for hash and array elements. I therefore consider exists a better test for unexpected auto-vivification.
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      defined does not distinguish between "never existed" and "is undef" for hash and array elements.

      Neither does exist. It checks if the element currently exists, not if the element has ever existed.

      Either way, so what?

      I therefore consider exists a better test for unexpected auto-vivification.

      That makes no sense. It doesn't prevent autovivification. It doesn't tell you whether something was autovivified or not. It doesn't help at all. 

      $ perl -le'{
    my @a;  $a[15][0];
    print "After autovivification:      ",
        exists($a[14])  || 0,
        exists($a[15])  || 0,
        defined($a[14]) || 0,
        defined($a[15]) || 0;
}{
    my @a;  $a[15] = [];
    print "After explicit vivification: ",
        exists($a[14])  || 0,
        exists($a[15])  || 0,
        defined($a[14]) || 0,
        defined($a[15]) || 0;
}'
After autovivification:      0101
After explicit vivification: 0101

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