Re^2: Difference between exists and defined

Your code example, the way you use Data::Dumper to show the contents of the array allows room for misinterpretation of what is realy happening here. I feel that a better code example is the following:

use strict ;
use warnings ;
use Data::Dumper ;

my @ar ;
$ar[0] = undef ;
$ar[2] = 3 ;

if ( exists $ar[0] ) {
    print "0 exists\n" ;
}
if ( exists $ar[1] ) {
    print "1 exists\n" ;
}
if ( exists $ar[2] ) {
    print "2 exists\n" ;
}
if ( exists $ar[3] ) {
    print "3 exists\n" ;
}
print Dumper(\@ar) ;
[download]

This prints:

0 exists
2 exists
[download]

As you can see element 0 DOES exists when it is explicitly set to undefined. Using Data::Dumper prints:

$VAR1 = [
          undef,
          undef,
          3
        ];
[download]

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Re^3: Difference between exists and defined by thanos1983 (Parson) on Apr 16, 2019 at 10:52 UTC
Hello Veltro, You are right my example was not clear if I do not add the following part: `#!/usr/bin/perl use strict; use warnings; use Data::Dumper; use feature 'say'; my @array; $array[0] = undef; # Assigns the value 'undef' to $array[0] $array[3] = 2; # print Dumper \@array; for my $value (@array) { say "Defined: \$array[$value]" if defined $value; } __END__ $ perl test.pl Defined: $array[2]` [download] The reason that this is happening is explained in the documentation exists: `Given an expression that specifies an element of a hash, returns true +if the specified element in the hash has ever been initialized, even +if the corresponding value is undefined.` [download] So if you check the array element with exists and you have manually defined undef to the element then exists will return True. :) Thanks for pointing it out. Hopefully it will avoid confusion for future reference. :) BR / Thanos Seeking for Perl wisdom...on the process of learning...not there...yet!	[reply] [d/l] [select]

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Re^3: Difference between exists and defined
by thanos1983 (Parson) on Apr 16, 2019 at 10:52 UTC

Hello Veltro,

You are right my example was not clear if I do not add the following part:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use feature 'say';

my @array;
$array[0] = undef; # Assigns the value 'undef' to $array[0]
$array[3] = 2;
# print Dumper \@array;

for my $value (@array) {
    say "Defined: \$array[$value]" if defined $value;
}

__END__

$ perl test.pl 
Defined: $array[2]
[download]

The reason that this is happening is explained in the documentation exists:

Given an expression that specifies an element of a hash, returns true 
+if the specified element in the hash has ever been initialized, even 
+if the corresponding value is undefined.
[download]

So if you check the array element with exists and you have manually defined undef to the element then exists will return True. :)

Thanks for pointing it out. Hopefully it will avoid confusion for future reference. :)

BR / Thanos

Seeking for Perl wisdom...on the process of learning...not there...yet!

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