How to slice only defined values?

Question

I can slice kes/values as next:

$item->%{ @cols }

But if some column does not exist at $item It will be created at resulting hash.

Can I slice only defined values?

Answer 1

You can check whether they exist.

$item->%{ grep {exists $item->{$_}} @cols }

should do the job slicing only the existing values.

Anyway - simply accessing these values should NOT autovivify them. Only if you Pass these values as parameters to some function and they are implicetly aliased there, they are autovivified.

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

my @cols =qw (a b c);
my $item = [{a => 1, c => 3}];

print Dumper({$item->[0]->%{ grep {exists $item->[0]->{$_}} @cols }});
print Dumper($item);

print Dumper({$item->[0]->%{ @cols }});
print Dumper($item);

print Dumper($item->[0]->%{ grep {exists $item->[0]->{$_}} @cols });
print Dumper($item);

print Dumper($item->[0]->%{ @cols }); ## Only here does autovivication take place
print Dumper($item);

Only the last print will generate the:

$VAR1 = [
          {
            'c' => 3,
            'a' => 1,
            'b' => undef
          }
        ];

indicating that b got autovivified.