Re: Autovivification and hash slices

The problem is that the parameters being passed to the user subroutine are being evaluated as they are passed, so the autovivify has already been done before the subroutine receives them. Apparently, (some wild-assed guess here) the built in join has some magic to check for existence before trying to evaluate.

If you pass a hash reference and key value separately to your myjoin, you have a chance to check for existence before the key can be autovivified. It isn't as slick as the builtin, but it's not too bad.

use Data::Dumper;
sub mydump($) {
    print Data::Dumper->Dump( [ $_[0] ], ['*hash'] );
}

sub myjoin {
    return unless exists($_[1]->{$_[2]});
    return join $_[0], $_[1]->{$_[2]};
}

my %hash;
my $s1 = join( ':', @hash{qw(key)} );
mydump( \%hash );
my $s2 = myjoin( ':', \%hash, 'key' );
mydump( \%hash );
[download]

Comment on Re: Autovivification and hash slices Download Code

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Re^2: Autovivification and hash slices by norbert.csongradi (Beadle) on Aug 17, 2011 at 13:55 UTC
Yeah, that is clear :). My problem is the modification of the original hash without touching the hash itself... and it only happens with hash slices :(	[reply]
Re^3: Autovivification and hash slices by thundergnat (Deacon) on Aug 17, 2011 at 14:20 UTC
Ah. Sorry, kind of glossed over the "slice" aspect there. All right, try this then. `use Data::Dumper; sub mydump($) { print Data::Dumper->Dump( [ $_[0] ], ['*hash'] ); } sub myjoin { my ($sep, $href, @keys) = @_; my @exists = grep { exists($href->{$_}) } @keys; return join $sep, @exists; } my %hash = (key1 => 'a'); my $s1 = join( ':', @hash{qw(key)} ); mydump( \%hash ); my $s2 = myjoin( ':', \%hash, qw/key1 key2 key3/ ); mydump( \%hash );` [download]	[reply] [d/l]