Re^4: Array dereference in foreach()

Thank you to for your detailed explanation. That makes sense. However I found another interesting feature.

my $h;
my $g;

$h->{abc} = $g->{lkj};

print "h: ", Dumper(\$h), "\n";
print "g: ", Dumper(\$g), "\n";
[download]

and the output is

h: $VAR1 = \{
            'abc' => undef
          };

g: $VAR1 = \{};
[download]

The left side ($h) is autovivified and undef is assigned to key 'abc'. It is correct. But what has happened on the right? Is it semi-autovivified or what? Empty anonymous hashref is assigned to $g. Where is 'lkj'? Is $g->{lkj} in lvalue context? Why there is no exception?

Regards

Comment on Re^4: Array dereference in foreach() Select or Download Code

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Re^5: Array dereference in foreach() by choroba (Cardinal) on Nov 16, 2017 at 10:20 UTC
That's how autovivification currently works. In order to assign the value to the LHS, you need to look what's in `$g->{lkj}`. To check for the existence of the key, you first need $g to be a hash reference, if it's undef, it autovivifies, else if it's not a hash ref, you get an exception (under strict). There's no need to create the lkj key in $g, as it wasn't used in an lvalue context, but if you tried to access `$g->{lkj}{mno}` or `$g->{lkj}[2]` ... Use `no autovivification;` [download] if you don't want the structures to be created. See autovivification. ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]
Re^6: Array dereference in foreach() by pme (Monsignor) on Nov 16, 2017 at 12:17 UTC
If $g is autovivified when it is on the right side of equal sign I just don't get why this throws exception. `my $a; my @b = @$a;` [download] and why this one works smoothly `my $a; foreach (@$a) { print "$_\n"; }` [download]	[reply] [d/l] [select]
Re^7: Array dereference in foreach() by choroba (Cardinal) on Nov 16, 2017 at 12:23 UTC
Note that $a gets autovivified into `[]` after `for (@$a)` -- that's because for creates a special context similar to lvalue (as its list's elements are aliased to the loop variable). ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]
Re^8: Array dereference in foreach() by pme (Monsignor) on Nov 16, 2017 at 12:55 UTC
Re^9: Array dereference in foreach() by choroba (Cardinal) on Nov 16, 2017 at 12:59 UTC
Some notes below your chosen depth have not been shown here
Re^7: Array dereference in foreach() by haukex (Archbishop) on Nov 16, 2017 at 12:43 UTC
why this throws exception ... `my @b = @$a;` ... and why this one works smoothly ... `foreach (@$a) { }` I hope that my earlier explanations, about why `my $x; my $y = @$x;` throws an error but `my $x; @$x = ();` does not, make sense? Do you see how in the second example, `@$x` is in "lvalue context", that is, it is on the left-hand side of the assignment? `foreach` is kind of special, it aliases the loop variable to the things it is looping over. In the following, see how `$val` becomes an alias for each of the variables in turn and the original values `$x,$y,$z` are modified via `$val`: `my ($x,$y,$z) = ('x','yy','zzz'); for my $val ($x,$y,$z) { $val = $val . length($val); } print "$x/$y/$z\n"; # prints "x1/yy2/zzz3"` [download] Loosely speaking, this is why the variables `foreach (...)` is looping over are treated by Perl as if they were on the left-hand side of an assignment (lvalue context), and why the autovivification behavior is applied to them.	[reply] [d/l] [select]
Re^5: Array dereference in foreach() by 1nickt (Canon) on Nov 16, 2017 at 12:19 UTC
Hi, on a side note, you don't need to pass to Data::Dumper a reference to the structure you are dumping, if that structure is already a reference. `my %hash = ( foo => 'bar' ); my $href = { baz => 'qux' }; print " hash: ", Dumper( \%hash ), "\n"; print "hashref: ", Dumper( $href ), "\n";` [download] Hope this helps! The way forward always starts with a minimal test.	[reply] [d/l]