Re^5: Array dereference in foreach()

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]

Comment on Re^5: Array dereference in foreach() Select or Download Code

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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
What is this "special context similar to lvalue"? Is there any difference to usual lvalue other than not throwing exception in case of array-dereferencing undef? However it is not special enough to work smoothly with dereferencing undef as hashref. This one throws exception. `my $a; foreach (keys %$a) { print "$_\n"; }` [download]	[reply] [d/l]
Re^9: Array dereference in foreach() by choroba (Cardinal) on Nov 16, 2017 at 12:59 UTC
Re^10: Array dereference in foreach() by pme (Monsignor) on Nov 16, 2017 at 13:03 UTC
Re^7: Array dereference in foreach() by haukex (Archbishop) on Nov 16, 2017 at 12:43 UTC
why this throws exception ... `my @b = @$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]