How to copy an list from a reference hash?

lazurwol has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks,
I am trying to use copy a list from a reference hash. I am passing the hash to a subroutine as a reference.
I can copy individual items from the hash without a problem but I cannot figure out how to copy the list contained in the refence.
This is what I have tried.

my %Point = ('E' => [0,1,2],  'ID' => 'X30Y30VX11VY15E15');
&SomeRoutine(\%Point);

sub SomeRoutine {
    $P = shift;
    $id = $$P{'ID'};
    @e = $@{$P{'E'}};
[download]

When I use this code the copy of the list returns back the following error:
Use of uninitialized value in hash element

Comment on How to copy an list from a reference hash? Download Code

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Re: How to copy an list from a reference hash? by davido (Cardinal) on Nov 30, 2004 at 22:24 UTC
Try this: `sub SomeRoutine { my $href = shift; my $id = $href->{'ID'}; my @e = @{$href->{'E'}}; #........ }` [download] Dave	[reply] [d/l]
Re: How to copy an list from a reference hash? by Velaki (Chaplain) on Nov 30, 2004 at 22:36 UTC
Running this with use strict reveals a helpful error: `Global symbol "%P" requires explicit package name` [download] in the line containing `@e = $@{$P{'E'}};` `$P` is a reference, whereas `$P{'E'}` refers to a simple hash variable, called `%P`. Since you wish to use the reference, you will need to dereference it back to the hash `%$P`, using either: `$$P{'E'}` [download] or `$P->{'E'}` [download] Now, since that is also a reference to an array, you will need to dereference that with; `@{$P->{'E'}}` [download] So the line which strict reports in error should be: `@e = @{$P->{'E'}};` [download] Hope that helped, `use strict; # it's the right thing to do` -v "Perl. There is no substitute."	[reply] [d/l] [select]
Re: How to copy an list from a reference hash? by injunjoel (Priest) on Nov 30, 2004 at 22:26 UTC
Greetings all, What about this? `sub SomeRoutine { $P = shift; $id = $P->{'ID'}; @e = @{$P->{'E'}}; }` [download] Personally I prefer the arrow notation for dereferencing. It just make more visual sense to me. I hope that helps, -InjunJoel "I do not feel obliged to believe that the same God who endowed us with sense, reason and intellect has intended us to forego their use." -Galileo	[reply] [d/l]
Re: How to copy an list from a reference hash? by jimbojones (Friar) on Nov 30, 2004 at 22:28 UTC
The arrow is your friend `use strict; use warnings; my %Point = ('E' => [0,1,2], 'ID' => 'X30Y30VX11VY15E15'); &SomeRoutine(\%Point); sub SomeRoutine { my $hashref = shift; my $id = $hashref->{'ID'}; my @e = @{$hashref->{'E'}}; print $id, "\n"; print @e, "\n"; } __DATA__ X30Y30VX11VY15E15 012` [download] HTH, -j	[reply] [d/l]