Question about arrays of hashes

Forgive a newbie question, but I'm hoping that someone can shed some
light for me on weirdness I'm seeing when trying to access a particular
hash from an array-of-hashes. I had a lot of difficulty assigning a
particular element to its own variable, but after some experimentation I
found that this works:


my @array = MyModule->getArrayOfHashes();
my $hash = $array[$index];
%hash = %$hash;


I have to first create the variable in scalar context, then do a funky
dereferencing thing while changing the context to hash. For one thing,
I'd like to better understand what is really going on here... I just
stumbled across this solution through experimentation and dumb luck, I
don't really understand it. Secondly, I'm wondering if there's some
shorthand technique for doing this that doesn't require three lines of
code. Thanks in advance!

Re: Question about arrays of hashes

Steve schrieb:
> my @array = MyModule->getArrayOfHashes();
> my $hash = $array[$index];
> %hash = %$hash;

Get n'th hash-Reference from the array: (MyModule->getArrayOfHashes())[n]

Get value of hash-key 'foo' from n'th hash-reference from the array:
(MyModule->getArrayOfHashes())[n]->{'foo'}

Re: Question about arrays of hashes

Steve wrote:
> Forgive a newbie question, but I'm hoping that someone can shed some
> light for me on weirdness I'm seeing when trying to access a particular
> hash from an array-of-hashes. I had a lot of difficulty assigning a
> particular element to its own variable, but after some experimentation I
> found that this works:
>
> my @array = MyModule->getArrayOfHashes();
> my $hash = $array[$index];
> %hash = %$hash;
>
> I have to first create the variable in scalar context, then do a
> funky dereferencing thing while changing the context to hash. For one
> thing, I'd like to better understand what is really going on here...

perldoc perlref

> I'm wondering if there's some shorthand technique for doing this that
> doesn't require three lines of code.

What's "this"? Copying one of the hashes to a named hash?

my %hash = %{ $array[$index] };

Or assigning a value to one of the anonymous hashes?

$array[$index]->{somekey} = 'somevalue';

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl

Re: Question about arrays of hashes

Steve wrote:
> Forgive a newbie question, but I'm hoping that someone can shed
> some light for me on weirdness I'm seeing when trying to access a
> particular hash from an array-of-hashes. I had a lot of difficulty
> assigning a particular element to its own variable, but after some
> experimentation I found that this works:
>
> my @array = MyModule->getArrayOfHashes();
> my $hash = $array[$index];
> %hash = %$hash;
>
> I have to first create the variable in scalar context, then do a
> funky dereferencing thing while changing the context to hash. For one
> thing, I'd like to better understand what is really going on here... I
> just stumbled across this solution through experimentation and dumb luck,
> I don't really understand it.

See the "perldoc"s for perlref, perlreftut, and perldsc. I don't how to
describe it better than they do. (If I did, I would propose replacing them
with my better description.)


> Secondly, I'm wondering if there's some
> shorthand technique for doing this that doesn't require three lines of
> code. Thanks in advance!

You need get the right element out of the list, which you do by
subscripting the list with the ()[] form:

my $hashref = (MyModule->getArrayOfHashes())[$index];

Although this does seem strange for me for semantic rather than syntactic
reasons. It seems like an odd OO module design that would lead me to
already know which index I want from a list when I don't yet have the list
but need to go to the module to get it. I know that is the way things work
with things like the "stat" function, but the stat function isn't OO.

Then you need to dereference that, which you do by %{} form:

my %hash=%{(MyModule->getArrayOfHashes())[$index]};

Although 90+% of the time, I would just get the $hashref and dereference
it as needed, rather than making a dereferenced copy up-front.

Xho

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