# @hash{@arr}=(); scalar into array then?

Just want to get a handle on what's going on here.  This
answer was given in response to remove duplicate entires.

Here is the construct under question.

@hash{@arr}=();
@arr = keys %hash;

My logic ....

@hash{'key'} is one value.
@hash{qw(A B C D)} is my view of his @hash{@arr}

So, @a = (qw( A B C)) initializes an array
@a = {qw( A B C)} initializes a hash.

Then, along the same line (my lines), you could do this then

\$a = {qw(A B C)}

and then

foreach (@a) {
print;
}

and get a normal array from that?  Except it
doesn't work.  Why can't you force \$a into
an array.  @a = {list} forced @a into %a!

Strighten me out.  Thanks.
LVL 1
###### Who is Participating?

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

OwnerCommented:
You were doing fine up til the line

@a = {qw( A B C)} initializes a hash.

While this is true, in a way, it isn't doing what you think. The construct

{qw(A B C) }

returns a reference to an anonymous hash and this reference is taken (even though you didn't write it precisely right) as the single element of the array @a. Data::Dumper will tell you that \@a looks like:

[
{
'A' => 'B',
'C' => undef
}
];

The constructions that you were given were using hash _slices_.

@hash{@arr} = ();

is a shortcut to initialize a hash (using the _slice_) with a set of keys and undefined values. For this purpose, the keys are all that is needed. In initializing the hash, duplicate keys replace any earlier instances of themselves, so to get a list of unique keys you do

@arr = keys %hash;

\$hash{'key'} is one value.
@hash{'key'} is a hash slice with only one value. It's equivalent to the simple \$hash{'key'} in many (most?) contexts. It's only when there's a multi-item list inside the braces that you must treat it as an array.

Experts Exchange Solution brought to you by

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Commented:
@hash{qw(A B C D)} = ()
is equivalent to
(\$hash{A},\$hash{B},\$hash{C},\$hash{D}) = ();

@hash{'key'} = localtime;
is not equivalent to
\$hash{'key'} = localtime;
OwnerCommented:
Ozo's right, of course. I just had trouble thinking up the right counterexample (been out of practice). The contexts where @hash{'key'} will be taken as equivalent to \$hash{'key'} will fall on the right hand side of an equals sign, but it would be better to use scalars when you want scalars and arrays (or slices) where you want arrays.
Author Commented:
Thanks both.
###### It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Perl

From novice to tech pro — start learning today.