cems22 has asked for the wisdom of the Perl Monks concerning the following question:

THANK YOU EVERYONE FOR THE GREAT REPLIES. I must go commune with myself now since it's truly perplexing when one must derefernece and when not to, and when the dereference is implicit.

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original question Below:

Commonly one can index out multiple keys simultaneously from a hash like this:

my (%H) = (1,100,2,200,3,300);
@A=  @H{ (1,3) };

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@A now contains (100,200)

But how do I do this when I have not the Hash itself but a reference to the hash?

I want to be able to write something like this:

my $Href = {1 => 100,3 =>300};
@A = @{$Href}->{ (1,3) };

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But that does not work.

It would be crazy to reconsitutute the hash (if the hash is enormous) so I don't consider the following a viable option:

$Href = {1 => 100,3 =>300};
%H = %{$Href};
@A = @H{ (1,3) };

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The following does work but it is a hideous kludge I think should not be neccessary:

$Href = {1 => 100,3 =>300};
my @A;
{ local %G;  *G = $Href; @A = @G->{ (1,3) } }

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Yuck...

SO what's the right way to slice index a a hash reference?

Code tags added by GrandFather

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Re: Looking up an array of keys with a Hash reference
by bobf (Monsignor) on Sep 25, 2006 at 04:13 UTC

    It looks like you're trying to dereference twice (@{$Href}->{ (1,3) }). Try this instead: 

    my @A = @{ $Href }{ 1, 3 };

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    I asked a similar question a while ago. The responses in that thread may be of interest, especially if you have more complex data structures.

    HTH

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Re: Looking up an array of keys with a Hash reference
by grep (Monsignor) on Sep 25, 2006 at 04:09 UTC
    You do it the same way you did for the non-ref hash. 
    my (%H) = (1,100,2,200,3,300);
@A= @H{ (1,3) };

#BECOMES
my $H = { 1 => 100, 2 => 200, 3 => 300};
my @A = @{ $H }{1,3};

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    So other than adding the curlys for the dereference it's the same.


    grep
    Mynd you, mønk bites Kan be pretti nasti...
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Re: Looking up an array of keys with a Hash reference
by NetWallah (Canon) on Sep 25, 2006 at 05:24 UTC
    The '@' will bind to the '$Href' even without the braces. 
      my @A = @$Href{ 1, 3 };

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         "For every complex problem, there is a simple answer ... and it is wrong." --H.L. Mencken

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Re: Looking up an array of keys with a Hash reference
by jwkrahn (Abbot) on Sep 25, 2006 at 04:10 UTC
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