Hash of Hashes

[perln00b]

I am new in Perl script. I get a real headache to understand
"hash of hashes". Could someone please explain to me? Here is some example codes:
"...
if($tempids{'IP'}{ $IP { $hostname } }{ $resourceID } !=1 )
{
$add_detail{'IP'}{$resourceID}{$IP{$hostname}} = 1;
}
...
"

Thanks,
Lucas

 

[KevinADC]

my %Flintstones = (
   Fred => { kids => 'Pebbles',
             wife => 'Wilma',
             pet => 'Dino',
           },
   Barney => { kids => 'Bam-Bam',
               wife => 'Betty',
               pet => 'None',
              },
);

print "Fred's wife is $Flintstones{Fred}{wife}.
Barney's wife is $Flintstones{Barney}{wife}.
Fred has a pet dinosuar named $Flintstones{Fred}{pet}.
Their childrens names are $Flintstones{Fred}{kids} and $Flintstones{Barney}{kids}.";

prints:

Fred's wife is Wilma.
Barney's wife is Betty.
Fred has a pet dinosuar named Dino.
Their childrens names are Pebbles and Bam-Bam.

 

[KevinADC]

Just like you enter a tall building on the bottom floor, (the building is the top level thinking in hash terms). You find the floor you need (the second level in hash terms), and finally you find the office you want (the bottom level of the hash). You can do that without even thinking about it!

Clinic -> 2nd Floor -> Office 2B = Dr. Smith

The hash is really just the same:

Flintstones -> Fred -> Wife = Wilma

 

[Corwinsw]

My representation is a tree, which name is the name of the hash and each branch is a key of that hash. If the branch has it's own branches, they also have names and that are the keys of their parent branch.
Try using Data:umper or the Perl debuger. to see that visually.

Corwin

 

[Corwinsw]

The thing which may be confusing in your case is that your first key is also hash value.
So $tempids is the tree, $IP { $hostname } is just the name of one of it's branches, and $resourceID is a little branch inheritor of the branch with name $IP { $hostname }. Then we chek if we have a bud on the last branch or something else, which is the value of the hash.

Corwin