CODEWITHSUNDEEP
perlhashperl-data-structures
Mike
Mike

Reputation: 60771

How do I create a hash of hashes in Perl?

Based on my current understanding of hashes in Perl, I would expect this code to print "hello world." It instead prints nothing.

%a=();

%b=();
$b{str} = "hello";  
$a{1}=%b;

$b=();
$b{str} = "world";
$a{2}=%b;

print "$a{1}{str}  $a{2}{str}";

I assume that a hash is just like an array, so why can't I make a hash contain another?

Upvotes: 8

Views: 4335

Answers (6)

Clarius
Clarius

Reputation: 1419

I needed to create 1000 employees records for testing a T&A system. The employee records were stored in a hash where the key was the employee's identity number, and the value was a hash of their name, date of birth, and date of hire etc. Here's how...

# declare an empty hash
%employees = ();

# add each employee to the hash
$employees{$identity} = {gender=>$gender, forename=>$forename, surname=>$surname, dob=>$dob, doh=>$doh};

# dump the hash as CSV
foreach $identity ( keys %employees ){
    print "$identity,$employees{$identity}{forename},$employees{$identity}{surname}\n";
}

Upvotes: 0

Alexandr Ciornii
Alexandr Ciornii

Reputation: 7392

  1. You should always use "use strict;" in your program.

  2. Use references and anonymous hashes.

use strict;use warnings;
my %a;

my %b;
$b{str} = "hello";  
$a{1}={%b};

%b=();
$b{str} = "world";
$a{2}={%b};

print "$a{1}{str}  $a{2}{str}";

{%b} creates reference to copy of hash %b. You need copy here because you empty it later.

Upvotes: 6

David Wall
David Wall

Reputation: 51

Short answer: hash keys can only be associated with a scalar, not a hash. To do what you want, you need to use references.

Rather than re-hash (heh) how to create multi-level data structures, I suggest you read perlreftut. perlref is more complete, but it's a bit overwhelming at first.

Upvotes: 4

dalton
dalton

Reputation: 3696

Mike, Alexandr's is the right answer.

Also a tip. If you are just learning hashes perl has a module called Data::Dumper that can pretty-print your data structures for you, which is really handy when you'd like to check what values your data structures have.

use Data::Dumper;
print Dumper(\%a);

when you print this it shows

$VAR1 = {
          '1' => {
                   'str' => 'hello'
                 },
          '2' => {
                   'str' => 'world'
                 }
        };

Upvotes: 2

cHao
cHao

Reputation: 86504

Perl likes to flatten your data structures. That's often a good thing...for example, (@options, "another option", "yet another") ends up as one list.

If you really mean to have one structure inside another, the inner structure needs to be a reference. Like so:

%a{1} = { %b };

The braces denote a hash, which you're filling with values from %b, and getting back as a reference rather than a straight hash.

You could also say

$a{1} = \%b;

but that makes changes to %b change $a{1} as well.

Upvotes: 1

mob
mob

Reputation: 118605

Hashes of hashes are tricky to get right the first time. In this case

$a{1} = { %b };
...
$a{2} = { %b };

will get you where you want to go.

See perldoc perllol for the gory details about two-dimensional data structures in Perl.

Upvotes: 6

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