How do I create a hash of hashes in Perl?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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'
                 }
        };

Answer 5

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.