How do I create a hash of hashes in Perl? - 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

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

Answer 6

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";
}