How to construct a hash of hashes

Question

I need to compare two hashes, but I can't get the inner set of keys...

my %HASH = ('first'=>{'A'=>50, 'B'=>40, 'C'=>30},
            'second'=>{'A'=>-30, 'B'=>-15, 'C'=>9});
foreach my $key (keys(%HASH))
{
   my %innerhash = $options{$key};
   foreach my $inner (keys(%innerhash))
   {
      print "Match: ".$otherhash{$key}->{$inner}." ".$HASH{$key}->{$inner};
   }
}

score 4 · Accepted Answer · answered Jul 15 '11 at 16:54

4

$options{$key} is a scalar (you can tell be the leading $ sigil). You want to "dereference" it to use it as a hash:

my %HASH = ('first'=>{'A'=>50, 'B'=>40, 'C'=>30},
            'second'=>{'A'=>-30, 'B'=>-15, 'C'=>9});
foreach my $key (keys(%HASH))
{
   my %innerhash = %{ $options{$key} };  # <---- note %{} cast
   foreach my $inner (keys(%innerhash))
   {
      print "Match: ".$otherhash{$key}->{$inner}." ".$HASH{$key}->{$inner};
   }
}

When you're ready to really dive into these things, see perllol, perldsc and perlref.

answered Jul 15 '11 at 16:54

mob

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I had no idea you could cast a variable in PERL, thanks so much! – Eric Fossum Jul 15 '11 at 17:07
1

@mob's answer is an application of "Use Rule 1" in "perldoc perlreftut" so you should start with that doc. – tadmc Jul 15 '11 at 18:24

score 1 · Answer 2 · answered Jul 15 '11 at 17:05

I'm guessing you say "options" there where you mean "HASH"?

Hashes only store scalars, not other hashes; each value of %HASH is a hash reference that needs to be dereferenced, so your inner loop should be:

foreach my $inner (keys(%{ $HASH{$key} })

Or:

my %HASH = ('first'=>{'A'=>50, 'B'=>40, 'C'=>30},
            'second'=>{'A'=>-30, 'B'=>-15, 'C'=>9});
foreach my $key (keys(%HASH))
{
    my $innerhash = $HASH{$key};
    foreach my $inner (keys(%$innerhash))
    {
        print "Match: ".$otherhash{$key}->{$inner}." ".$innerhash->{$inner};
    }
}

How to construct a hash of hashes

2 Answers2