Re^2: Accessing an AoHoAoH

[bradcathey]

Thanks calin, that did it! I guess I'm still a little confused about when to use parens and when to use brackets. In reading The Perl Cookbook and Programming Perl, it appears that I might have been confusing the building of a HoA and an AoH, the former using the brackets, and the latter using parens.

[jeffa]

As a "i just now made this up rule of thumb", use braces when parens won't do. Parens are just a fragile container for lists, they will flatten by default:

my @one_d = ( (1,2,3), (4,5,6), (7,8,9) );

is really just a single list. You have to use braces:

my @two_d = ( [1,2,3], [4,5,6], [7,8,9] );

I prefer to use a reference to an anonymous array for the outside container, maily because there is no mixing of parens and braces, square or curly:

my $two_d = [ [1,2,3], [4,5,6], [7,8,9] ];

When in doubt, always consult Data::Dumper. Always! If you had used Data::Dumper on your data structure, you would have seen that the problem was within it:

print Dumper \@AoH;

__END__
(it's an AoHoH ... not an AoHoAoH)
$VAR1 = [
          {
            'page' => {
                        'paragraph' => 'lesson1'
                      },
            'chapter' => 'Basic',
            'HASH(0x8638dec)' => undef
          },
          {
            'HASH(0x86fee04)' => undef,
            'page' => {
                        'paragraph' => 'lesson3'
                      },
            'chapter' => 'Advanced'
          }
        ];

Finally, print out a copy of References quick reference. And don't forget about Data::Dumper! :)

[bradcathey]

Great rule of thumb, jeffa. BTW, you have this knack for catching me with my zipper down. I always use Data::Dumper...er...almost all time. Thanks for the reminder to use it all the time and for the Quick Ref.

[Jasper]

As a "i just now made this up rule of thumb", use braces when parens won't do.

I think I'd rather the rot was "try braces when parens didn't do what you thought they would".

Ditto for curlies and parens when dealing with hashes. Although looking at why, maybe with Data::Dumper, as you say, or looking at what perl told us when we tried to run the thing!!!, is always best. Otherwise we might as well be monkeys bashing away at our keyboards.

[bart]

I guess I'm still a little confused about when to use parens and when to use brackets.

Parens build a list, brackets build an anonymous array, and return a reference to it. Hashes can only contain scalars as values, so a list won't ever do (except when it contains only one item — but then you don't need a list). An array reference is a scalar, so that will work. As it's the only thing that actually does work properly, Perl's syntax has been optimized to ease access to array references as hash and array values.

$x->{'foo'}[1] actually means: use $x as a hash reference, get the value associated with the string 'foo'. Use this as an array reference and access the second element (with index 1) from it.

Actually you can even split this up into:

$aref = $x->{'foo'};
$aref->[1]

provided you didn't need autovivification, which were to happen in case $x->{'foo'} was undefined, in the former case — but not in the latter.

[Roy Johnson]

If you want to nest something, use brackets. Nested parentheses are meaningless in lists; only the outermost set defines any structure.