Looping through known elements in a hash of hashes of arrays - arrays

Question

I have a question I am hoping someone could help with (simplified for the purposes of explaining my question).
I have the following hash of hashes of arrays (I think that is what it is anyway?)
Data structure
{
Cat => {
Height => ["Tiny"],
},
Dog => {
Colour => ["Black"],
Height => ["Tall"],
Weight => ["Fat", "Huge"],
},
Elephant => {
Colour => ["Grey"],
Height => ["Really Big"],
Weight => ["Fat", "Medium", "Thin"],
},
}
What I am trying to do
The program below will print the whole data structure. I want to use this kind of way to do it
my %h;
for my $animal (keys %h) {
print "$animal\n";
for my $attribute ( keys %{$h{$animal}} ) {
print "\t $attribute\n";
for my $i (0 .. $#{$h{$animal}{$attribute}} ) {
print "\t\t$h{$animal}{$attribute}[$i]\n";
}
}
}
The problem I am having
I am trying to access a particular part of the data structure. For example, I want to only print out the Height arrays for each animal as I do not care about the other Colour, Weight attributes in this example.
I'm sure there is a simple answer to this, and I know I need to specify the Height part, but what is the correct way of doing it? I have tried multiple ways that I thought would work without success.

Answer 1

In your code, instead of looping over all the attributes with
for my $attribute ( keys %{ $h{$animal} } ) { ... }
just use the one you are interested in. Like this
for my $animal (keys %h) {
print "$animal\n";
for my $attribute ( 'Height' ) {
print "\t $attribute\n";
for my $i (0 .. $#{$h{$animal}{$attribute}} ) {
print "\t$h{$animal}{$attribute}[$i]\n";
}
}
}
I would choose to loop over the contents of the heights array rather than the indices, making the code look like this:
for my $animal (keys %h) {
print "$animal\n";
print "\t\t$_\n" for #{ $h{$animal}{Height} };
}

Answer 2

Taking a quick look at your data structure: It's a hash of hashes of arrays! Wow. Mind officially blown.
Here's a quick way of printing out all of the data:
use feature qw(say);
# Working with a Hash of Hash of Arrays
for my $animal (keys %h) {
say "Animal: $animal";
# Dereference: Now I am talking about a hash of arrays
my %animal_attributes = %{ $h{$animal} };
for my $attribute (keys %animal_attributes) {
# Dereference: Now I am talking about just an array
my #attribute_value_list = #{ $animal_attributes{$attribute} };
say "\tAttribute: $attribute - " . join ", ", #attribute_value_list;
}
}
Note I use dereferencing. I don't have to do the dereference, but it makes the code a bit easier to work with. I don't have to think of my various levels. I know my animal is a hash of attributes, and those attributes are an array of attribute values. By using dereferencing, it allows me to keep things straight.
Now, let's say you want to print out only a list of desirable attributes. You can use the exists function to see if that attribute exists before trying to print it out.
use feature qw(say);
use constant DESIRED_ATTRIBUTES => qw(weight height sex_appeal);
# Working with a Hash of Hash of Arrays
for my $animal (keys %h) {
say "Animal: $animal";
# Dereference: Now I am talking about a hash of arrays
my %animal_attributes = %{ $h{$animal} };
for my $attribute ( DESIRED_ATTRIBUTES ) {
if ( exists $animal_attributes{$attribute} ) {
# Dereference: Now I am talking about just an array
my #attribute_value_list = #{ $animal_attributes{$attribute} };
say "\tAttribute: $attribute - " . join ", ", #attribute_value_list;
}
}
}
Same code, I just added an if clause.
When you get into these complex data structures, you might be better off using Object Oriented design. Perl has an excellent tutorial on OOP Perl. If you used that, you could have defined a class of animals and have various methods to pull out the data you want. It makes maintenance much easier and allows you to bravely create even more complex data structures without worrying about tracking where you are.

Answer 3

I think sometimes it's easier to use the value directly, if it is a reference to another structure. You could do something like:
my $height = "Height";
while (my ($animal, $attr) = each %h) {
print "$animal\n";
print "\t$height\n";
print "\t\t$_\n" for #{ $attr->{$height} };
}
Using the value of the main keys, you can skip over one step of references and go straight at the Height attribute. The output below is after the format you had in your original code.
Output:
Elephant
Height
Really Big
Cat
Height
Tiny
Dog
Height
Tall

Answer 4

Assuming your variable is called %h:
foreach my $animal (keys %h) {
my $heights = $h{$animal}->{Height}; #gets the Height array
print $animal, "\n";
foreach my $height( #$heights ) {
print " ", $height, "\n";
}
}

Answer 5

I think I have worked it out and found what I was doing wrong?
This is how I think it should be:
my %h;
for my $animal (keys %h) {
print "$animal\n";
for my $i (0 .. $#{$h{$animal}{Height}} ) {
print "\t\t$h{$animal}{Height}[$i]\n";
}
}