Question

Using hashes in a chaining manner confuses me a lot. For instance, I read the following Perl codes, how to understand them piece by piece?

$model->{result}->{forcast}->[$index]->{label}  = 1;

$Neg{$examples->{result}->[$index]->{title}} = 1

In addition, why some items has $ , like $index; while others do not have, like label.

$index is wrapped in [ ] while others are wrapped in { }, what are the differences here?

Is $Neg{$examples->{result}->[$index]->{title}} = 1 equivalent to $Neg{$examples->{result}->[$index]->{title}} = 1

Answer 1

Consider:

$model->{result}->{forcast}->[$index]->{label}  = 1;

->[] is used to dereference an array reference.
->{} is used to dereference a hash reference.

Let us scan it from the left:

• $model is a hash reference (due to it being used in the context: $model->{})
• result is a hash key (as it does not have a $ sigil prepended)
• $model->{result} is again a hash reference
• $model->{result}->{forcast} is an array reference (due to it being used in the context: $model->{result}->{forcast}->[])
• $index is a variable set by the user that possibly contains the index of an array item
• $model->{result}->{forcast}->[$index] is a hash reference
• label is a hash key
• $model->{result}->{forcast}->[$index]->{label} sets 1 as the value for the hash key

Hash keys can be barewords, which will be automatically quoted. So, specifying the hash key as result or 'result' are the same.

perldoc perldsc is the cookbook for data structures. Data::Dumper is very helpful in viewing such data structures.