CODEWITHSUNDEEP
perlhash
Amy Ndy
Amy Ndy

Reputation: 35

Perl code ! What does it do (Hash of hashes)?

I'm currently working on some code written by the previous internship student. I'm not familiar to Perl so I have some problems in understanding what his code actually do. So it looks like: 

$Hash{Key1}{Key2}++;

The original code was: 

$genotypes_parent2_array{$real_genotype}{$individu_depth}++;

I use to see hashes in this form $Hash{Key} in order to get the value but I struggle with this one. Any help out there ? Thanks!

Upvotes: 0

Views: 82

Answers (2)

Jens
Jens

Reputation: 69460

%Hash is a hash of hashes.

That codes add 1 to the value of $Hash{Key1}{Key2} which is the value of a hash element.

Upvotes: -1

Dave Cross
Dave Cross

Reputation: 69314

%genotypes_parent2_array is a hash (so that's not a very good name for the variable!) Each value in the hash is a hash reference. So effectively you have a hash of hashes.

$genotypes_parent2_array{$real_genotype} looks up the key $real_genotype in the hash. And that value is (as we said above) a hash reference. If you have a hash reference, then you can look up values in the referenced hash using an arrow. So we can get to a value in the second-level hash using code like this:

$genotypes_parent2_array{$real_genotype}->{$individu_depth}

However, Perl has a nice piece of syntactic sugar. When you have two pairs of "look-up brackets" next to each other (as we have here) you can omit the arrow. So you can get exactly the same effect with:

$genotypes_parent2_array{$real_genotype}{$individu_depth}

And that's what we have here. We look up the key $real_genotype in the hash %genotypes_parent2_array. This gives us a hash reference. We then look up the key $individu_depth in the referenced array and that gives us the value in the second-level hash. Your code then increments that value.

The manual page perldoc perldsc is a good introduction to using references to build complex data structures in Perl. In addition, I find Data::Dumper very useful for showing what a complex data structure looks like.

Upvotes: 4

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