A Hash of Hashes

A student is not unlike a new car, although less reliable and probably washed less frequently. Still, a student possesses a set of characteristics, such as their class year, major of study, and grade point average. In fact, we can wrap all of these properties into a hash. We can then say that this hash is in fact a value whose key is the student's name. Yes, a hash of a hash.

Consider the plight of Nick Plato, estranged descendent of the famous Athenian Plato line. Nick's student hash might look something like:

%students=("Nick Plato"=>{"year"=>"2",
"GPA"=>"2.5",
"major"=>"Phys. Ed.",
"email"=>"nplato\@school.edu"});

The students hash contains one key, "Nick Plato", whose value is a hash. We can reference, for instance, Nick's GPA as follows:

$students{"Nick Plato"}{"GPA"}

You can't have a party of one, so let's throw a couple of more student hashes into the mix.

$students{"Mary Pitts"}=({"year"=>"4",
"GPA"=>"4","major"=>"Economics",
"email"=>"mpitts\@school.edu"});
$students{"Sally Cummings"}=({"year"=>"1",
"GPA"=>"3.3","major"=>"Undecided",
"email"=>"scummings\@school.edu"});

As we've seen before, you can add keys to a hash through simple assignment. In this case, we're again addings keys whose values are themselves hashes. Now suppose that we wish to output all records for each student, sorted by descending GPA, with the records internally sorted by alphabet.

#Double-Sort students by GPA and keys
print "\nAll students sorted by GPA:\n";
foreach $student 
(reverse sort {$students{$a}{GPA}<=>$students{$b}{GPA} } 
keys (%students)) 
{print "$student\n";
foreach $recorditem (sort keys %{$students{$student} }) 
{print "\t$recorditem: $students{$student}{$recorditem}\n"}
}

The list of all students is represented by the function keys (%students). We want to sort this list by associated GPA, however. Thus a custom reverse sort is executed where we compare the "GPA" key values within the student hash ($students{$a} and $students{$b}). As we iterate over each student hash, then, we retrieve the list of records within the student hash (keys %{$students{$student} }) and sort these by the default alphabetic scheme. Each record is then output, in the syntax we saw when accessing Nick Plato's GPA.

A Hash of Lists
The Perl You Need to Know
Reprieve