A List of Hashes

If you're brain isn't twisted enough yet, let's consider a list wherein each item is a hash. Who would do such a thing? Well, maybe a used car dealership for example, and not only because of its mind-bending qualities. Imagine that we have a list of "pre-owned" cars for sale. Each car possesses a number of important sale characteristics such as its make, model, year, color, mileage, and price. These properties are ideally suited for a hash of key-value pairs. So, we can sum up each sale auto as a hash, and we can compile all of our sale autos into a list.

@carsforsale=({"make"=>"Toyota",
"model"=>"Avalon",
"year"=>"1998",
"color"=>"black",
"mileage"=>"23000",
"price"=>"13500"}); 

Initially, we've created a list carsforsale with one item. That one item is a hash, defined within the curly braces, with all relevent key-value pairs. We can pluck out any one of these values, such as the make of the first car on the list (Toyota):

$carsforsale[0]{"make"}

A trade-in arrives on the lot and we're ready to hose it down and add it to our list of cars for sale. First, we might create a temporary hash to contain its characteristics, and then push that hash onto our carsforsale list:

%temp=("make"=>"Nissan",
"model"=>"Maxima",
"year"=>"2000",
"color"=>"green",
"mileage"=>"5520",
"price"=>"18750"); 

push(@carsforsale,{%temp});

You don't have to use a temporary hash as the middleman, but it makes the code that much more legible, relatively speaking. Now, what can we do with a list of hashes such as this? One idea is to simply print it all out ... that is, create a catalog of used cars for sale. Leveraging our knowledge of sorting, we can create output which is sorted by ascending price. We'll also want to sort the hash keys, such as "make", "model", "year", and so forth, otherwise they may appear in a different order for each car, since hash keys are intrinsically non-ordered.

 #Double-sort by price and key
 print "\nCars for sale sorted by price:\n";
 $count=0;
 foreach $saleitem 
 (sort {$a->{price}<=>$b->{price} } @carsforsale)
{$count++;
 print "Sale item $count:\n";
 foreach $hash_key (sort keys %{$saleitem})
 {print "\t$hash_key: $saleitem->{$hash_key}\n"}
 }

The outer foreach loop iterates over each hash (sale item) in the carsforsale list. These items are sorted using the sort function with custom code that evaluates the value of the "price" key in each hash. Within each hash, the keys are sorted alphabetically and then output with their corresponding values.

We might spin-off the master carsforsale list into a sub-list by manufacturer; for instance, a list of hashes for only those cars made by Toyota.

@toyotas=();
foreach $car (@carsforsale) 
{ if ($car->{make} eq "Toyota") 
{ push(@toyotas,$car) }
}

The resulting toyotas list is simply a subset of the original carsforsale list of hashes, and can be managed or output the same way.

A List of Lists
The Perl You Need to Know
A Hash of Lists