A List of Lists

What if each item in a list was itself a list? Freaky, and quite possible. Consider a list of car models for one manufacturer ... for instance, "avalon", "camry", and "celica". Now we might have another list for another manufacturer, such as "accord, civic, prelude". And we want to store each of these lists as items of a list. Here is what the initial list declaration might look like:

@carmodels=(["avalon","camry","celica"],
["accord","civic","prelude"],
["929","626","miata"]);

The list carmodels is declared like a normal list, but each item is a list itself, contained within square brackets. The syntax, especially which brackets are used where (square, curly, or parentheses), becomes crucial in manipulating lists and hashes. Because carmodels is a list of lists, it is also sometimes known as a two-dimensional array. To refer to a single scalar item in carmodels you need to specify two dimensions:

$carmodels[1][2]

The above scalar would reference the value "prelude", which is item index 2 of the list at index 1 of carmodels. Now, suppose you want to append another list of car models to carmodels ... this means adding another list to the list. We can use the push function to add a new item to the list, but note the syntax for pushing a list as an item onto the list:

push(@carmodels,["sentra,stanza,altima"]);

The square brackets around the list to be pushed are crucial; if you were to leave them off then "sentra", "stanza", and "altima" would each be added as individual scalar items to carmodels, rather than as a single list item. Now, imagine that you want to add a column to a list; that is, add a new item onto one of the second-dimension lists, such as "tercel" onto the list at index 0 (e.g. the Toyota list).

push (@{$carmodels[0]},"tercel");

Our syntax grows ever more obscure! Push requires a destination list as its first paramter ... and we know that the first item of carmodels is a list. The syntax for communicating that is, as seen, @{$listname[index]}. In other words, "the list at index within the list listname". The end result will be that "tercel" will be added onto the first list, immediately after "celica". Similarly, we use the @{$listname[index]} constructor to iterate over a list-within-a-list. The following will traverse into each list and output the car models contained within:

#iterate over list of lists
$index=0;
foreach $modellist (@carmodels) {
$index++;
print "List index $index:\n";
foreach $model (@{$modellist})
{print "\t$model\n"}
}

Now, if you've got that under your belt, you're probably asking yourself, "Yeah but how can I double-sort this list of lists?" What does that mean? Okay, it means that we wish to sort carmodels by the first item in each list, then we wish to output each second-dimension list sorted with respect to itself. Doing so combines everything we've seen above with list sorting from earlier. Hang on:

#double-sort list of lists
print "\nDouble-Sorted List of Lists:\n";
foreach $modellist 
(sort {@{$a}[0] cmp @{$b}[0]} @carmodels) 
{foreach $model (sort @{$modellist}) 
{print "$model "} 
print"\n";}

Yields:

626 929 miata
accord civic prelude
avalon camry celica tercel
altima maxima sentra

You can see that the resulting list is sorted in both dimensions, by rows and by columns. Time for an aspirin break.

All Sorts of Sorts: Hashes
The Perl You Need to Know
A List of Hashes