Programming Perl, 3rd Edition

Chapter 9. Data Structures

Perl provides for free many of the data structures that you have to build yourself in other programming languages. The stacks and queues that budding computer scientists learn about are both just arrays in Perl. When you push and pop (or shift and unshift) an array, it's a stack; when you push and shift (or unshift and pop) an array, it's a queue. And many of the tree structures in the world are built only to provide fast, dynamic access to a conceptually flat lookup table. Hashes, of course, are built into Perl, and provide fast, dynamic access to a conceptually flat lookup table, only without the mind-numbingly recursive data structures that are claimed to be beautiful by people whose minds have been suitably numbed already.

But sometimes you want nested data structures because they most naturally model the problem you're trying to solve. So Perl lets you combine and nest arrays and hashes to create arbitrarily complex data structures. Properly applied, they can be used to create linked lists, binary trees, heaps, B-trees, sets, graphs, and anything else you can devise. See Mastering Algorithms with Perl (O'Reilly, 1999), the Perl Cookbook (O'Reilly, 1998), or CPAN, the central repository for all such modules. But simple combinations of arrays and hashes may be all you ever need, so they're what we'll talk about in this chapter.

Arrays of Arrays

There are many kinds of nested data structures. The simplest kind to build is an array of arrays, ...