Thursday, 12 March 2015

Abstract Data Type: Richard Linklist-er

In week 8, the topic we are forced required to write about is an ADT called a "linked-list". Which sounds an awful lot like the surname of the director "Richard Linklater". And since I just watched "Dazed and Confused", I wanted to reference this admittedly shallow relation between the two words. Alright, now to move on. In this post, I will try to explain, to the best of my abilities, what a linked list is.

A linked-list is, to be succinct, a collection of individual node objects, each with two attributes: one called "data", which is the actual piece of data being stored in the linked list at that position, and one called "nxt", which stores the next node in the sequence (or None if it's the last node of the linked list). The first node is stored in an overarching data structure called a "LinkedList". In short, the linked-list shell lets you find the first node, and each node (including the first) lets you access the very next one (or last one).

Now, on the surface, linked-lists may seem trivial- especially when compared to tree data structures. After all, the whole premise of a tree is that each node has multiple children nodes. So if you are at any node in the tree, you can directly access a large set of nodes on the tree without having to go through them all sequentially. This is why we often use them over lists when storing comparable types of data (a topic I will cover in a later post). Now, for any node in a linked-list, we can only access the element directly behind it. In this sense, it seems nearly indistinguishable from a tree with a branching factor of one, which kinda defeats the point (as we'll see in a later post). And from my correspondences with my peers, this sentiment isn't uncommon. And speaking of correspondences with my peers, here's a link to all my interactions with other SLOG's. Just throwing that out there, if you just so happen to be a marking TA...

Anyways, how can I convince all of them that linked lists are worth learning about? Well, lets think of them in a different way. In much the same way we learn the fundamentals of programming using a specific language (in this case python), we can also learn the fundamentals of many of our built in data types by understanding analogous, non-built in ones. In this case, I think a case can be made that understanding linked-lists gives us a better appreciation for what is perhaps my favorite built in data type- List.

List is an extremely useful data type for solving problems. You can put various types of objects in, and like magic, they doesn't disappear into the abyss of computer memory. You can also identify objects that you have previously put into the list (called elements), so that you can do things with them later. You can immediately identify the first and last elements of the list. You can also identify the element in the list with a certain "position" called an index. Once you have identified the element you desire, you can use it just like any ordinary object in the shell. For example, if its an int, you can apply the arithmetic operators over it. Perhaps the most important feature of a list is the ability to evaluate every single element in the collection sequentially, from one end to the other. This process, called an iteration over the list, is very commonly used in the implementation of various types of algorithms- such as searching and sorting. It's hard to imagine what programming would be like without this ability.

Interestingly, a linked list is capable of doing these exact things. If you don't believe me, than go back and re-read my definition for a linked-list. Recall how, for any node in a linked list, we can immediately access the node to its right- which is identified by the attribute "nxt". We can also identify the beginning of the linked list immediately, since that piece of data is stored in the linked list object itself. And, although not immediately obvious, we can identify the very last node of the sequence by simply identifying the node which has a "nxt" attribute set to "None". Because we can identify the first and last element or a linked list, as well as the next element from any given position, we care capable of iterating over a linked list, just like a regular list. In fact, with little effort, one can even write an algorithm which identifies elements in a linked-list based on their index number- all you need to do is identify the first node, access it's "nxt" attribute, and than add one to some counter, and repeat the process for the next node until the value of the counter is equal to the value inputted, and than return the value of that node. This process perfectly simulates the concept of indexing in a List (at least for index values greater than or equal to zero).

Rightly or wrongly, I think that linked-list's are far more relevant to computer science than many of my peers appear to. Now, this is not to say that linked lists have no pragmatic value at all, outside of being an educational aid. Perhaps linked-lists are more efficient at storing certain pieces of data, such as the length of the linked-list. The point I wish to make is that they are useful for another reason. Which is because they take a relatively complicated process, and break it down into much more palatable pieces. And that's something a computer science course can never do too much.

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