Beat Box

Learning Goals

Follow an interaction pattern
Write readable code that adheres to Ruby convention
Write tests
Distinguishing between classes and instances of those classes
Use and implement iteration or recursion techniques
Host code on Github

Overview

In this project we’re going to do some silly things with sound. Specifically, we’re going to make a very basic drum machine program.

However to add some additional depth, let’s also use this project as a chance to explore one of the fundamental data structures in computer science – the Linked List.

Drum Machine 101 – Making Sounds

Go into your Terminal and try this:

$ say -r 500 "ding, dah, oom, oom, ding, oom, oom, oom, ding, dah, oom, oom, ding, dah, oom, oom, ding, dah, oom, oom "

Yeah. That’s what we’re looking for. Now try it from Ruby:

$ pry
> `say -r 500 "ding, dah, oom, oom"`

Note that the backticks allow you to run terminal commands from within Ruby.

The exact command that you need to run may differ based on what version of OS X you have installed on your computer. The commands above will work on 10.13.

Linked Lists

Linked Lists are one of the most fundamental Computer Science data structures. A Linked List models a collection of data as a series of “nodes” which link to one another in a chain.

In a singly-linked list (the type we will be building) you have a head, which is a node representing the “start” of the list, and subsequent nodes which make up the remainder of the list.

The list itself can hold a reference to one thing – the head node.

Each node can hold a single element of data and a link to the next node in the list.

The last node of the list is often called its tail.

Using sweet ASCII art, it might look like this:

List -- (head) --> ["hello" | -]-- (link) --> ["world" | -]-- (link) --> ["!" | ]

The three nodes here hold the data “hello”, “world”, and “!”. The first two nodes have links which point to other nodes. The last node, holding the data “!”, has no reference in the link spot. This signifies that it is the end of the list.

In other lower level languages, something called a pointer is what is used to ensure that a single link knows about the next link. In Ruby, we don’t use pointers, so the link is literally its node. When we get to a node which is the last node, we call it the tail, and its link is nil.

A linked list should be able to do the following:

Insert elements
Pop an element from the end
Push an element onto the beginning
Remove the (first occurrence all occurrences) of an element by data content
Remove an element by position
Add an element at an arbitrary position
Add an element after a known node
Find whether a data element is or is not in the list
Find the distance between two nodes

Tips

A linked list is not an array. While it may perform many of the same functions as an array, its structure is conceptually very different.
There are only 3 types of “state” that need to be tracked for a linked list – the head of the list, the data of each node, and the “next node” of each node.
In object-oriented programming, “state” is generally modeled with instance variables
There are two main ways to implement Linked Lists: iteration and recursion. Iterative solutions use looping structures (while, for) to walk through the nodes in the list. Recursive solutions use methods which call themselves to walk through nodes. It would be ideal to solve it each way.
Most of your methods will be defined on the List itself, but you will need to manipulate one or more Nodes to implement them.
TDD will be your friend in implementing the list. Remember to start small, work iteratively, and test all of your methods.
An empty list has nil as its head.
The tail of a list is the node that has nil as its next node.

Constraints

Make sure that your code is well tested for both expected cases and edge cases. Try popping more elements than there are in the list. Try seeing if an empty list includes anything. Try inserting elements at a position beyond the length of the list.
Avoid using other ruby collections (Arrays, Hashes, etc) for the storage of your beats. That’s where you’re supposed to use the linked list. But having Arrays elsewhere in your code, or using methods that return arrays (like .split) are totally ok.

Resources

Need some help on Linked Lists? You can check out some of the following resources:

https://www.youtube.com/watch?v=oiW79L8VYXk
http://www.sitepoint.com/rubys-missing-data-structure/