Project: Linked Lists

Introduction

In Computer Science, one of the most basic and fundamental data structures is the linked list, which functions similarly to an array. The principal benefit of a linked list over a conventional array is that the list elements can easily be inserted or removed without reallocation of any other elements.

In some programming languages, the size of an array is a concern and one of the ways to overcome that problem and allow dynamically allocated data is using linked lists.

Luckily in Ruby, arrays aren’t limited to a certain size, and both insertion and deletion can be done trivially at any index using the appropriate built in array method, so you don’t have to think about overcoming those limitations.

So if array size, array insertion and array deletion are not limitations in Ruby, are linked lists really necessary? The short answer to that is it depends; however, it’s the simplest of the dynamic data structures and it will give you a solid foundation, so you can understand more complex data structures like graphs and binary trees with more ease.

Structure of a linked list

A linked list is a linear collection of data elements called nodes that “point” to the next node by means of a pointer.

Each node holds a single element of data and a link or pointer to the next node in the list.

A head node is the first node in the list, a tail node is the last node in the list. Below is a basic representation of a linked list:

[ NODE(head) ] -> [ NODE ] -> [ NODE(tail) ] -> nil

For a more thorough explanation, use these resources:

Assignment

You will need two classes:

LinkedList class, which will represent the full list.
Node class, containing two instance variables, #value and #next_node, set both as nil by default.

Build the following methods in your linked list class:

#append(value) adds a new node containing value to the end of the list.
#prepend(value) adds a new node containing value to the start of the list.
#size returns the total number of nodes in the list.
#head should return the value of the first node in the list. If the list is empty, it should return nil.
#tail should return the value of the final node in the list. If the list is empty, it should return nil.
#at(index) should return the value of the node at the given index. If there’s no node at the given index, it should return nil.
#pop should remove the head node from the list and return that node’s value. If called on an empty list, it should just return nil.
#contains?(value) returns true if the passed in value is in the list and otherwise returns false.
#index(value) returns the index of the node containing the value. If the value isn’t found, it should return nil. If more than one node has a value matching the given value, it should return the index of the first node with the matching value.
#to_s represent your LinkedList objects as strings, so you can print them out and preview them in the console. If the list is empty, it should return an empty string. The format should be: ( value ) -> ( value ) -> ( value ) -> nil.

Extra credit

#insert_at(index, *values) should insert new nodes with the given values at the given index. For example:
```
 # list that's already set up
 puts list
 # => "( 1 ) -> ( 2 ) -> ( 3 ) -> null"

 list.insert_at(1, 10, 11)
 puts list
 # => "( 1 ) -> ( 10 ) -> ( 11 ) -> ( 2 ) -> ( 3 ) -> null"
```
If the method is called with an index that is out of bounds (below 0 or above the list’s size), raise an IndexError.
#remove_at(index) that removes the node at the given index. If the given index is out of bounds (below 0 or greater than or equal to the list’s size), raise an IndexError.

Extra Credit Tip: When you insert or remove a node, consider how it will affect the existing nodes. Some of the nodes will need their #next_node link updated.

Test it out

Let’s test out the Linked List you made!

Create a main.rb file and make sure it requires the file containing LinkedList. This is where we’ll test the list.

Create an instance of LinkedList and populate it with nodes:

list = LinkedList.new

list.append('dog')
list.append('cat')
list.append('parrot')
list.append('hamster')
list.append('snake')
list.append('turtle')

Add puts list to the end of the file and run it. Because of the naming, this should use the #to_s method you wrote earlier.
If everything is working, the output should be ( dog ) -> ( cat ) -> ( parrot ) -> ( hamster ) -> ( snake ) -> ( turtle ) -> nil. Feel free to use different values to test if you like.

Ruby Course

Introduction

Structure of a linked list

Assignment

Extra credit

Test it out

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