Collection basics

Arrays

An Array is a variable that can store more than one variable under the same name. In it’s purest form it is a continuous memory block storing variables sequentially. Each variable in an Array is accessible using an index number. They are a linear lists with a fixed number of spaces available for variables.

JavaScript Arrays are not Arrays in the pure sense of the word as they are auto extending.

You can create an Array like this:

// Create an empty `Array` with 5 slots for values
var players = new Array(5);

// See the empty Array
console.log(players);

// Populate the Array like this
player[0] = "David Beckman";
player[1] = "Ryan Geeks";

// see the values in the Array using indexes
console.log(players[0]);
console.log(players[1]);

JavaScript Arrays allow this to happen:

// Add a 6th element to the Array with only 5 allocated places
player[5] = "Thierry Hungry";

// What happened here?
console.log(player[5]);

// We can even do funny thing like adding a 9th element
player[8] = "Wayne Rooky";

When adding new values to a JavaScript Array it auto-extends. In programming languages with pure Arrays they don’t auto-extend. You can only add as many values into an Array as allocated at Array initialization time. To extend an Array you would have to create a new, bigger, Array and then copy the values over to the new Array. In JavaScript this is not the case, Lists in JavaScript are Array-like, but not pure Arrays. Something to bear in mind in your journey ahead as a programmer.

Arrays and Objects are fundamental building blocks for creating other data structures.

Linked List

A Linked List is an ordered set of data nodes, each node containing data and a link to the next node (and sometimes to the previous node).

Does this sounds familiar?

This is what you did in the previous section. You created a Node Object and then use it to create a list of inter-connected Node objects resulting in a list. It is cumbersome to use though: we can fix this by using a LinkedList object.

A usable Linked List

As is hard to use the Node Object on it’s own we need to create a LinkedList object that is easier to use.

Use TDD to create a LinkedList object that have:

• an add function that adds an item to the list;
• a length function that returns the number of items in the list;
• a next function that returns the next item in the list, which can be used for looping over the list. It should return null if the end of the list is reached;
• a first function that returns the first item in the list;
• a last function that returns the last item in the list.

Now add this:

• a find function that finds all items in a list that matches a given condition. It should take in a function as a parameter. This function will take in the current Node and will return true/false if the condition is met.

Use your LinkedList object to and create a list of Categories and Product names for Nelisa.