# Beginner Data Structures

## Hello there!

### Let's get started

Welcome to your first lesson about Data Structures!  Now that you know how to
obtain data with Ruby you're also going to be able to store it in collections. 
Arrays and Hashes are two key classes in Ruby that are able to help with 
storing your data!  Let's get going!

Click the icon like this (on the bottom of the screen) to get started:

![Not this one! The one below!](/icon_button/arrow_right)

### Lesson Controls

Before we move on, Here's a refresher on the controls you can use to move around
in the Lesson.

![](/icon_button/arrow_left)
__back__: goes back one page

![](/icon_button/arrow_right)
__continue__: goes to the next page

![](/icon_button/menu)
__menu__: makes it easy to jump around to any lesson

![](/icon_button/x)
__close__: closes the tutor

Don't forget! Press this to move to the next part:

![](/icon_button/arrow_right)

Have at it!

### What are Arrays?

An array can be thought of as a dresser where each drawer is a numbered spot. 
This  dresser is a collection of slots where one thing can be in each place. 
Arrays  are  used to store data like numbers, strings, or even other Arrays! 
Ruby makes it really easy  to create and store things inside of them.  Let's get
going!

### What can we do with them?

    my_array = [1,2,"shoes"]

Try typing this and press the Run button.

You created an array with the numbers 1, 2, and the string 'shoes' in it. 
Things in the array can be gotten by typing the array's variable name
(`my_array`) and square brackets (`[]`) with a number inside of the brackets. 
This number can be though of as the address to that spot in the array.  It's
good to remember that arrays start at 0 in Ruby.

    alert my_array[0]

What thing in the array are you going to get back?  Type this in and press the
'Run' button.

### Arrays in action!

When using Arrays we need to know a few things first.  Arrays in Ruby can expand
to the size that you need them.  So if you wanted to put the string 'cat' at
spot 99(which would be the 100th item in the array) we could put: 

    my_array[99] = "cat"

If there is nothing in a spot you will have 'nil' filling it.

If we wanted to print out everything in an array we could do something like
this: 

    my_new_array = ["cat","dog","mouse"]

Then we would put: 

    my_new_array.each {|animal| alert animal}

Type this all in and press the 'Run' button.

### Arrays in even more action!

We've seen what we can do with arrays, but what other things can they do to help
us?  What if we had an array of numbers and we wanted to sort it?  Try typing
this this and running it: 

    num_array = [4,3,22,19,100,45]

    alert num_array.sort

That was really easy to sort it from lowest to highest!  What if we want it from
highest to lowest though?  Type this in next and press the 'Run' button.

    alert num_array.reverse

The array class has so many methods that you can call upon it.  Take a look on
the Ruby API: http://www.ruby-doc.org/core/classes/Array.html

## The Hash

### What are Hashes?

Now that we've gotten an introduction to Arrays we can also learn about Hashes! 
Hashes in other languages are sometimes called Dictionaries.  Well, what do they
do?  Like in a dictionary you are able to look up a word or 'key' which
corresponds to a 'value'.  You separate the key and value with a hashrocket
(=>).  Just like Arrays you can access a certain key  by typing the hashes
variable name and the key in square brackets ([]).    Let's try working with a
hash!

    my_hash = { "dog" => "puppy", "cat" => "kitten" }

    alert my_hash["dog"]

Try typing this all in and press the 'Run' button!

### Working with Hashes

So what else are we able to do with hashes?  Let's try something will help us
see if something is in the hash as a key or value  The methods `has_key?` and
`has_value?` are exactly what we're looking for!

    new_hash = { "1" => "one", "2" => "two"}

    alert new_hash.has_key?("1")

    alert new_hash.has_value?("one")

Try typing these in and press the 'Run' button for each of the methods!

### Let's tie these Hashes and Arrays together!

We've looked at two different data structures that are able to hold data for us
and let us access certain parts of the collections.  Both Arrays and Hashes are
commonly used by programmers and are great to  have knowledge about!  Both of
these data structures have so many methods that can be read about in the  Ruby
Documentation: http://www.ruby-doc.org/core/.  Now with Arrays and Hashes you
should be able to keep your data organized and usable!