Arrays

An array is a collection of data that can contain any combination of variables, numbers, strings, or other Ruby objects (including other arrays).

Introduction

At the beginning of this section, you learned how to create and manipulate numbers and strings and how to assign them to variables. In real-world development, where you'll be working with dozens (and even hundreds!) of variables, working with numbers and strings individually is tedious, if not impossible.

One way Ruby allows you to represent a collection of data is with arrays, which you can think of as ordered lists. Rather than working with individual variables, numbers, or strings, an array allows you to create and manipulate an ordered and indexed collection of these data. The individual variables, numbers, or strings within an array are known as elements. An array can contain any combination of variables, numbers, strings, or other Ruby objects (including other arrays), although it is advisable to keep similar data types in any one array.

Learning outcomes

By the end of this lesson, you should be able to do the following:

• Describe what an array is, and explain why it's useful.

• Describe two different ways of creating a new array.

• Explain how to access array elements using different methods.

• Explain three different ways to add data to an array.

• Explain how to remove elements from an array.

Creating arrays

Here are two basic arrays:

num_array = [1, 2, 3, 4, 5]
str_array = ["This", "is", "a", "small", "array"]

Both arrays have five elements separated by commas. The first array contains integers, while the second array contains strings.

Arrays are commonly created with an array literal, which is simply a special syntax used to create instances of an array object. To create a new array using an array literal, use square brackets ([]).

An array can also be created by calling the Array.new method. When you call this method, you can also include up to 2 optional arguments (initial size and default value):

Array.new               #=> []
Array.new(3)            #=> [nil, nil, nil]
Array.new(3, 7)         #=> [7, 7, 7]
Array.new(3, true)      #=> [true, true, true]

Accessing elements

Every element in an array has an index, which is a numerical representation of the element's position in the array. Like most other programming languages, Ruby arrays use zero-based indexing, which means that the index of the first element is 0, the index of the second element is 1, and so on. Accessing a specific element within an array is as simple as calling myArray[x], where x is the index of the element you want. Calling an invalid position will result in nil. Ruby also allows the use of negative indices, which return elements starting from the end of an array, starting at [-1].

str_array = ["This", "is", "a", "small", "array"]

str_array[0]            #=> "This"
str_array[1]            #=> "is"
str_array[4]            #=> "array"
str_array[-1]           #=> "array"
str_array[-2]           #=> "small"

Finally, Ruby provides the #first and #last array methods, which should be self-explanatory. In addition, these methods can take an integer argument, e.g., myArray.first(n) or myArray.last(n), which will return a new array that contains the first or last n elements of myArray, respectively.

str_array = ["This", "is", "a", "small", "array"]

str_array.first         #=> "This"
str_array.first(2)      #=> ["This", "is"]
str_array.last(2)       #=> ["small", "array"]

Adding and removing elements

Adding an element to an existing array is as simple as using the #push method or the shovel operator <<. Both methods will add elements to the end of an array and return that array with the new elements. The #pop method will remove the element at the end of an array and return the element that was removed.

num_array = [1, 2]

num_array.push(3, 4)      #=> [1, 2, 3, 4]
num_array << 5            #=> [1, 2, 3, 4, 5]
num_array.pop             #=> 5
num_array                 #=> [1, 2, 3, 4]

The methods #shift and #unshift are used to add and remove elements at the beginning of an array. The #unshift method adds elements to the beginning of an array and returns that array (much like #push). The #shift method removes the first element of an array and returns that element (much like #pop).

num_array = [2, 3, 4]

num_array.unshift(1)      #=> [1, 2, 3, 4]
num_array.shift           #=> 1
num_array                 #=> [2, 3, 4]

It's also useful to know that both #pop and #shift can take integer arguments:

num_array = [1, 2, 3, 4, 5, 6]

num_array.pop(3)          #=> [4, 5, 6]
num_array.shift(2)        #=> [1, 2]
num_array                 #=> [3]

Adding and subtracting arrays

What do you think will be the outcome of [1, 2, 3] + [3, 4, 5]?

If you guessed [1, 2, 3, 3, 4, 5], congratulations! Adding two arrays will return a new array built by concatenating them, similar to string concatenation. The concat method works the same way.

a = [1, 2, 3]
b = [3, 4, 5]

a + b         #=> [1, 2, 3, 3, 4, 5]
a.concat(b)   #=> [1, 2, 3, 3, 4, 5]

To find the difference between two arrays, you can subtract them using -. This method returns a copy of the first array, removing any elements that appear in the second array.

[1, 1, 1, 2, 2, 3, 4] - [1, 4]  #=> [2, 2, 3]

Basic methods

Ruby gives you many methods to manipulate arrays and their contents (over 150!), many of which are beyond the scope of this lesson. For full documentation, go to http://ruby-doc.org/, click on "Core API", scroll down to the Classes section, and click on "Array". There, you'll find the most up-to-date documentation on the various methods available to Ruby arrays along with explanations.

As previously mentioned, there are many methods available to Ruby arrays, and the behavior of some methods change depending on a variety of factors, such as if they take arguments or not. Therefore, ruby-doc.org will be your best friend in maximizing your aptitude with arrays. So visit soon, and visit often.

Calling the #methods method on an array will also yield a long list of the available methods.

num_array.methods       #=> A very long list of methods

Here is a brief look at some other common array methods you might run into:

[].empty?               #=> true
[[]].empty?             #=> false
[1, 2].empty?           #=> false

[1, 2, 3].length        #=> 3

[1, 2, 3].reverse       #=> [3, 2, 1]

[1, 2, 3].include?(3)   #=> true
[1, 2, 3].include?("3") #=> false

[1, 2, 3].join          #=> "123"
[1, 2, 3].join("-")     #=> "1-2-3"

Assignment

1. Read Launch School's chapter on Arrays, following along with the exercises using irb or any other REPL, such as repl.it.

2. Read The Definitive Guide to Ruby Arrays by Jesus Castello.

3. Complete the array exercises from the ruby-exercises repo that you previously cloned.

4. Let's get some practice reading documentation. You'll feel just like a real programmer! What do you think the methods #clear, #insert, #sample, #shuffle, and #uniq do? Look these methods up in the Array class documentation. Were you close?

Additional resources

This section contains helpful links to other content. It isn't required, so consider it supplemental for if you need to dive deeper into something.

• Look over the latest Ruby API documentation on arrays by clicking on "Core API" and searching for "Array". There, you'll find the most up-to-date documentation on the various methods available to the Array object along with explanations and examples.

• If you need a different take on arrays, read through this article by zetcode.

• Arrays also allow for set operations, which you can read about here. Don't worry about the simple bookshelf example. Classes and Rails will be covered later!

Knowledge check

This section contains questions for you to check your understanding of this lesson. If you're having trouble answering the questions below on your own, review the material above to find the answer.

• What is an array and why is it useful?

• What are two ways to create a new empty array?

• What are some ways of accessing the elements in an array?

• What are 3 ways to add data to an array?

• How can you remove elements from an array?