Using Arrays

Overview

This lesson will give a deeper dive on how to create, manipulate, and retrieve data from arrays.

Objectives

1. Create and populate an array.

2. Add items to an array using different methods.

3. Remove items from an array using different methods.

4. Operate on an array using the sort, reverse, and include methods.

5. Identify elements in an array based on their index number.

6. Retrieve items from an array.

7. Reference the Ruby documentation on arrays.

Video

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Intro to Ruby Arrays

Creating an Array

There are a few different ways to make a new array. You can use the literal constructor or the class constructor.

The Literal Constructor

The Class Constructor

my_array = Array.new
#  └── []

Advanced: Don't worry about the class constructor right now. We'll learn much more about this later on. We're introducing it briefly here because you may encounter this syntax if you read through other resources you might find online.

To make an array that isn't empty, you can separate each item, known as an element, by a , ("comma") and wrap all the elements inside [``] ("square brackets").

puppies = ["bulldog", "terrier", "poodle"]
#  └── ["bulldog", "terrier", "poodle"]

random_numbers = [ 2, 5, 6, 8, 30050]
#  └── [ 2, 5, 6, 8, 30050]

mixed = ["this", "array", 7, 20, "has", 45, "integers", "&", "strings", 309]
#  └── ["this", "array", 7, 20, "has", 45, "integers", "&", "strings", 309]

It is possible to create an array that contains disparate data types. In other words, you could create an array like the one above, that stores both strings and integers. This is generally discouraged, however. It's best to keep your arrays populated with only one kind of element.

Adding Items to an Array

If an array is a storage container for a list of data, then we can imagine adding and removing individual items. Let's take a look at how we can add elements to an array.

Shovel Method

The shovel method employs the "shovel" operator (<<) and allows you to add ("shovel") items onto the end of an array:

famous_cats = ["lil' bub", "grumpy cat", "Maru"]

famous_cats << "nala cat"

puts famous_cats.inspect
# > ["lil' bub", "grumpy cat", "Maru", "nala cat"]

Note: The .inspect method returns a string containing a human-readable representation of an object. In this case, the list of the strings held in the array.

The shovel method (<<) is the preferred syntax for adding elements to an array, however you might see other methods used in examples online:

The .push Method

Calling .push on an array with an argument will add that element to the end of the array. It has the same effect as the shovel method explained above:

famous_cats = ["lil' bub", "grumpy cat", "Maru"]

famous_cats.push("nala cat")

puts famous_cats.inspect
# > ["lil' bub", "grumpy cat", "Maru", "nala cat"]

The .unshift Method

To add an element to the front of an array, you can call the .unshift method on it with an argument of the element you wish to add:

famous_cats = ["lil' bub", "grumpy cat", "Maru"]

famous_cats.unshift("nala cat")

puts famous_cats.inspect
# > ["nala cat", "lil' bub", "grumpy cat", "Maru"]

Removing Items From an Array

The .pop Method

Calling .pop on an array will remove the last item from the end of the array. The .pop method will also supply the removed element as its return:

famous_cats = ["lil' bub", "grumpy cat", "Maru"]
maru_cat = famous_cats.pop

puts famous_cats.inspect 
# > ["lil' bub", "grumpy cat"]
puts maru_cat
# > Maru

The .shift Method

Calling .shift on an array will remove the first item from the front of the array. The .shift method will also supply the removed element as a return:

famous_cats = ["lil' bub", "grumpy cat", "Maru"]
lil_bub = famous_cats.shift

puts famous_cats.inspect
# > ["grumpy cat", "Maru"]
puts lil_bub
# > lil' bub

Note: If you want to remove items that are not at the beginning or end of an array, use the Ruby documentation to figure out how.

Retrieving Items from Array

When you write out a list on a notepad, you typically write each item on its own line. Whether or not the list is explicitly numbered, the list has a numerology to it based on the sequence of the lines that the items are listed upon.

Just like the items in our notepad lists, elements in an array are associated with a number that represents their order. In programming, this number is called an index. While humans typically start their lists at "1", arrays begin their indexes at 0 (zero). So, the first item in an array will always be "at index 0". If we have an array of famous (fictional) cats:

famous_cats = ["Cheshire Cat", "Puss in Boots", "Garfield"]

The "Cheshire Cat" is at index 0 in the array, "Puss in Boots" is a index 1, and "Garfield" is at index 2. Indexes will always be one less than the count.

To access one of these items in the famous_cats array, we can type the name of the array immediately followed by the relevant index number wrapped in square brackets ([]).

famous_cats =  ["Cheshire Cat", "Puss in Boots", "Garfield"]

famous_cats[1]
#  └── "Puss in Boots"

famous_cats[0]
#  └── "Cheshire Cat"

famous_cats[2]
#  └── "Garfield"

We can also access array elements by using negative index numbers. The last item of an array is considered to be stored at an index of -1. Let's give it a shot:

famous_cats[-1]
# └── "Garfield"

Bonus: Using Methods

We can also use the #first method on an array to access the first element:

famous_cats.first
# └── "Cheshire Cat"

We can use the #last method to access the last element:

famous_cats.last
# └── "Garfield

A Note on Index Numbers

What happens when we try to access the element stored in an index that doesn't exist? In other words, let's say we have our famous_cats array that contains three elements. That means that our array contains an element at indexes 0, 1, and 2. What happens if we try to access an element at index 3? An index element that doesn't exist.

Let's take a look:

It returns nil!

Advanced: Finding An Element's Index With .index()

To discover the index number of an element within an array, we can use the .index() method. Calling .index() on an array with an argument inside the parentheses will return the first index number of an element matching that argument. If no elements match the argument, then this method will return nil.

famous_cats = ["Cheshire Cat", "Puss in Boots", "Garfield"]

famous_cats.index("Puss in Boots")
#  └── 1

famous_cats.index("Maru")
#  └── nil

This is not an operation you will perform very often. Arrays are used to store data and usually you will use the index number of an item to access it, not the other way around.

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