Arrays

Overview

In this lesson, we'll introduce you to arrays.

Learning Objectives

• Define and create an array
• Access items in an array
• Change items in an array

Video

Arrays

Variables can store String and Int values. For example, you could create a variable to represent an ingredient:

let ingredient = "Jalapeno"

Variables only allow you to store one item of information at a time. What if we were making a shopping list and needed all the ingredients wanted to make a sandwich?

You could create multiple variables to represent the ingredients:

let ingredient1 = "Bread"
let ingredient2 = "Butter"
let ingredient3 = "Cheese"
let ingredient4 = "Lettuce"
let ingredient5 = "Tomatoes"

And then write code that passes these variables one at a time. This will get messy quickly. What if you forget an ingredient or need to add a new one? In the real world, you would write down all your ingredients on one shopping list. With Swift, like most programming languages, you achieve the same using an array. An array allows a variable to contain a collection of values of the same type.

Creating an Array

Here's how to create an array of values in Swift which represents the ingredients of a sandwich.

var list = ["Bread", "Butter", "Cheese", "Lettuce", "Tomatoes"]

In this case, you are creating a variable with the var keyword called 'list'. Swift can infer that [String] is the correct type for the list variable, based on its content.

Type the above into a Playground and if you option click the list variable you can see Swift's guess.

If you wanted to be more precise when creating an array, you can declare the type of values allowed in the array explicitly:

var shoppingList: [String] = ["Bread", "Butter", "Cheese", "Lettuce", "Tomatoes"]

Here you are creating a variable with the var keyword called 'shoppingList', setting the type of values allowed explicitly to be of String and then adding five string values to the array.

Or alternatively (but less used):

To be clear, there is no difference between this method of creating an array and the previous, but simpler method.

Often an array is initialized first and populated later. Say for example you want to make a shopping list for your weekly shop, but need to wait for your family before filling it with items.

var shoppingList: [String]

// Waiting for family to come home…
// They're back, let's make a list!

shoppingList = ["Bread", "Butter", "Cheese", "Lettuce", "Tomatoes"]

Here you create an array variable called 'shoppingList' that will accept values of type String. You then assign five string values to the variable later in the program.

Constant Arrays

As you have used the var keyword in all the examples so far, these collections of values are mutable. This means you can add, remove or change items in the array. As with other variable types in Swift, to make an array constant, unchangeable or immutable, use the let keyword.

For example, a shopping list may change, but the ingredients for a cheese sandwich never will:

let cheeseSandwich: [String] = ["Bread", "Butter", "Cheese", "Lettuce", "Tomatoes"]

Other Values in an Array

As you would expect, arrays can contain values other than strings, and are created the same way. For example, to create an array recording attendance at your Saturday afternoon sandwich meet:

var afternoonAttendance: [Int] = [10, 5, 15, 2, 12]

While you can create arrays that contain different values, each array may only contain values of the same type. For example, you cannot have an array of mixed number and string values.

Try this array in a Playground and see what happens:

var mixedArray = [10, "cheese", 15, 2, "Bread"]

Accessing Items in an Array

Like any series of items in a list, items in an array have a number associated with them that represents their place within it. In programming, this is called an 'index'. Remember, that whereas we start counting from 1, many programming languages start from 0, making the first item in an array actually 'at index 0'.

Continuing with our example, you want to know what the second item on the shopping list is, with a normal list this would be:

But, as an array starts from 0, to be precise, it's actually:

In the shoppingList array, if you want to print the value of the item in the second position, starting from 0, this is position 1. Assign the value in that position to a variable, and print the value of the variable:

let secondItem = shoppingList[1]

print(secondItem)
// prints "Butter"

The syntax above is called subscript. The name of the array is shoppingList and you want to access the second item in the array by using square brackets and the position of the value, i.e. [1].

Arrays share similar concepts to other collection types available in Swift and you will cover these later in the course. For now it's important to know that an array is an ordered collection of values, i.e. that every value has a particular place (or index) in the collection.

Changing Items in an Array

You have decided instead to make a ham sandwich, so you need to change the 'Cheese' value in the array. Again you need to know the position of 'Cheese' and change it by assigning a new value.

What index is the 'Cheese' value at?

Remember, that Swift counts from '0', so the value of 'Cheese' is actually at position '2', three values into the array.

Assign the new value of 'Ham' at this index:

shoppingList[2] = "Ham"

If you now print the array, the values will have changed:

print(shoppingList)
// prints ["Bread", "Butter", "Ham", "Lettuce", "Tomatoes"]

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