Variables and Constants

In almost any program that we create, we will need to store data at some point. In Swift, you can store data in two ways: variables and constants. What is the difference between these two? When do we use which?

Swift Basics

Fall 2023

In almost any program that we create, we will need to store data at some point. In Swift, we can store data in two ways: variables and constants. We can think of both variables and constants as a box holding some value inside. However, there is one key difference between these two. A variable can change its value whenever we want. On the contrary, a constant can hold a value once and can never be changed again.

It may seem pointless to have both variables and constants; however, there are many advantages. If Xcode knows that a value will never change, it will optimize our program to make it run faster. Another advantage is that if we were to make a mistake and change a value of a constant when we don’t need to, Xcode will tell us and our code will not compile.

Variables

To create a variable, we use the var keyword.

1var instructor = "Vin"

To change the value of the variable, we can simply do the following.

1var instructor = "Vin"
2instructor = "Richie"

Let’s try this in the Xcode playground.

To change the value of the variable, we can simply do the following.

Notice how we do not need to use the var keyword the second time. We should only use the var keyword if we are declaring a new variable. We can test this out in the Xcode playground.

Constants

Now, what if we wanted to use a constant instead of a variable? All we would need to do is to use the let keyword instead.

As we can see, changing the instructor variable to a constant caused Xcode to get angry. The error message clearly informs us that we are attempting to change the value of a constant.