Constants

When we need to store a value that should never change, we use the keyword let. Unlike var, which allows us to modify a variable’s value, let creates a constant—a value that stays the same throughout the program.

In our previous post, we learned that variables can change. But sometimes, we want to make sure a value remains fixed and cannot be accidentally modified. This is where let comes in.

Let’s understand the let keyword — what it is, why it’s useful, and how to use it with different examples.

What is let?

The let keyword is used to create constants. A constant is like a box that you can put something in once, but once it’s inside, you can’t change it.

Syntax of let

Here’s how we declare a constant:

let constantName = value

• let tells Swift we are creating a constant.
• constantName is the name we give to our constant.
• value is the fixed data that the constant will store.

Unlike variables (var), once we assign a value to a let constant, you cannot change it.

Defining a Constant

Here’s an example to declare a constant:

let pi = 3.14159

print(pi) // Output: 3.14159

In this example:

• We declare pi as a constant using let.
• We assign it the value 3.14159.
• The value of pi cannot be changed later in the program.

If we try to change it, Swift will throw an error:

pi = 3.14 // Error: Cannot assign to value: 'pi' is a 'let' constant

Here are some example scenarios where the let can be used.

Using let for fixed data

Let’s say we’re building a program that calculates a circle’s area. Since π (pi) is a fixed mathematical constant, it makes sense to use let:

let pi = 3.14159

let radius = 5

let area = pi * Double(radius) * Double(radius)

print(area) // Output: 78.53975

Here, pi and radius are constants because their values won’t change. This helps us prevent accidental modifications in our code.

Using let for user information

Imagine we have an app where a user’s date of birth never changes:

let birthYear = 1990

print("The user was born in (birthYear).")

// Output: The user was born in 1990.

Since a birth year will never change once set, let is the perfect choice here.

Constants and Type Inference

Just like with var, Swift can automatically figure out the type of a constant based on the assigned value:

let appName = "SwiftGuide"

Here, Swift knows that appName is a String because we assigned it a text value. We don’t need to specify String explicitly.

Preventing Unintentional Changes

One of the biggest advantages of using let is that it protects our data from being modified by mistake.

Consider this example with a student’s ID number:

let studentID = 12345

If we try to change it:

studentID = 67890 // Error: Cannot assign to value: 'studentID' is a 'let' constant