Swift values and variables

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Goal

You will read and write the two Swift keywords that hold values: let and var. After this page you will be able to declare typed and inferred values and tell the compiler when something is allowed to change.

Prerequisites

• Xcode installed

let, var, and type inference

Swift has two ways to name a value:

• let makes a constant. Once set, the name always points to the same value.
• var makes a variable. The name can be reassigned later.

Swift is statically typed: every value has a type known at compile time. You can write the type yourself or let the compiler infer it from the assigned value.

The three built-in types you will use constantly in SwiftUI are String, Int, and Bool. Arrays are written [Element], so an array of strings is [String].

For the full Swift language reference, see the Swift book. This page covers only the pieces the rest of the tier needs.

Try it in a playground

In Xcode, choose File → New → Playground, pick Blank under the iOS tab, save it as SwiftBasics.playground. Replace the contents with:

import Foundation

let name: String = "Mei"
var streak: Int = 3
let isActive: Bool = true

streak = streak + 1

let items: [String] = ["Read", "Walk", "Sleep"]
let firstItem = items[0]

print(name, streak, isActive, firstItem)

Run the playground by clicking the play button next to the last line, or press Shift-Enter. The bottom panel shows:

Mei 4 true Read

Two things to notice:

• firstItem has no explicit type. Swift infers String from items[0].
• Trying to reassign name = "Aarav" would fail with a compiler error, because name is let. Try it — you will see Cannot assign to value: 'name' is a 'let' constant.

When to pick which

Default to let. Only reach for var when you actually need to reassign. SwiftUI views read constants more than they read variables — most properties on a view are let-ish by default.

Next → Functions and trailing closures