Lesson 02 of 20

8 mins read

Constants (let), Variables (var) & Type Inference

In Plain English

Swift strongly encourages immutability. Learn when to use let (immutable constant) versus var (mutable variable), how type inference detects types automatically, explicit type annotations, and core primitives (Int, Double, Float, String, Bool).

The Mental Model

"A constant (let) is like engraving your name in stone—once carved, it can never be altered. A variable (var) is like writing on a whiteboard with a dry-erase marker."

Deep Dive: How It Works

let vs var: Always default to let unless you specifically know the value must change.

Type Inference: Swift automatically determines variable types (e.g. let score = 100 infers Int).

Explicit Type Annotation: var rating: Double = 4.5 specifies the type explicitly when inference is ambiguous.

Type Safety: Swift will not implicitly convert types (you cannot add Int and Double without explicit Double(intVal) conversion).

Syntax Blueprint

let maxRetries = 3 // Immutable
var currentAttempt = 0 // Mutable
let timeout: Double = 15.5

Use let for constants and var for variables, optionally adding : Type annotations.

Core Rules to Remember

Immutability First: Using let makes code safer, faster, and clearer to both the compiler and other developers.

Strong Static Typing: Once declared, a variable can never hold a value of a different data type.

No Implicit Coercion: Convert numbers explicitly: Double(intVal) or Int(doubleVal).

Common Beginner Traps & How to Fix Them

Mistake:Trying to mutate a value declared with let (e.g. let count = 0; count += 1).

Why it happens: let constants are read-only after initialization.

How to fix: Change let to var if the value legitimately needs to mutate over time.

Pro Tip: The Swift compiler will give you a warning if you declare a variable with var but never mutate it, reminding you to change it to let.

Live Interactive Example

Hit Run Code to see it live

Type Inference and Explicit Conversion

Swift 5.10 / LLVM

Output Console

Click "Run Code" to view the rendered output.

How it works: Double(basePrice) converted Int to Double for multiplication, and status was updated because it was declared with var.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate Circle Area with Constants

Declare a constant let radius: Double = 5.0.
Declare a constant let pi: Double = 3.14159.
Calculate let area = pi * radius * radius.
Print "Area: " followed by area.

Sandbox Output

Click "Run & Check" to test your solution.

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