Variables and Types

Swift has a rich type system that balances safety with expressiveness. This lesson covers how to declare variables and constants, the built-in types, type inference, and type conversions.

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Constants and Variables

In Swift, you declare constants with let and variables with var:

let maximumAttempts = 10   // constant — cannot be changed
var currentAttempt = 1     // variable — can be reassigned

currentAttempt = 2         // OK
// maximumAttempts = 20    // Error: cannot assign to a 'let' constant

Best Practice: Prefer let over var whenever possible. The compiler will warn you if you use var for a value that is never mutated.

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Type Annotations

Swift uses type inference, but you can add explicit type annotations:

let name: String = "Alice"
let age: Int = 30
let pi: Double = 3.14159
let isActive: Bool = true

When the type can be inferred from the value, you can omit the annotation:

let name = "Alice"     // inferred as String
let age = 30           // inferred as Int
let pi = 3.14159       // inferred as Double
let isActive = true    // inferred as Bool

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