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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Numeric Types

Integer Types

Type	Size	Range
Int8	8-bit	-128 to 127
Int16	16-bit	-32,768 to 32,767
Int32	32-bit	-2.1 billion to 2.1 billion
Int64	64-bit	Very large range
Int	Platform-native	64-bit on modern platforms
UInt	Unsigned platform-native	0 to very large

Best Practice: Use Int for most integer values, even when the values are known to be non-negative. This promotes code consistency and interoperability.

Floating-Point Types

Type	Size	Precision
Float	32-bit	~6 decimal digits
Double	64-bit	~15 decimal digits

Default: Swift infers Double for floating-point literals.

Numeric Literals

let decimal = 17
let binary = 0b10001       // binary notation
let octal = 0o21           // octal notation
let hexadecimal = 0x11     // hexadecimal notation

// Underscores for readability
let oneMillion = 1_000_000
let justOverOneMillion = 1_000_000.000_001

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Strings and Characters

String Basics

let greeting = "Hello, World!"
let empty = ""
let alsoEmpty = String()