Basics

Hello World

import Foundation
print("Hello, World!")

Constants & Variables

let name = "Swift"       // constant (immutable)
var count = 0            // variable (mutable)
count += 1
let pi: Double = 3.14   // explicit type annotation

Comments

// single-line comment
/* multi-line
   comment */
/// documentation comment (Markdown supported)

Types

Basic Types

Int	Platform-sized integer (64-bit on modern systems)
Double	64-bit floating point (preferred over Float)
Float	32-bit floating point
Bool	`true` / `false`
String	Unicode string, value type
Character	Single extended grapheme cluster

Type Inference & Conversion

let score = 95              // inferred as Int
let gpa = 3.8              // inferred as Double
let total = Double(score) + gpa  // explicit conversion
let label = "Score: \(score)"    // string interpolation

Tuples

let point = (x: 3, y: 5)
print(point.x)            // named access
let (x, y) = point        // decompose
let (first, _) = point    // ignore second value

Type Aliases

typealias Coordinate = (Double, Double)
let origin: Coordinate = (0.0, 0.0)

Control Flow

If / Else

if score > 90 { print("A") }
else if score > 80 { print("B") }
else { print("C") }

Switch

switch grade {
case "A": print("excellent")
case "B", "C": print("passing")
default: print("unknown")
}

Loops

for i in 0..<5 { }            // half-open range
for name in names { }          // collection
for (i, val) in list.enumerated() { }
while condition { }
repeat { } while condition     // do-while

Guard

func process(value: Int?) {
    guard let v = value, v > 0 else { return }
    print(v)   // v is unwrapped and in scope
}

Functions

Basic Function

func greet(name: String) -> String {
    return "Hello, \(name)!"
}
greet(name: "Alice")

Argument Labels

func move(from start: Int, to end: Int) -> Int {
    return end - start
}
move(from: 0, to: 10)  // external labels
func add(_ a: Int, _ b: Int) -> Int { a + b }

Default & Variadic Parameters

func join(_ items: String..., separator: String = ", ") -> String {
    items.joined(separator: separator)
}
join("a", "b", "c")

inout Parameters

func double(_ x: inout Int) { x *= 2 }
var num = 5
double(&num)   // num is now 10

Closures

Closure Syntax

let double = { (x: Int) -> Int in return x * 2 }
let nums = [3, 1, 2]
let sorted = nums.sorted { $0 < $1 }
let mapped = nums.map { $0 * 10 }

Trailing Closure

UIView.animate(withDuration: 0.3) {
    view.alpha = 0.0
}

Capturing Values

func makeCounter() -> () -> Int {
    var count = 0
    return { count += 1; return count }
}
let counter = makeCounter()  // counter() => 1, 2, ...

Classes & Structs

Struct (Value Type)

struct Point {
    var x: Double
    var y: Double
}
var p = Point(x: 1, y: 2)  // auto memberwise init

Class (Reference Type)

class Vehicle {
    var speed: Double = 0
    init(speed: Double) { self.speed = speed }
}
class Car: Vehicle { var gear: Int = 1 }

Struct vs Class

struct	Value type, copied on assignment, no inheritance
class	Reference type, shared by reference, supports inheritance
mutating	Required keyword for struct methods that modify `self`
deinit	Class-only deinitializer (called before deallocation)

Protocols

Defining & Conforming

protocol Drawable {
    var description: String { get }
    func draw()
}
struct Circle: Drawable { /* implement required members */ }

Protocol Extensions

extension Drawable {
    func log() { print("Drawing: \(description)") }
}
// all Drawable conformers get log() for free

Common Protocols

Equatable	`==` and `!=` comparison
Comparable	`<`, `>`, `<=`, `>=` ordering
Hashable	Can be used as Dictionary key or in Set
Codable	Encodable + Decodable (JSON, Plist)
CustomStringConvertible	Custom `description` property
Identifiable	Requires `id` property (SwiftUI)

Optionals

Declaring Optionals

var name: String? = "Alice"  // may contain String or nil
var age: Int? = nil          // currently nil
let count: Int = 5           // non-optional, never nil

Unwrapping

if let n = name { print(n) }       // optional binding
guard let n = name else { return }  // guard
let n = name ?? "Unknown"           // nil coalescing
let n = name!                       // force unwrap (crashes if nil)

Optional Chaining

let count = user?.address?.zip?.count
// returns nil if any link in the chain is nil
user?.save()   // called only if user is non-nil

Optional Map

let length = name.map { $0.count }   // Int?
let upper = name.flatMap { $0.isEmpty ? nil : $0.uppercased() }

Enums

Basic Enum

enum Direction {
    case north, south, east, west
}
var heading = Direction.north
heading = .east   // type inferred

Associated Values

enum Result {
    case success(data: String)
    case failure(code: Int, message: String)
}
if case .failure(let code, _) = r { print(code) }

Raw Values

enum Planet: Int {
    case mercury = 1, venus, earth, mars
}
let p = Planet(rawValue: 3)   // Optional(.earth)
print(Planet.earth.rawValue)  // 3

Methods on Enums

enum Suit: String, CaseIterable {
    case hearts, diamonds, clubs, spades
}
Suit.allCases.forEach { print($0.rawValue) }

Error Handling

Defining Errors

enum NetworkError: Error {
    case badURL
    case timeout(seconds: Int)
    case serverError(code: Int)
}

Throwing & Catching

func fetch(url: String) throws -> Data {
    guard url.hasPrefix("https") else { throw NetworkError.badURL }
    return Data()
}
do { let data = try fetch(url: "https://example.com") }
catch { print("Error: \(error)") }

try Variants

try	Must be inside `do-catch`, propagates error
try?	Returns optional, `nil` on error
try!	Force try, crashes on error
throws	Function can throw errors
rethrows	Throws only if closure argument throws

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