Swift: Swiftの関数定義とパラメータ：初心者から上級者へ

func calculate(price: Double, count: Int, rate: Double) -> Double {
    return price * Double(count) * (1 - rate)
}
// At the call site — parameter meaning is guesswork
let result = calculate(price: 100, count: 3, rate: 0.1)

func calculate(price: Double, count: Int, discount rate: Double) -> Double {
    return price * Double(count) * (1 - rate)
}
// Call site is clear: "discount" tells you it's a discount rate
let result = calculate(price: 100, count: 3, discount: 0.1)

func sayHello() {
    print("Hello!")
}
sayHello() // Call

func add(a: Int, b: Int) -> Int {
    return a + b
}
let sum = add(a: 5, b: 3)
print("5 + 3 = \(sum)")

// ============================================
// Calculate order total including tax
// ============================================
func orderAmount(price: Double, quantity: Int) -> Double {
    return price * Double(quantity)
}
let amount = orderAmount(price: 29.99, quantity: 3)
print("Order amount: $\(amount)")

// ============================================
// Three argument label styles compared
// ============================================
// 1. Default — label equals name
func greet1(name: String) {
    print("Hello, \(name)")
}
// 2. Custom external label
func greet2(to name: String) {
    print("Hello, \(name)")
}
// 3. Omit external label (underscore)
func greet3(_ name: String) {
    print("Hello, \(name)")
}
greet1(name: "Alice")
greet2(to: "Bob")
greet3("Charlie")

func makeCoffee(size: String, sugar: Int = 1) -> String {
    return "\(size) coffee with \(sugar) sugar(s)"
}
print(makeCoffee(size: "Large"))           // Uses default sugar
print(makeCoffee(size: "Medium", sugar: 0)) // Custom sugar

func sum(_ numbers: Double...) -> Double {
    var total = 0.0
    for number in numbers {
        total += number
    }
    return total
}
print(sum(1, 2, 3))       // 3 arguments
print(sum(10, 20))         // 2 arguments
print(sum())               // 0 arguments

// ============================================
// inout parameter: swapping two variables
// ============================================
func swapValues(_ a: inout Int, _ b: inout Int) {
    let temp = a
    a = b
    b = temp
}
var x = 10
var y = 20
swapValues(&x, &y)
print("x = \(x), y = \(y)")

// ============================================
// Variadic + inout combined: compute average and track calls
// ============================================
func average(_ scores: Double..., record callCount: inout Int) -> Double {
    callCount += 1
    guard !scores.isEmpty else { return 0 }
    var total = 0.0
    for score in scores {
        total += score
    }
    return total / Double(scores.count)
}
var callTimes = 0
print("Average: \(average(85, 90, 78, record: &callTimes))")
print("Average: \(average(100, 95, record: &callTimes))")
print("Function called \(callTimes) times")

// ============================================
// Full example: Order discount calculator
// Combines argument labels, defaults, variadic, inout
// ============================================
import Foundation
// 1. Compute subtotal (variadic)
func subtotal(_ prices: Double...) -> Double {
    var total = 0.0
    for price in prices {
        total += price
    }
    return total
}
// 2. Apply discount (default parameter + argument label)
func applyDiscount(to total: Double, rate: Double = 0.1) -> Double {
    return total * (1 - rate)
}
// 3. Log orders (inout parameter)
func logOrder(_ description: String, counter: inout Int) {
    counter += 1
    print("[\(counter)] \(description)")
}
// 4. Simulate order processing
var orderCount = 0
let item1 = subtotal(29.99, 49.99)
let item2 = subtotal(9.99, 14.99, 24.99)
logOrder("Item 1 subtotal: $\(item1)", counter: &orderCount)
logOrder("Item 2 subtotal: $\(item2)", counter: &orderCount)
let total1 = applyDiscount(to: item1, rate: 0.15)
let total2 = applyDiscount(to: item2)
let finalTotal = total1 + total2
logOrder("Final total: $\(finalTotal)", counter: &orderCount)