Parameters and Argument Labels

Swift functions have unusually expressive parameters. Learning them makes Apple's frameworks much easier to read.

Argument labels vs parameter names

Each parameter can have two names: an argument label used when calling, and a parameter name used inside the function:

func send(_ message: String, to recipient: String) {
    print("To \(recipient): \(message)")
}

send("Hi!", to: "Ada")

To Ada: Hi!

• to recipient — callers write to:; inside, the value is called recipient. The call reads like a sentence: send "Hi!" to Ada.
• _ message — _ means no label at the call site.

Swift's API design guidelines favor call sites that read naturally in English, which is why you see labels like insert(_:at:) and replacingOccurrences(of:with:).

Default values

func greet(_ name: String, greeting: String = "Hello") -> String {
    "\(greeting), \(name)!"
}

print(greet("Ada"))
print(greet("Ada", greeting: "Welcome"))

Hello, Ada!
Welcome, Ada!

Variadic parameters

... after a type accepts any number of values, available inside as an array:

func average(_ numbers: Double...) -> Double {
    guard !numbers.isEmpty else { return 0 }
    return numbers.reduce(0, +) / Double(numbers.count)
}

print(average(2, 4, 9))

5.0

In-out parameters

Parameters are constants inside a function. To let a function change a variable you pass in, mark it inout and call it with &:

func addBonus(to score: inout Int) {
    score += 10
}

var myScore = 50
addBonus(to: &myScore)
print(myScore)

60

Use inout sparingly — returning a new value is usually clearer.

Common mistakes

• Trying to change a normal parameter inside a function: cannot assign to value: 'score' is a 'let' constant.
• Passing arguments in the wrong order. Labeled arguments must follow the order the function declares.

Practice

Write formatPrice(_ amount: Double, currency: String = "USD") that returns a string like "12.5 USD". Call it once with only an amount and once with currency: "EUR".

func formatPrice(_ amount: Double, currency: String = "USD") -> String {
    "\(amount) \(currency)"
}

print(formatPrice(12.5))
print(formatPrice(12.5, currency: "EUR"))

12.5 USD
12.5 EUR