Lesson 10 of 20

9 mins read

Functions, Argument Labels & In-Out Parameters

In Plain English

Swift functions are designed for expressive, English-like readability at call sites. Learn parameter naming (argument labels vs parameter names), omitting labels with underscore (_), default parameters, variadic inputs, and mutating caller values with inout.

The Mental Model

"Argument labels make calling a function read like an English sentence: greet(person: "Alice", from: "Cupertino") sounds natural and makes code self-documenting."

Deep Dive: How It Works

Argument Labels vs Parameter Names: func greet(to recipient: String) -> String; to is used at callsite, recipient inside body.

Omitted Argument Labels (_): func calculate(_ a: Int, _ b: Int) allows calling calculate(10, 20) without labels.

In-Out Parameters (inout): Passes variables by reference (copy-in copy-out); callers must prefix arguments with ampersand (&).

Variadic Parameters: func sum(_ numbers: Double...) collects arbitrary arguments into an array [Double].

Syntax Blueprint

func swapValues(_ a: inout Int, _ b: inout Int) {
    let temp = a; a = b; b = temp;
}
swapValues(&x, &y)

Use inout for mutating parameters and prefix passed variables with & at the call site.

Core Rules to Remember

Readable Call Sites: Argument labels clarify parameter intent directly at the invocation point.

Default Parameters: func connect(host: String, port: Int = 8080) allows omitting parameters with defaults.

In-Out Reference Semantics: inout parameters allow functions to mutate the caller original variable directly.

Common Beginner Traps & How to Fix Them

Mistake:Passing a let constant or literal to an inout parameter (e.g. swapValues(&10, &20)).

Why it happens: inout requires mutable variable storage (var) to write back the mutated value.

How to fix: Ensure variables passed to inout are declared with var.

Pro Tip: Use _ for the first argument label when the function name already implies the argument (e.g. print("Hello") rather than print(item: "Hello")).

Live Interactive Example

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Argument Labels, Default Values and In-Out Swap

Swift 5.10 / LLVM

Output Console

Click "Run Code" to view the rendered output.

How it works: buildGreeting read naturally at the call site, and inout swapped x and y in place.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate Discounted Price Function

Write a function calculateTotal(price: Double, taxRate: Double = 0.08) -> Double.
Return price * (1.0 + taxRate).
In top-level code, call calculateTotal(price: 100.0) and print "Total: $\(result)".

Sandbox Output

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