Functions, parameters, and return values | Swift

A function gives a name to a piece of behavior. Good functions make code easier to read, test, and reuse.

Declaring a function

Use func, then name the function, list parameters, and optionally return a value:

func greeting(for name: String) -> String {
    return "Hello, \(name)"
}

let message = greeting(for: "Maya")

The -> String means the function returns a String.

Argument labels

Swift separates parameter names from argument labels. Argument labels make call sites read like small sentences:

func sendEmail(to address: String, subject: String) {
    print("Sending \(subject) to \(address)")
}

sendEmail(to: "maya@example.com", subject: "Welcome")

Inside the function, you use address. At the call site, you use to:.

Use _ when a label would add noise:

func square(_ number: Int) -> Int {
    number * number
}

square(4)

Default parameters

Default values keep simple calls short while still allowing customization:

func log(_ message: String, level: String = "info") {
    print("[\(level)] \(message)")
}

log("Started")
log("Failed", level: "error")

Functions should do one job

Swift makes small functions pleasant. Prefer a function that validates one thing, formats one value, or performs one action over a large function that mixes rules, networking, and UI updates.

func isValidEmail(_ email: String) -> Bool {
    email.contains("@") && email.contains(".")
}

This function is not a perfect email validator, but it has a clear job and a clear result.

What to carry forward

functions name reusable behavior
parameters define inputs
return types define outputs
argument labels make call sites readable
default parameters reduce overloads and repeated code
small functions are easier to test and change

Next, you will learn Swift’s most important safety feature: optionals.