Function Basics

A function is a named recipe of steps you can run whenever you want, just by calling its name.

In this page:

A function is defined with func, a name, parameters in parentheses, and an optional return type after ->.

Example: Declaring and Calling a Function

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

A function that only performs an action, without producing a value, simply omits the -> return type entirely.

Example: Functions Without a Return Value

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func printBanner(title: String) {
    print("=== \(title) ===")
}
printBanner(title: "Welcome")

A function can take several parameters, each with its own name and type, separated by commas.

Example: Multiple Parameters

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func add(a: Int, b: Int) -> Int {
    return a + b
}
print(add(a: 3, b: 4))

Common Mistakes

Forgetting the -> arrow before the return type, writing func add(a: Int, b: Int) Int instead of func add(a: Int, b: Int) -> Int.
Omitting return inside a multi-statement function body; only single-expression bodies can skip it implicitly.
Calling a function without its argument labels when they are required, e.g. calling add(a, b) instead of add(a: a, b: b).

Chapter Summary

Functions are declared with func name(parameters) -> ReturnType { }.
Parameters and their types are listed in parentheses, separated by commas.
A function with no meaningful return value omits the -> ReturnType part (implicitly returning Void).
Calling a function uses its name followed by arguments in parentheses, using labels by default.

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