Functions

Enemy 1 of 5

A function packages a piece of logic under a name so you can run it again without retyping the same code. Define one with func, followed by a name and a parameter list in parentheses. Each parameter needs a type, and if the function returns a value, the return type follows an arrow (-> Type) after the parameters — for example, func double(_ n: Int) -> Int declares a function named double taking one Int parameter and returning an Int.

The leading underscore before n (_ n: Int) is a distinctly Swift detail: by default, Swift functions require callers to write the parameter's name as a label at the call site (like double(n: 5)), which can make code very self-documenting for functions with several parameters. The underscore explicitly opts out of that requirement for a given parameter, letting you call the function as double(5) instead. You'll see both styles in real Swift code — labeled parameters when clarity matters (especially with multiple parameters of the same type), and underscore-prefixed ones when the meaning is already obvious from context.

return sends a value back to the caller and immediately stops the function from executing any further code in that block.