func sayHello() {
    print("Hello")
}

sayHello()     // "Hello"

func square (num: Int) -> Int {
    return num * num
}

square(num: 20)     // 400

func greet(person: String) -> String {
    let greeting = "Hello, " + person + "!"
    return greeting
}

greet(person: "xx")    // "Hello, xx!"

func minMax(array: [Int]) -> (min: Int, max: Int) {
    var currentMin = array[0]
    var currentMax = array[0]
    for value in array[1..<array.count] {
        if value < currentMin {
            currentMin = value
        } else if value > currentMax {
            currentMax = value
        }
    }
    return (currentMin, currentMax)
}

let bounds = minMax(array: [8, -6, 2, 109, 3, 71])
print("min is \(bounds.min) and max is \(bounds.max)")

reference:

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html