Functions

A function takes inputs and gives outputs. In Swift, we must tell the compiler the types of our inputs and the type of our output.

func sayHello(to name: String) -> String {
    return "Hello, \(name)!"
}

sayHello(to: "Mr.Whiskers")
sayHello(to: "Tunses")
sayHello(to: "Kitty")

sayHello took a String and returned a String.

func addTwoNumbers(_ a: Int, _ b: Int) -> Int {
    return a + b
}
addTwoNumbers(10, 20)

If you add a _ before the parameter names you can exclude them when you call the function.

Void Functions

func foo() -> Void {
    print("Hello, World!")
}

func bar() {
    print("Hello, World!")
}

foo()
bar()

These functions don’t return any data. Such functions are said to return Void.
Note that you do not have to write -> Void always, such as with bar.

One-line Functions

func diceRoll() -> Int {
     Int.random(in: 1...6)
}
diceRoll()

Functions that are one line can omit the return keyword.

Testing Functions

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

square(3) == 9  // true
square(2) == 5  // false

let result: Bool = square(2) == 4

assert(square(3) == 9)
// assert(false)  // Uncomment to see a failing assertion

• The expressions square(3) == 9 and square(2) == 5 evaluate to a Bool.
• Use assert to crash on a false condition, signaling an error.

Exercises

1. Coffee Shop Order

   • Write a function

     func submitOrder(for customer: String, item: String) -> String
     

     that returns "Order submitted for <customer> for a <item>".
   • Test with:

     assert(submitOrder(for: "Kailey", item: "Latte") == "Order submitted for Kailey for a Latte")
     assert(submitOrder(for: "Alex",  item: "Espresso") == "Order submitted for Alex for a Espresso")
     

2. Order Total Calculator

   • Write a function

     func calculateTotal(prices: [Double]) -> Double
     

     that returns the sum of all prices.
   • Test with:

     assert(calculateTotal(prices: [3.5, 4.0, 2.75]) == 10.25)
     

3. Discount Application

   • Write a function

     func applyDiscount(to total: Double, percent: Double) -> Double
     

     that returns the total after applying the discount percentage.
   • Test with:

     assert(applyDiscount(to: 50, percent: 10) == 45)
     

4. Tip Calculator

   • Write a function

     func addTip(to total: Double, tipPercent: Double) -> Double
     

     that adds a tip (percent) to the total.
   • Test with:

     assert(addTip(to: 40, tipPercent: 15) == 46)
     

5. Receipt Formatter

   • Write a function

     func formatReceipt(items: [String], total: Double) -> String
     

     that returns a multi-line receipt with each item on its own line, then "Total: $<total>".
   • Test with sample items and total.

6. Bonus – Process Multiple Orders

   • Write a function

     func processOrders(orders: [(customer: String, item: String, price: Double)]) -> String
     

     that returns a summary string listing each order on its own line and a final "Combined total: $<sum>" line.
   • Test with at least three different orders.