colinfwalsh / swift-optionalFunc-readme-ios-0616

Optionals & Functions

Don't watch the clock; do what it does. Keep going. -Sam Levenson

Learning Objectives

• Return Optional types from functions
• Accept and work with Optional function parameters

Returning Optionals From Functions

You've learned about Optional values in the past couple of lessons. In a previous unit, you also learned how to write functions that return values. Do you think it's possible to combine these two bits of knowledge? Can you write a function that returns an Optional?

You can! An Optional is really no different than any other type of value. Just like you can return an Int or a String from a function, you can also return an Optional as well. And you do it just as you probably expect! Let's see this in action.

Imagine this: You are tasked with writing a function, firstItem, that takes in an array of strings and returns the first item in the list. The return value of the function would be of type String, wouldn't it? You might write a function like this:

func firstItem(items: [String]) -> String {
    return items[0]
}

That works really well when you have a list of items:

var fruits = ["apple", "banana", "grape"]
firstItem(fruits)
// Returns "apple"

What if your list is empty, though?

var fruits = []
firstItem(fruits)
// Uh oh! You get an index out of range error.

If the list is empty, you get an error that the index is out of range! In an empty list, there is no first item.

Now that you know about Optionals, it should be fairly easy to fix up this function. The first item is optional: It may or may not exist. So your firstItem function can be rewritten to possibly return the first item. There might not be a first item, though, in which case your function will return nil. The return value of your function, then, is of type String? (an optional String), instead of String.

Can you rewrite firstItem so it doesn't crash if the array of strings passed to it is empty?

You probably ended up with something like this:

func firstItem(items: [String]) -> String? {
    if items.isEmpty {
        return nil
    }
    return items[0]
}

Try calling your new firstItem function with both a non-empty and empty list. What happens?

var fruits = ["apple", "banana", "grape"]
firstItem(fruits)
// Returns "apple"
fruits = []
firstItem(fruits)
// Returns nil

If you enter the above code, along with your definition of firstItem, into a new playground, you can see what the return value of each line will be. Remember that you can also use optional unwrapping to print a message, depending on whether firstItem returns a string or returns nil. Try running this in a playground:

fruits = ["apple", "banana", "grape"]
if let fruit = firstItem(fruits) {
    print("The first fruit is \(fruit)")
} else {
    print("There are no fruits!")
}
// Prints "The first fruit is apple"

fruits = []
if let fruit = firstItem(fruits) {
    print("The first fruit is \(fruit)")
} else {
    print("There are no fruits!")
}
// Prints "There are no fruits!"

Pretty neat, huh? You'll find that many functions and methods in Swift's standard library are designed to return Optionals, and you'll probably write many of your own functions that will return Optionals, too. They're very useful!

Optional Function Parameters

Not only can functions return Optional values, they can take Optionals as parameter types, as well. Doing so is just as straightforward as you would expect.

Imagine you are writing a function that takes in a person's first, middle, and last names, and constructs a properly-formatted string with all three of those names. Some people don't have middle names, though, so perhaps that parameter should be optional.

No problem! You can just specify the type as String? instead of String, and everything will be groovy. You will, of course, have to deal with the possibility of a nil value being passed in, but that should be easy for you to handle with optional unwrapping or similar techniques you have learned about.

Here is an example for building a full name string from the given parts:

func produceFullName(withFirstName firstName: String, middleName: String?, lastName: String) -> String {
    var name = firstName
    if let middleName = middleName {
        name = "\(name) \(middleName)"
    }
    name = "\(name) \(lastName)"
    return name
}

Notice that firstName and lastName are declared as Strings—they are not optional—whereas middleName is instead a String?. In the body of the function, optional unwrapping is used to detect if middleName is not nil and include it in the string if it isn't (that is, if a value exists for middleName). Pretty easy, right?

Go ahead and add that to your playground file, then try calling it a few different ways. What happens if you call produceFullName using the following code?

produceFullName(withFirstName: "Jon", middleName: "Bon", lastName: "Jovi")
// Returns "Jon Bon Jovi"

produceFullName(withFirstName: "Mary", middleName: nil, lastName: "Doe")
// Returns "Mary Doe"

Optional parameters to functions can be very useful and powerful. Of course, if your function takes an Optional as a parameter, you are forced to deal with cases in which nil is passed, so you should only take an Optional parameter when you are prepared to deal with cases in which the value is nil.

View this lesson on Learn.co