Day 12

About 11 min

Day 12 관련

Null references – literally when a variable has no value – were invented by Tony Hoare way back in 1965. When asked about it in retrospect, he said “I call it my billion-dollar mistake” because they lead to so many problems.

This is the last day that you’ll be learning the fundamentals of Swift, and it’s devoted exclusively to Swift’s solution to null references, known as optionals. These are a really important language feature, but they can hurt your brain a little – don’t feel bad if you need to repeat some videos a few times.

Today you have 11 one-minute videos to watch, and you’ll meet unwrapping, optional chaining, typecasting, and more. Once you’ve watched each video and completed any optional extra reading you wanted, there’s a short test to help make sure you’ve understood what was taught.

1. Handling missing data

100 Days of Swift - Day 12 - 1. Handling missing data

1. Handling missing data

We’ve used types such as Int to hold values like 5. But if you wanted to store an age property for users, what would you do if you didn’t know someone’s age?

You might say “well, I’ll store 0”, but then you would get confused between new-born babies and people whose age you don’t know. You could use a special number such as 1000 or -1 to represent “unknown”, both of which are impossible ages, but then would you really remember that number in all the places it’s used?

Swift’s solution is called optionals, and you can make optionals out of any type. An optional integer might have a number like 0 or 40, but it might have no value at all – it might literally be missing, which is nil in Swift.

To make a type optional, add a question mark after it. For example, we can make an optional integer like this:

var age: Int? = nil

That doesn’t hold any number – it holds nothing. But if we later learn that age, we can use it:

age = 38

1. Handling missing data - Additional

Optional: Why does Swift have optionals?
Test: Handling missing data

2. Unwrapping optionals

100 Days of Swift - Day 12 - 2. Unwrapping optionals

2. Unwrapping optionals

Optional strings might contain a string like “Hello” or they might be nil – nothing at all.

Consider this optional string:

var name: String? = nil

What happens if we use name.count? A real string has a count property that stores how many letters it has, but this is nil – it’s empty memory, not a string, so it doesn’t have a count.

Because of this, trying to read name.count is unsafe and Swift won’t allow it. Instead, we must look inside the optional and see what’s there – a process known as unwrapping.

A common way of unwrapping optionals is with if let syntax, which unwraps with a condition. If there was a value inside the optional then you can use it, but if there wasn’t the condition fails.

For example:

if let unwrapped = name {
    print("\(unwrapped.count) letters")
} else {
    print("Missing name.")
}

If name holds a string, it will be put inside unwrapped as a regular String and we can read its count property inside the condition. Alternatively, if name is empty, the else code will be run.

2. Unwrapping optionals - Additional

Optional: Why does Swift make us unwrap optionals?
Test: Unwrapping optionals

3. Unwrapping with guard

100 Days of Swift - Day 12 - 3. Unwrapping with guard

3. Unwrapping with guard

An alternative to if let is guard let, which also unwraps optionals. guard let will unwrap an optional for you, but if it finds nil inside it expects you to exit the function, loop, or condition you used it in.

However, the major difference between if let and guard let is that your unwrapped optional remains usable after the guard code.

Let’s try it out with a greet() function. This will accept an optional string as its only parameter and try to unwrap it, but if there’s nothing inside it will print a message and exit. Because optionals unwrapped using guard let stay around after the guard finishes, we can print the unwrapped string at the end of the function:

func greet(_ name: String?) {
    guard let unwrapped = name else {
        print("You didn't provide a name!")
        return
    }

    print("Hello, \(unwrapped)!")
}

Using guard let lets you deal with problems at the start of your functions, then exit immediately. This means the rest of your function is the happy path – the path your code takes if everything is correct.

3. Unwrapping with guard - Additional

Optional: When to use guard let rather than if let
Test: Unwrapping with guard

4. Force unwrapping

100 Days of Swift - Day 12 - 4. Force unwrapping

4. Force unwrapping

Optionals represent data that may or may not be there, but sometimes you know for sure that a value isn’t nil. In these cases, Swift lets you force unwrap the optional: convert it from an optional type to a non-optional type.

For example, if you have a string that contains a number, you can convert it to an Int like this:

let str = "5"
let num = Int(str)

That makes num an optional Int because you might have tried to convert a string like “Fish” rather than “5”.

Even though Swift isn’t sure the conversion will work, you can see the code is safe so you can force unwrap the result by writing ! after Int(str), like this:

let num = Int(str)!

Swift will immediately unwrap the optional and make num a regular Int rather than an Int?. But if you’re wrong – if str was something that couldn’t be converted to an integer – your code will crash.

As a result, you should force unwrap only when you’re sure it’s safe – there’s a reason it’s often called the crash operator.

4. Force unwrapping - Additional

Optional: When should you force unwrap optionals in Swift?
Test: Force unwrapping

5. Implicitly unwrapped optionals

100 Days of Swift - Day 12 - 5. Implicitly unwrapped optionals

5. Implicitly unwrapped optionals

Like regular optionals, implicitly unwrapped optionals might contain a value or they might be nil. However, unlike regular optionals you don’t need to unwrap them in order to use them: you can use them as if they weren’t optional at all.

Implicitly unwrapped optionals are created by adding an exclamation mark after your type name, like this:

let age: Int! = nil

Because they behave as if they were already unwrapped, you don’t need if let or guard let to use implicitly unwrapped optionals. However, if you try to use them and they have no value – if they are nil – your code crashes.

Implicitly unwrapped optionals exist because sometimes a variable will start life as nil, but will always have a value before you need to use it. Because you know they will have a value by the time you need them, it’s helpful not having to write if let all the time.

That being said, if you’re able to use regular optionals instead it’s generally a good idea.

5. Implicitly unwrapped optionals - Additional

6. Nil coalescing

100 Days of Swift - Day 12 - 6. Nil coalescing

6. Nil coalescing

The nil coalescing operator unwraps an optional and returns the value inside if there is one. If there isn’t a value – if the optional was nil – then a default value is used instead. Either way, the result won’t be optional: it will either be the value from inside the optional or the default value used as a backup.

Here’s a function that accepts an integer as its only parameter and returns an optional string:

func username(for id: Int) -> String? {
    if id == 1 {
        return "Taylor Swift"
    } else {
        return nil
    }
}

If we call that with ID 15 we’ll get back nil because the user isn’t recognized, but with nil coalescing we can provide a default value of “Anonymous” like this:

let user = username(for: 15) ?? "Anonymous"

That will check the result that comes back from the username() function: if it’s a string then it will be unwrapped and placed into user, but if it has nil inside then “Anonymous” will be used instead.

6. Nil coalescing - Additional

Optional: When should you use nil coalescing in Swift?
Test: Nil coalescing

7. Optional chaining

100 Days of Swift - Day 12 - 7. Optional chaining

7. Optional chaining

Swift provides us with a shortcut when using optionals: if you want to access something like a.b.c and b is optional, you can write a question mark after it to enable optional chaining: a.b?.c.

When that code is run, Swift will check whether b has a value, and if it’s nil the rest of the line will be ignored – Swift will return nil immediately. But if it has a value, it will be unwrapped and execution will continue.

To try this out, here’s an array of names:

let names = ["John", "Paul", "George", "Ringo"]

We’re going to use the first property of that array, which will return the first name if there is one or nil if the array is empty. We can then call uppercased() on the result to make it an uppercase string:

let beatle = names.first?.uppercased()

That question mark is optional chaining – if first returns nil then Swift won’t try to uppercase it, and will set beatle to nil immediately.

7. Optional chaining - Additional

Optional: Why is optional chaining so important?
Test: Optional chaining

8. Optional try

100 Days of Swift - Day 12 - 8. Optional try

8. Optional try

Back when we were talking about throwing functions, we looked at this code:

enum PasswordError: Error {
    case obvious
}

func checkPassword(_ password: String) throws -> Bool {
    if password == "password" {
        throw PasswordError.obvious
    }

    return true
}

do {
    try checkPassword("password")
    print("That password is good!")
} catch {
    print("You can't use that password.")
}

That runs a throwing function, using do, try, and catch to handle errors gracefully.

There are two alternatives to try, both of which will make more sense now that you understand optionals and force unwrapping.

The first is try?, and changes throwing functions into functions that return an optional. If the function throws an error you’ll be sent nil as the result, otherwise you’ll get the return value wrapped as an optional.

Using try? we can run checkPassword() like this:

if let result = try? checkPassword("password") {
    print("Result was \(result)")
} else {
    print("D'oh.")
}

The other alternative is try!, which you can use when you know for sure that the function will not fail. If the function does throw an error, your code will crash.

Using try! we can rewrite the code to this:

try! checkPassword("sekrit")
print("OK!")

8. Optional try - Additional

Optional: When should you use optional try?
Test: Optional try

9. Failable initializers

100 Days of Swift - Day 12 - 9. Failable initializers

9. Failable initializers

When talking about force unwrapping, I used this code:

let str = "5"
let num = Int(str)

That converts a string to an integer, but because you might try to pass any string there what you actually get back is an optional integer.

This is a failable initializer: an initializer that might work or might not. You can write these in your own structs and classes by using init?() rather than init(), and return nil if something goes wrong. The return value will then be an optional of your type, for you to unwrap however you want.

As an example, we could code a Person struct that must be created using a nine-letter ID string. If anything other than a nine-letter string is used we’ll return nil, otherwise we’ll continue as normal.

Here’s that in Swift:

struct Person {
    var id: String

    init?(id: String) {
        if id.count == 9 {
            self.id = id
        } else {
            return nil
        }
    }
}

9. Failable initializers - Additional

Optional: Why would you want a failable initializer?
Test: Failable initializers

19. Typecasting

100 Days of Swift - Day 12 - 19. Typecasting

19. Typecasting

Swift must always know the type of each of your variables, but sometimes you know more information than Swift does. For example, here are three classes:

class Animal { }
class Fish: Animal { }

class Dog: Animal {
    func makeNoise() {
        print("Woof!")
    }
}

We can create a couple of fish and a couple of dogs, and put them into an array, like this:

let pets = [Fish(), Dog(), Fish(), Dog()]

Swift can see both Fish and Dog inherit from the Animal class, so it uses type inference to make pets an array of Animal.

If we want to loop over the pets array and ask all the dogs to bark, we need to perform a typecast: Swift will check to see whether each pet is a Dog object, and if it is we can then call makeNoise().

This uses a new keyword called as?, which returns an optional: it will be nil if the typecast failed, or a converted type otherwise.

Here’s how we write the loop in Swift:

for pet in pets {
    if let dog = pet as? Dog {
        dog.makeNoise()
    }
}

19. Typecasting - Additional

Optional: When is type casting useful in Swift?
Test: Typecasting

11. Optionals summary

100 Days of Swift - Day 12 - 11. Optionals summary

11. Optionals summary

You’ve made it to the end of the tenth part of this series, so let’s summarize:

Optionals let us represent the absence of a value in a clear and unambiguous way.
Swift won’t let us use optionals without unwrapping them, either using if let or using guard let.
You can force unwrap optionals with an exclamation mark, but if you try to force unwrap nil your code will crash.
Implicitly unwrapped optionals don’t have the safety checks of regular optionals.
You can use nil coalescing to unwrap an optional and provide a default value if there was nothing inside.
Optional chaining lets us write code to manipulate an optional, but if the optional turns out to be empty the code is ignored.
You can use try? to convert a throwing function into an optional return value, or try! to crash if an error is thrown.
If you need your initializer to fail when it’s given bad input, use init?() to make a failable initializer.
You can use typecasting to convert one type of object to another.

11. Optionals summary - Additional

Test: Optionals