Optional chaining with ?. in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this optional chaining example?

Consider the following Swift code. What will be printed when it runs?

Swift

class Person {
    var pet: Pet?
}

class Pet {
    var name: String
    init(name: String) {
        self.name = name
    }
}

let person = Person()
if let petName = person.pet?.name {
    print(petName)
} else {
    print("No pet name")
}

A Empty string

B nil

C Runtime error

D No pet name

💡 Hint

Think about what happens when you try to access a property of a nil optional using optional chaining.

The person has no pet assigned, so person.pet is nil. Using optional chaining person.pet?.name returns nil safely, so the else branch runs and prints "No pet name".

❓ Predict Output

intermediate

What does this optional chaining expression return?

Given the code below, what is the value of 'length' after execution?

Swift

class Book {
    var title: String?
}

let book = Book()
book.title = "Swift Guide"
let length = book.title?.count
print(length ?? "No length")

A No length

B 11

C Optional(11)

D nil

💡 Hint

Optional chaining returns an optional value even if the property is non-optional.

book.title is a String? with value "Swift Guide". Using optional chaining book.title?.count returns an Int? with value 11 wrapped in Optional. Printing length shows Optional(11).

❓ Predict Output

advanced

What is the output of this nested optional chaining?

Analyze the code and determine what will be printed.

Swift

class Engine {
    var horsepower: Int
    init(horsepower: Int) {
        self.horsepower = horsepower
    }
}

class Car {
    var engine: Engine?
}

let car = Car()
car.engine = Engine(horsepower: 150)

if let hp = car.engine?.horsepower {
    print("Horsepower: \(hp)")
} else {
    print("No engine horsepower")
}

A No engine horsepower

B Horsepower: 150

C nil

D Runtime error

💡 Hint

Check if the engine property is nil or not before accessing horsepower.

car.engine is assigned an Engine instance with horsepower 150. Optional chaining safely unwraps engine and accesses horsepower, printing "Horsepower: 150".

❓ Predict Output

advanced

What error does this code produce?

What happens when you run this Swift code?

Swift

class House {
    var address: String
    init(address: String) {
        self.address = address
    }
}

var house: House?
house!.address = "123 Main St"

A Runtime error: Unexpectedly found nil while unwrapping an Optional value

B No output, runs fine

C Runtime error: Index out of range

D Compile-time error: Use of unresolved identifier 'house'

💡 Hint

What happens if you force unwrap a nil optional?

house is nil. Using house! tries to force unwrap nil, causing a runtime crash with the message "Unexpectedly found nil while unwrapping an Optional value".

🧠 Conceptual

expert

How many items are in the resulting array after filtering with optional chaining?

Consider this Swift code snippet. How many elements does 'validLengths' contain after execution?

Swift

class Document {
    var text: String?
    init(text: String?) {
        self.text = text
    }
}

let docs = [Document(text: "Hello"), Document(text: nil), Document(text: "Swift"), Document(text: nil)]

let validLengths = docs.compactMap { $0.text?.count }

A 2

B 4

C 0

D 3

💡 Hint

compactMap removes nil values after optional chaining.

Only two Document instances have non-nil text: "Hello" (length 5) and "Swift" (length 5). compactMap collects these lengths, so validLengths contains 2 elements.

Practice

1. What does the ?. operator do in Swift when used with optionals?

easy

A. It forces unwraps the optional and crashes if nil.

B. It converts the optional to a non-optional value automatically.

C. It safely accesses a property or method and returns nil if the optional is nil.

D. It assigns a default value if the optional is nil.

Solution

1. Step 1: Understand optional chaining purpose

   The ?. operator is used to safely access properties or methods on an optional value.

2. Step 2: Behavior when optional is nil

   If the optional is nil, the chaining stops and returns nil instead of crashing.

3. Final Answer:

   It safely accesses a property or method and returns nil if the optional is nil. -> Option C

4. Quick Check:

   Optional chaining = safe access with nil return [OK]

Hint: Remember: ? stops if nil, no crash happens [OK]

Common Mistakes:

• Confusing optional chaining with forced unwrapping
• Thinking it unwraps optionals automatically
• Assuming it provides default values

2. Which of the following is the correct syntax to safely access the name property of an optional person object using optional chaining?

easy

A. person->name

B. person!.name

C. person.name?

D. person?.name

Solution

1. Step 1: Identify correct optional chaining syntax

   Optional chaining uses ?. between the optional variable and the property.

2. Step 2: Check each option

   person?.name uses person?.name, which is correct syntax. person!.name forces unwraps with !. person.name? places ? incorrectly. person->name uses invalid operator.

3. Final Answer:

   person?.name -> Option D

4. Quick Check:

   Optional chaining syntax = variable?.property [OK]

Hint: Use ?. right before the property to chain safely [OK]

Common Mistakes:

• Using ! instead of ? for optional chaining
• Placing ? after the property name
• Using invalid operators like ->

3. Consider the following Swift code:

class Person {
  var pet: Pet?
}
class Pet {
  var name: String
  init(name: String) { self.name = name }
}

let person = Person()
let petName = person.pet?.name
print(petName)

What will be printed?

medium

A. Optional(nil)

B. nil

C. An empty string

D. Runtime error

Solution

1. Step 1: Analyze the optional chaining result

   Since person.pet is nil (not assigned), person.pet?.name returns nil of type String?.

2. Step 2: Understand print output of optional nil

   Printing an optional value that is nil in Swift shows Optional(nil) because the variable itself is an optional.

3. Final Answer:

   Optional(nil) -> Option A

4. Quick Check:

   print(String? nil) = Optional(nil) [OK]

Hint: Printing optional nil shows 'Optional(nil)', not 'nil' [OK]

Common Mistakes:

• Thinking print shows nil
• Expecting empty string output
• Thinking it causes runtime error

4. The following Swift code causes a compile error. What is the mistake?

struct Car {
  var owner: Person?
}

let car = Car()
let ownerName = car.owner?.name!

medium

A. Using forced unwrap (!) after optional chaining without parentheses

B. Missing initialization of car variable

C. Incorrect use of optional chaining operator

D. Person type does not have a name property

Solution

1. Step 1: Identify forced unwrap misuse

   The code uses car.owner?.name! which tries to force unwrap the property without parentheses.

2. Step 2: Correct syntax for forced unwrap after optional chaining

   Forced unwrap must be applied to the entire optional chaining expression with parentheses like (car.owner?.name)!.

3. Final Answer:

   Using forced unwrap (!) after optional chaining without parentheses -> Option A

4. Quick Check:

   Force unwrap after optional chaining needs parentheses [OK]

Hint: Wrap optional chain in () before ! to force unwrap [OK]

Common Mistakes:

• Forgetting parentheses around optional chain before !
• Assuming optional chaining can be forced unwrapped directly
• Ignoring compiler error details

5. Given the following classes:

class Address {
  var city: String?
}
class User {
  var address: Address?
}

let user = User()

Which code safely prints the city name if it exists, or "Unknown" if city or address is nil?

hard

A. print(user.address.city ?? "Unknown")

B. print(user.address?.city ?? "Unknown")

C. print(user.address!.city!)

D. print(user.address?.city!)

Solution

1. Step 1: Understand optional chaining with nil coalescing

   Using user.address?.city safely accesses city, returning nil if address or city is nil.

2. Step 2: Use nil coalescing operator to provide default

   The ?? "Unknown" provides "Unknown" if the optional is nil, avoiding crashes.

3. Final Answer:

   print(user.address?.city ?? "Unknown") -> Option B

4. Quick Check:

   Optional chaining + ?? = safe access with default [OK]

Hint: Combine ?. with ?? to provide default value safely [OK]

Common Mistakes:

• Forcing unwraps causing runtime crashes
• Not handling nil for nested optionals
• Using optional chaining without default fallback