Class declaration syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Swift class instance property

What is the output of this Swift code?

Swift

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

let dog = Animal(name: "Buddy")
print(dog.name)

A Buddy

B Animal

C name

D Compilation error

💡 Hint

Look at how the property 'name' is set and accessed.

The class Animal has a property 'name' initialized with 'Buddy'. Printing dog.name outputs 'Buddy'.

❓ Predict Output

intermediate

Output when accessing a class method

What will this Swift code print?

Swift

class Calculator {
    func add(_ a: Int, _ b: Int) -> Int {
        return a + b
    }
}

let calc = Calculator()
print(calc.add(3, 4))

A Compilation error

B 34

C 7

D 0

💡 Hint

The add method returns the sum of two numbers.

The add method adds 3 and 4, returning 7 which is printed.

❓ Predict Output

advanced

Output of class inheritance and method override

What is the output of this Swift code?

Swift

class Vehicle {
    func sound() -> String {
        return "Vroom"
    }
}

class Car: Vehicle {
    override func sound() -> String {
        return "Beep"
    }
}

let myCar = Car()
print(myCar.sound())

A Vroom

B Beep

C Compilation error

D Runtime error

💡 Hint

Check which method is called on the Car instance.

Car overrides the sound method to return 'Beep'. So printing myCar.sound() outputs 'Beep'.

❓ Predict Output

advanced

Output of a class with a computed property

What will this Swift code print?

Swift

class Rectangle {
    var width: Double
    var height: Double
    var area: Double {
        return width * height
    }
    init(width: Double, height: Double) {
        self.width = width
        self.height = height
    }
}

let rect = Rectangle(width: 3, height: 4)
print(rect.area)

A 12.0

B 7.0

C Compilation error

D 0.0

💡 Hint

The area is width multiplied by height.

The computed property area returns 3 * 4 = 12.0 which is printed.

🧠 Conceptual

expert

Understanding class reference behavior in Swift

Consider this Swift code. What is the value of 'b.name' after execution?

Swift

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

let a = Person(name: "Alice")
let b = a
b.name = "Bob"
print(a.name)

A Runtime error

B Alice

C Compilation error

D Bob

💡 Hint

Classes are reference types in Swift.

Both 'a' and 'b' refer to the same Person instance. Changing b.name changes a.name too.

Practice

1. What is the correct way to start declaring a class named Car in Swift?

easy

A. function Car() { }

B. def Car() { }

C. class Car { }

D. struct Car { }

Solution

1. Step 1: Recognize Swift class syntax

   In Swift, classes start with the keyword class followed by the class name and curly braces.

2. Step 2: Match the correct option

   class Car { } uses class Car { }, which is the correct syntax for declaring a class named Car.

3. Final Answer:

   class Car { } -> Option C

4. Quick Check:

   Swift class declaration = class ClassName { } [OK]

Hint: Look for 'class' keyword followed by name and braces [OK]

Common Mistakes:

• Using 'def' or 'function' which are not Swift keywords
• Confusing struct with class
• Missing curly braces

2. Which of the following is the correct syntax to declare a class with an initializer in Swift?

easy

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

B. class Person { var name: String; init(name: String) { self.name = name } }

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

D. class Person { var name: String init(name: String) { name = name } }

Solution

1. Step 1: Understand property declaration order

   In Swift, properties should be declared before the initializer for clarity and correctness.

2. Step 2: Check initializer syntax

   The initializer must assign the parameter to the property using self.name = name.

3. Final Answer:

   class Person { var name: String; init(name: String) { self.name = name } } -> Option B

4. Quick Check:

   Properties before init, use self.property = parameter [OK]

Hint: Declare properties before init; assign with self.property = parameter [OK]

Common Mistakes:

• Declaring properties after init causing errors
• Assigning parameter to self incorrectly
• Missing semicolon or braces

3. What will be the output of this Swift code?

class Animal {
    var name: String
    init(name: String) {
        self.name = name
    }
    func speak() -> String {
        return "Hello, I am \(name)"
    }
}

let dog = Animal(name: "Buddy")
print(dog.speak())

medium

A. Hello, I am name

B. Error: Missing initializer

C. Hello, I am

D. Hello, I am Buddy

Solution

1. Step 1: Understand class initialization

   The class Animal has a property name initialized via init(name: String). The instance dog is created with name "Buddy".

2. Step 2: Analyze the speak() method output

   The speak() method returns a string with the name property interpolated. So it returns "Hello, I am Buddy".

3. Final Answer:

   Hello, I am Buddy -> Option D

4. Quick Check:

   String interpolation with property = Hello, I am Buddy [OK]

Hint: Check property value passed to init and used in method [OK]

Common Mistakes:

• Confusing property name with string literal
• Forgetting to call speak() method
• Assuming default values without init

4. Identify the error in this Swift class declaration:

class Book {
    var title: String
    init(title: String) {
        title = title
    }
}

medium

A. Property 'title' is not initialized correctly

B. Missing class keyword

C. Initializer has wrong name

D. No error, code is correct

Solution

1. Step 1: Check property assignment in initializer

   The initializer assigns title = title, which assigns the parameter to itself, not to the property.

2. Step 2: Correct assignment syntax

   It should be self.title = title to assign the parameter value to the property.

3. Final Answer:

   Property 'title' is not initialized correctly -> Option A

4. Quick Check:

   Use self.property = parameter to assign [OK]

Hint: Use self.property = parameter to avoid confusion [OK]

Common Mistakes:

• Assigning parameter to itself instead of property
• Forgetting self keyword
• Assuming implicit assignment

5. You want to create a Swift class Rectangle with properties width and height, and a method area() that returns the area. Which of the following is the correct complete class declaration?

hard

A. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
func area() -> Double {
return width * height
}
}

B. class Rectangle {
var width: Double
var height: Double
func area() -> Double {
return width * height
}
init(width: Double, height: Double) {
width = width
height = height
}
}

C. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
width = width
height = height
}
func area() -> Double {
return width * height
}
}

D. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
func area() {
width * height
}
}

Solution

1. Step 1: Check property initialization

   Properties width and height must be assigned using self.width = width and self.height = height in the initializer.

2. Step 2: Verify method return type and body

   The area() method must return a Double and use return width * height.

3. Final Answer:

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } func area() -> Double { return width * height } } -> Option A

4. Quick Check:

   Use self.property in init and return in methods [OK]

Hint: Assign properties with self. and return values in methods [OK]

Common Mistakes:

• Assigning parameters to themselves without self.
• Missing return statement in method
• Incorrect method return type