Convenience initializers in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class with convenience initializer

What will be printed when the following Swift code runs?

Swift

class Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
    convenience init() {
        self.init(wheels: 4)
    }
}

let car = Vehicle()
print(car.wheels)

A 4

B 0

C Compilation error

D nil

💡 Hint

Remember that the convenience initializer calls the designated initializer with 4 wheels.

The convenience initializer calls init(wheels: 4), so the wheels property is set to 4. Printing car.wheels outputs 4.

❓ Predict Output

intermediate

Value of property after using convenience initializer with parameters

What is the value of 'name' after running this Swift code?

Swift

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

let p = Person()
print(p.name)

A ""

B "Anonymous"

C nil

D Compilation error

💡 Hint

The convenience initializer sets the name to "Anonymous".

The convenience initializer calls init(name: "Anonymous"), so p.name is "Anonymous".

❓ Predict Output

advanced

Output of subclass using convenience initializer

What will be printed when this Swift code runs?

Swift

class Animal {
    var species: String
    init(species: String) {
        self.species = species
    }
    convenience init() {
        self.init(species: "Unknown")
    }
}

class Dog: Animal {
    var name: String
    init(name: String) {
        self.name = name
        super.init(species: "Dog")
    }
    convenience init() {
        self.init(name: "Buddy")
    }
}

let dog = Dog()
print(dog.name + " is a " + dog.species)

A Buddy is a Dog

B Buddy is a Unknown

C Runtime error

D Compilation error

💡 Hint

The Dog convenience initializer calls init(name: "Buddy") which sets species to "Dog".

Dog's convenience init calls init(name: "Buddy"), which sets name and calls super.init(species: "Dog"), so output is "Buddy is a Dog".

❓ Predict Output

advanced

Error type from incorrect convenience initializer

What error does this Swift code produce?

Swift

class Box {
    var size: Int
    init(size: Int) {
        self.size = size
    }
    convenience init() {
        self.size = 10
    }
}

A Error: Convenience initializer cannot have parameters

B No error, compiles fine

C Error: Missing return type

D Error: 'self' used before calling designated initializer

💡 Hint

In convenience initializers, you must call a designated initializer before setting properties.

The convenience initializer tries to set self.size before calling a designated initializer, which is not allowed and causes a compile-time error.

🧠 Conceptual

expert

Correct order of initialization in Swift convenience initializer

Which sequence correctly describes the order of steps when a convenience initializer is called in Swift?

A 1, 3, 2, 4

B 2, 1, 3, 4

C 1, 2, 3, 4

D 3, 2, 1, 4

💡 Hint

Convenience initializers must call a designated initializer before properties are set.

First, the convenience initializer is called (1), then it calls a designated initializer (2), which initializes all stored properties (3), and finally returns the fully initialized instance (4).

Practice

1. What is the main purpose of a convenience initializer in Swift?

easy

A. To provide an easier way to create an object by calling a designated initializer

B. To override a superclass initializer

C. To create a completely new object without calling any other initializer

D. To deallocate an object before it is destroyed

Solution

1. Step 1: Understand the role of convenience initializers

   Convenience initializers are shortcuts to create objects more easily by calling a designated initializer.

2. Step 2: Compare options with this role

   Only To provide an easier way to create an object by calling a designated initializer describes this purpose correctly; others describe unrelated concepts.

3. Final Answer:

   To provide an easier way to create an object by calling a designated initializer -> Option A

4. Quick Check:

   Convenience initializer = shortcut calling designated initializer [OK]

Hint: Convenience initializers always call a designated initializer [OK]

Common Mistakes:

• Thinking convenience initializers create objects independently
• Confusing convenience initializers with deinitializers
• Believing convenience initializers override superclass initializers

2. Which of the following is the correct syntax for a convenience initializer in Swift?

easy

A. convenience func init(name: String) { self.init() }

B. init convenience(name: String) { self.init() }

C. func convenience init(name: String) { self.init() }

D. convenience init(name: String) { self.init() }

Solution

1. Step 1: Recall the correct syntax for convenience initializers

   In Swift, the keyword convenience comes before init without any function keyword.

2. Step 2: Check each option

   convenience init(name: String) { self.init() } matches the correct syntax. Options B, C, and D misuse keywords or order.

3. Final Answer:

   convenience init(name: String) { self.init() } -> Option D

4. Quick Check:

   Keyword order: convenience before init [OK]

Hint: Remember: 'convenience' keyword comes before 'init' [OK]

Common Mistakes:

• Placing 'convenience' after 'init'
• Using 'func' keyword with initializers
• Incorrect keyword order causing syntax errors

3. Consider the following Swift code:

class Car {
    var color: String
    init(color: String) {
        self.color = color
    }
    convenience init() {
        self.init(color: "Red")
    }
}
let myCar = Car()

What is the value of myCar.color?

medium

A. nil

B. "Red"

C. "" (empty string)

D. Compilation error

Solution

1. Step 1: Understand the initializers in the Car class

   The designated initializer sets the color property. The convenience initializer calls the designated one with "Red".

2. Step 2: Analyze the object creation

   Creating myCar with Car() calls the convenience initializer, which sets color to "Red".

3. Final Answer:

   "Red" -> Option B

4. Quick Check:

   Convenience init sets color to "Red" [OK]

Hint: Convenience init calls designated init with default values [OK]

Common Mistakes:

• Assuming color is empty or nil without initialization
• Thinking convenience init does not call designated init
• Expecting compilation error due to missing default init

4. Identify the error in this Swift class using a convenience initializer:

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

medium

A. Convenience initializer must call a designated initializer, not assign properties directly

B. Designated initializer must be marked as convenience

C. Property 'name' must be optional

D. No error, code is correct

Solution

1. Step 1: Recall rules for convenience initializers

   A convenience initializer must call another initializer in the same class, not assign properties directly.

2. Step 2: Check the convenience init implementation

   Here, self.name = "Unknown" is assigned directly, which is not allowed.

3. Final Answer:

   Convenience initializer must call a designated initializer, not assign properties directly -> Option A

4. Quick Check:

   Convenience init must call designated init [OK]

Hint: Convenience init cannot set properties directly; must call designated init [OK]

Common Mistakes:

• Assigning properties directly in convenience initializer
• Marking designated initializer as convenience
• Making properties optional unnecessarily

5. You want to create a Swift class Rectangle with a designated initializer that takes width and height, and a convenience initializer that creates a square by taking only one side length. Which of the following implementations correctly uses a convenience initializer?

hard

A. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.width = side
self.height = side
}
}

B. class Rectangle {
var width: Double
var height: Double
convenience init(width: Double, height: Double) {
self.width = width
self.height = height
}
init(side: Double) {
self.width = side
self.height = side
}
}

C. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.init(width: side, height: side)
}
}

D. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.height = side
}
}

Solution

1. Step 1: Identify designated and convenience initializers

   The designated initializer takes width and height. The convenience initializer should call the designated one.

2. Step 2: Check each option for correct usage

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } convenience init(side: Double) { self.init(width: side, height: side) } } correctly defines the designated init and the convenience init calls it with equal sides. Others either misuse convenience keyword or assign properties directly.

3. Final Answer:

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } convenience init(side: Double) { self.init(width: side, height: side) } } -> Option C

4. Quick Check:

   Convenience init calls designated init with side values [OK]

Hint: Convenience init must call designated init with all required parameters [OK]

Common Mistakes:

• Marking designated initializer as convenience
• Assigning properties directly in convenience initializer
• Not calling designated initializer from convenience initializer