Adding initializers via extension in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of initializer added via extension

What is the output of this Swift code that adds an initializer via extension?

Swift

struct Point {
    var x: Int
    var y: Int
}

extension Point {
    init() {
        self.x = 0
        self.y = 0
    }
}

let p = Point()
print("(\(p.x), \(p.y))")

A (x: 0, y: 0)

B (nil, nil)

C Compilation error

D (0, 0)

💡 Hint

Think about what the extension initializer sets the properties to.

The extension adds an initializer that sets x and y to 0, so the output is (0, 0).

❓ Predict Output

intermediate

Value of property after extension initializer

What is the value of the property 'name' after creating an instance using the extension initializer?

Swift

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

extension Person {
    convenience init() {
        self.init(name: "Unknown")
    }
}

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

A Empty string

B nil

C Unknown

D Compilation error

💡 Hint

Check what the convenience initializer passes to the designated initializer.

The convenience initializer calls the designated initializer with "Unknown", so p.name is "Unknown".

🔧 Debug

advanced

Why does this extension initializer cause a compile error?

Consider this Swift code. Why does the extension initializer cause a compile error?

Swift

struct Rectangle {
    var width: Int
    var height: Int
}

extension Rectangle {
    init(area: Int) {
        self.width = area
        // Missing height initialization
    }
}

A All stored properties must be initialized before use, height is missing.

B Extensions cannot add initializers to structs.

C The init method must be marked as convenience.

D The code is valid and compiles without errors.

💡 Hint

Check if all stored properties are initialized in the initializer.

Swift requires all stored properties to be initialized before the initializer ends. Here, height is not initialized, causing a compile error.

📝 Syntax

advanced

Which extension initializer syntax is correct?

Which of the following extension initializers for struct Circle is syntactically correct?

Swift

struct Circle {
    var radius: Double
}

// Choose the correct extension initializer syntax:

A

extension Circle {
    init(diameter: Double) {
        self.radius = diameter / 2
    }
}

B

extension Circle {
    convenience init(diameter: Double) {
        self.radius = diameter / 2
    }
}

C

extension Circle {
    init(diameter: Double) {
        radius = diameter / 2
    }
}

D

extension Circle {
    convenience init(diameter: Double) {
        radius = diameter / 2
    }
}

💡 Hint

Remember that convenience initializers are only for classes, not structs.

Structs cannot have convenience initializers, so options A and D are invalid. Also, self. is required to assign to properties, so C is invalid.

🚀 Application

expert

How many initializers does this class have after extension?

Given this Swift class and its extension, how many initializers does the class have in total?

Swift

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

extension Vehicle {
    convenience init() {
        self.init(wheels: 4)
    }
}

A 1

B 2

C 3

D 0

💡 Hint

Count both the original and extension initializers.

The class has one designated initializer and one convenience initializer added by the extension, totaling 2 initializers.