Memberwise initializer in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using a memberwise initializer?

Consider the following Swift struct and code:

struct Book {
    var title: String
    var pages: Int
}

let myBook = Book(title: "Swift Guide", pages: 300)
print(myBook.pages)

What will be printed?

Swift

struct Book {
    var title: String
    var pages: Int
}

let myBook = Book(title: "Swift Guide", pages: 300)
print(myBook.pages)

A 300

B Swift Guide

C 0

D Compilation error

💡 Hint

Memberwise initializer sets properties in the order you provide them.

The memberwise initializer automatically creates an initializer with parameters matching the struct's properties. Here, pages is set to 300, so printing myBook.pages outputs 300.

❓ Predict Output

intermediate

What error does this Swift code produce?

Look at this Swift struct and code:

struct Car {
    var brand: String
    var year: Int
}

let myCar = Car(brand: "Toyota")

What error will this code cause?

Swift

struct Car {
    var brand: String
    var year: Int
}

let myCar = Car(brand: "Toyota")

A No error, compiles fine

B Type mismatch error

C Missing argument for parameter 'year' in call

D Cannot assign value of type 'String' to type 'Int'

💡 Hint

Memberwise initializer requires all properties to be initialized unless default values are provided.

The memberwise initializer expects both brand and year parameters. Omitting year causes a compile-time error about the missing argument.

🔧 Debug

advanced

Why does this Swift struct fail to compile?

Examine this Swift code:

struct Person {
    var name: String
    var age: Int

    init(name: String) {
        self.name = name
    }
}

let p = Person(name: "Anna", age: 25)

Why does this code fail to compile?

Swift

struct Person {
    var name: String
    var age: Int

    init(name: String) {
        self.name = name
    }
}

let p = Person(name: "Anna", age: 25)

A Custom init disables memberwise initializer, so 'age' parameter is missing

B Cannot assign 'age' after init is defined

C Structs cannot have custom initializers

D No error, compiles fine

💡 Hint

Adding a custom initializer removes the automatic memberwise initializer.

When you add a custom initializer, Swift does not generate the memberwise initializer automatically. So Person(name:age:) does not exist, causing a compile error.

📝 Syntax

advanced

Which option correctly uses the memberwise initializer for this Swift struct?

Given this struct:

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

Which code correctly creates a Rectangle instance using the memberwise initializer?

Swift

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

A let rect = Rectangle(width = 5.0, height = 10.0)

B let rect = Rectangle(5.0, 10.0)

C let rect = Rectangle(width: 5, height: "10")

D let rect = Rectangle(width: 5.0, height: 10.0)

💡 Hint

Memberwise initializer requires parameter labels matching property names.

Option D uses the correct syntax with parameter labels and matching types. Option D misses labels, A uses assignment syntax which is invalid, and D has a type mismatch.

🚀 Application

expert

How many properties does this Swift struct instance have after initialization?

Consider this Swift struct with default values:

struct Laptop {
    var brand: String
    var ramGB: Int = 8
    var storageGB: Int
}

let myLaptop = Laptop(brand: "Apple", storageGB: 256)

How many properties does myLaptop have initialized after this code runs?

Swift

struct Laptop {
    var brand: String
    var ramGB: Int = 8
    var storageGB: Int
}

let myLaptop = Laptop(brand: "Apple", storageGB: 256)

A 2

B 3

C 1

D Compilation error

💡 Hint

Default values count as initialized properties.

The struct has three properties: brand, ramGB, and storageGB. The memberwise initializer requires brand and storageGB as parameters. ramGB uses its default value 8. So all three properties are initialized.