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.

Practice

1. What does a memberwise initializer in Swift do for a struct?

easy

A. It creates a function to print all properties.

B. It deletes all properties from the struct.

C. It automatically creates an initializer that sets all properties at once.

D. It converts the struct into a class.

Solution

1. Step 1: Understand memberwise initializer purpose

   A memberwise initializer lets you create an instance by setting all properties in one call.

2. Step 2: Recall Swift default behavior for structs

   Swift automatically provides this initializer for structs without custom initializers.

3. Final Answer:

   It automatically creates an initializer that sets all properties at once. -> Option C

4. Quick Check:

   Memberwise initializer = sets all properties at once [OK]

Hint: Memberwise initializer sets all struct properties at once [OK]

Common Mistakes:

• Thinking it prints properties
• Confusing with class initializers
• Assuming it deletes properties

2. Which of the following is the correct way to use a memberwise initializer for this struct?

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

easy

A. let myBook = Book(title -> "Swift Guide", pages -> 300)

B. let myBook = Book("Swift Guide", 300)

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

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

Solution

1. Step 1: Recall memberwise initializer syntax

   It uses parameter labels matching property names: title: and pages:.

2. Step 2: Check each option

   let myBook = Book(title: "Swift Guide", pages: 300) uses correct syntax with labels and values. Others use wrong syntax or missing labels.

3. Final Answer:

   let myBook = Book(title: "Swift Guide", pages: 300) -> Option D

4. Quick Check:

   Memberwise initializer syntax = labels and values [OK]

Hint: Use property names as labels in initializer call [OK]

Common Mistakes:

• Omitting labels in initializer
• Using = instead of : in parameters
• Using arrows -> instead of colons

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

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

let p = Point(x: 5, y: 10)
print(p.x + p.y)

medium

A. 510

B. 15

C. x: 5, y: 10

D. Error: No memberwise initializer

Solution

1. Step 1: Understand memberwise initializer usage

   The struct Point has two Int properties and uses the memberwise initializer to set x=5 and y=10.

2. Step 2: Calculate the print output

   p.x + p.y = 5 + 10 = 15, so print outputs 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add property values after initializing with memberwise initializer [OK]

Common Mistakes:

• Concatenating numbers as strings
• Expecting a print of property names
• Assuming no initializer exists

4. Identify the error in this code using a memberwise initializer:

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

let myCar = Car(brand: "Toyota")

medium

A. Missing value for 'year' property in initializer

B. Incorrect property name 'brand'

C. Cannot create instance of struct

D. Syntax error in struct declaration

Solution

1. Step 1: Check memberwise initializer requirements

   All properties must be initialized. 'year' is missing in the initializer call.

2. Step 2: Identify error cause

   Since 'year' is not given a value, the code will cause a compile error.

3. Final Answer:

   Missing value for 'year' property in initializer -> Option A

4. Quick Check:

   All properties must be set in memberwise initializer [OK]

Hint: Provide all properties when using memberwise initializer [OK]

Common Mistakes:

• Forgetting to initialize all properties
• Assuming default values without setting
• Misnaming properties in initializer

5. Given this struct with a default property value:

struct User {
  var name: String
  var age: Int = 18
}

Which initializer call is valid and why?

hard

A. let u = User(name: "Anna") // age uses default value

B. let u = User(name: "Anna", age: 20) // must always provide age

C. let u = User() // no parameters allowed

D. let u = User(age: 18) // name can be omitted

Solution

1. Step 1: Understand default property values in structs

   Properties with default values can be omitted in memberwise initializer calls.

2. Step 2: Analyze each initializer call

   let u = User(name: "Anna") // age uses default value provides 'name' only; 'age' uses default 18, which is valid. Others omit required 'name' or incorrectly require all parameters.

3. Final Answer:

   let u = User(name: "Anna") // age uses default value -> Option A

4. Quick Check:

   Default property values allow skipping parameters [OK]

Hint: Skip properties with default values in initializer call [OK]

Common Mistakes:

• Trying to omit non-default properties
• Assuming all properties must be provided
• Calling initializer with no parameters when required