Memberwise initializer in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a struct with a memberwise initializer.

Swift

struct Person {
    var name: String
    var age: Int
}

let person = Person([1]: "Alice", age: 30)

Drag options to blanks, or click blank then click option'

Aage

BPerson

Cname

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'age' as the first label instead of 'name'.

Trying to call 'init' explicitly.

The memberwise initializer uses the property names as parameter labels. Here, name is the correct label for the first argument.

2fill in blank

medium

Complete the code to create an instance of the struct using the memberwise initializer.

Swift

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

let car = Car(make: "Toyota", [1]: 2020)

Drag options to blanks, or click blank then click option'

Ayear

Binit

Cmake

Dmodel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'model' instead of 'year'.

Repeating 'make' as the label.

The memberwise initializer requires the property names as labels. Here, year is the correct label for the second argument.

3fill in blank

hard

Fix the error in the code by completing the memberwise initializer call.

Swift

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

let book = Book(title: "Swift Guide", [1]: 250)

Drag options to blanks, or click blank then click option'

Apages

Bcount

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' or 'size' instead of 'pages'.

Omitting the label for the second argument.

The memberwise initializer uses the property names as labels. The property is called pages, so that label must be used.

4fill in blank

hard

Fill both blanks to create a struct instance using the memberwise initializer.

Swift

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

let rect = Rectangle([1]: 10.5, [2]: 20.0)

Drag options to blanks, or click blank then click option'

Awidth

Blength

Cheight

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' or 'size' instead of 'width' or 'height'.

Swapping the labels.

The memberwise initializer uses the property names as labels. The properties are width and height.

5fill in blank

hard

Fill all three blanks to create a struct instance with the memberwise initializer.

Swift

struct Student {
    var name: String
    var grade: Int
    var passed: Bool
}

let student = Student([1]: "John", [2]: 9, [3]: true)

Drag options to blanks, or click blank then click option'

Aname

Bgrade

Cpassed

Dscore

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'score' instead of 'grade' or 'passed'.

Mixing up the labels.

The memberwise initializer uses the property names as labels. The properties are name, grade, and passed.

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