Initializers and designated init in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a designated initializer for the class.

Swift

class Person {
    var name: String
    init([1]: String) {
        self.name = name
    }
}

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

Ainit

Bself

Cvalue

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' or 'init' as parameter names causes errors.

Using a different parameter name without assigning it properly.

The parameter name in the initializer must match the property name to assign it correctly.

2fill in blank

medium

Complete the code to call the designated initializer from a convenience initializer.

Swift

class Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
    convenience init() {
        self.[1](wheels: 4)
    }
}

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

Asuper

Bself

Cinit

Dconvenience

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' without '.init' causes errors.

Using 'super' in a convenience initializer of the same class.

Convenience initializers call the designated initializer using 'self.init'.

3fill in blank

hard

Fix the error in the initializer by completing the missing keyword.

Swift

class Animal {
    var species: String
    [1] init(species: String) {
        self.species = species
    }
}

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

Arequired

Bfinal

Coverride

Dconvenience

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'convenience' instead of 'required' changes initializer type.

Omitting the keyword when subclassing requires it.

The 'required' keyword ensures subclasses implement this initializer.

4fill in blank

hard

Fill both blanks to create a designated initializer with a default parameter value.

Swift

class Book {
    var title: String
    var pages: Int
    init(title: String, pages: [1] = [2]) {
        self.title = title
        self.pages = pages
    }
}

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

AInt

B100

CString

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using String type for pages causes type errors.

Omitting the default value or using zero when 100 is expected.

The parameter 'pages' is of type Int with a default value 100.

5fill in blank

hard

Fill all three blanks to complete the subclass initializer calling the superclass designated initializer.

Swift

class Employee: Person {
    var id: Int
    init(name: String, id: [1]) {
        self.id = id
        super.[2](name: [3])
    }
}

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

AString

Binit

Cname

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using String type for id causes errors.

Calling 'super' without '.init' is invalid.

Passing wrong argument name to superclass initializer.

The subclass initializer takes an Int id, calls 'super.init' with the name parameter.

Practice

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

easy

A. To fully initialize all properties of a class before use

B. To provide default values for optional properties only

C. To create a copy of an existing object

D. To deallocate memory when an object is no longer needed

Solution

1. Step 1: Understand what initializers do

   Initializers set starting values for new objects to ensure they are ready to use.

2. Step 2: Identify the role of designated initializers

   Designated initializers fully initialize all properties of a class, making the object safe and complete.

3. Final Answer:

   To fully initialize all properties of a class before use -> Option A

4. Quick Check:

   Designated initializer = full property initialization [OK]

Hint: Designated init always sets all properties fully [OK]

Common Mistakes:

• Confusing designated init with convenience init
• Thinking initializers copy objects
• Mixing initialization with deallocation

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

easy

A. func init() { }

B. init() { }

C. constructor() { }

D. initialize() { }

Solution

1. Step 1: Recall Swift initializer syntax

   In Swift, initializers use the keyword init without a return type or func keyword.

2. Step 2: Identify the correct syntax

   The correct syntax is init() { }. Other options use incorrect keywords or syntax.

3. Final Answer:

   init() { } -> Option B

4. Quick Check:

   Swift designated init syntax = init() { } [OK]

Hint: Swift init uses just 'init', no func or constructor [OK]

Common Mistakes:

• Using 'func' keyword with init
• Using 'constructor' or 'initialize' instead of 'init'
• Adding return types to init

3. Consider this Swift class:

class Person {
  var name: String
  var age: Int

  init(name: String) {
    self.name = name
    self.age = 0
  }
}

What will be the value of age after creating let p = Person(name: "Anna")?

medium

A. Name of the person

B. nil

C. Undefined

D. 0

Solution

1. Step 1: Analyze the initializer code

   The initializer sets self.name = name and explicitly sets self.age = 0.

2. Step 2: Determine the value of age after initialization

   Since age is set to 0 inside the initializer, the new object will have age = 0.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   Explicit age assignment = 0 [OK]

Hint: Check initializer assignments for property values [OK]

Common Mistakes:

• Assuming age is nil because not passed as parameter
• Thinking age is uninitialized or undefined
• Confusing name with age value

4. Identify the error in this Swift class initializer:

class Car {
  var model: String
  var year: Int

  init(model: String) {
    model = model
    year = 2020
  }
}

medium

A. Initializer missing return type

B. Class must inherit from NSObject

C. Properties are not initialized using self keyword

D. year property cannot be assigned a value

Solution

1. Step 1: Check property assignments in initializer

   The code assigns model = model and year = 2020 without self., so it assigns parameters to themselves, not properties.

2. Step 2: Understand correct property initialization

   Properties must be assigned with self.model = model and self.year = 2020 to set the instance variables.

3. Final Answer:

   Properties are not initialized using self keyword -> Option C

4. Quick Check:

   Use self.property = value to initialize [OK]

Hint: Use self. to assign properties inside init [OK]

Common Mistakes:

• Forgetting self. causes no property assignment
• Adding return type to init
• Thinking inheritance affects property assignment

5. You want to create a Swift class Book with properties title (String) and pages (Int). The designated initializer should allow setting both properties, but if pages is not provided, it defaults to 100. Which is the best way to write this initializer?

hard

A. init(title: String, pages: Int = 100) { self.title = title; self.pages = pages }

B. init(title: String) { self.title = title; self.pages = 100 }

C. init(title: String, pages: Int) { self.title = title }

D. init(title: String, pages: Int?) { self.title = title; self.pages = pages! }

Solution

1. Step 1: Understand default parameter values in Swift

   Swift allows default values in parameters, so pages: Int = 100 means pages defaults to 100 if not passed.

2. Step 2: Check initializer completeness and safety

   The initializer init(title: String, pages: Int = 100) { self.title = title; self.pages = pages } sets both properties properly and safely with a default value. The version with only a title parameter cannot set pages explicitly. The version taking both parameters but only assigning title misses the pages assignment. The version using an optional Int? for pages with force unwrap is unsafe.

3. Final Answer:

   init(title: String, pages: Int = 100) { self.title = title; self.pages = pages } -> Option A

4. Quick Check:

   Default parameter = safe default value [OK]

Hint: Use default parameter values for optional init args [OK]

Common Mistakes:

• Forgetting to assign all properties
• Using force unwrap (!) unsafely
• Not providing default value in parameter