Convenience initializers in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a convenience initializer that sets a default name.

Swift

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

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

A"Unknown"

BUnknown

Cname

Dself.name

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use quotes around the string.

Trying to assign directly to self.name in the convenience initializer.

The convenience initializer calls the designated initializer with a default string "Unknown" as the name.

2fill in blank

medium

Complete the convenience initializer to set both name and age with default values.

Swift

class Person {
    var name: String
    var age: Int
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
    convenience init() {
        self.init(name: [1], age: 0)
    }
}

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

Aname

BNoName

C"NoName"

Dself.name

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a variable name instead of a string literal.

Omitting quotes around the string.

The convenience initializer calls the designated initializer with a default name "NoName" and age 0.

3fill in blank

hard

Fix the error in the convenience initializer call to the designated initializer.

Swift

class Rectangle {
    var width: Double
    var height: Double
    init(width: Double, height: Double) {
        self.width = width
        self.height = height
    }
    convenience init(size: Double) {
        self.init(width: [1], height: size)
    }
}

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

Asize

Bself.size

Cwidth

Dheight

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self.size' which is not defined.

Using 'width' which is not a parameter here.

The convenience initializer uses the parameter 'size' for both width and height by passing it directly.

4fill in blank

hard

Fill both blanks to complete the convenience initializer that calls the designated initializer with default values.

Swift

class Car {
    var make: String
    var year: Int
    init(make: String, year: Int) {
        self.make = make
        self.year = year
    }
    convenience init() {
        self.init(make: [1], year: [2])
    }
}

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

A"Toyota"

B2020

C2010

D"Honda"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string without quotes for make.

Using a year value that is not the default.

The convenience initializer calls the designated initializer with default make "Toyota" and year 2020.

5fill in blank

hard

Fill all three blanks to create a convenience initializer that sets default values and calls the designated initializer.

Swift

class Book {
    var title: String
    var author: String
    var pages: Int
    init(title: String, author: String, pages: Int) {
        self.title = title
        self.author = author
        self.pages = pages
    }
    convenience init() {
        self.init(title: [1], author: [2], pages: [3])
    }
}

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

A"Untitled"

B"Unknown"

C100

D"Anonymous"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up author and title values.

Using strings without quotes.

The convenience initializer calls the designated initializer with default title "Untitled", author "Unknown", and pages 100.

Practice

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

easy

A. To provide an easier way to create an object by calling a designated initializer

B. To override a superclass initializer

C. To create a completely new object without calling any other initializer

D. To deallocate an object before it is destroyed

Solution

1. Step 1: Understand the role of convenience initializers

   Convenience initializers are shortcuts to create objects more easily by calling a designated initializer.

2. Step 2: Compare options with this role

   Only To provide an easier way to create an object by calling a designated initializer describes this purpose correctly; others describe unrelated concepts.

3. Final Answer:

   To provide an easier way to create an object by calling a designated initializer -> Option A

4. Quick Check:

   Convenience initializer = shortcut calling designated initializer [OK]

Hint: Convenience initializers always call a designated initializer [OK]

Common Mistakes:

• Thinking convenience initializers create objects independently
• Confusing convenience initializers with deinitializers
• Believing convenience initializers override superclass initializers

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

easy

A. convenience func init(name: String) { self.init() }

B. init convenience(name: String) { self.init() }

C. func convenience init(name: String) { self.init() }

D. convenience init(name: String) { self.init() }

Solution

1. Step 1: Recall the correct syntax for convenience initializers

   In Swift, the keyword convenience comes before init without any function keyword.

2. Step 2: Check each option

   convenience init(name: String) { self.init() } matches the correct syntax. Options B, C, and D misuse keywords or order.

3. Final Answer:

   convenience init(name: String) { self.init() } -> Option D

4. Quick Check:

   Keyword order: convenience before init [OK]

Hint: Remember: 'convenience' keyword comes before 'init' [OK]

Common Mistakes:

• Placing 'convenience' after 'init'
• Using 'func' keyword with initializers
• Incorrect keyword order causing syntax errors

3. Consider the following Swift code:

class Car {
    var color: String
    init(color: String) {
        self.color = color
    }
    convenience init() {
        self.init(color: "Red")
    }
}
let myCar = Car()

What is the value of myCar.color?

medium

A. nil

B. "Red"

C. "" (empty string)

D. Compilation error

Solution

1. Step 1: Understand the initializers in the Car class

   The designated initializer sets the color property. The convenience initializer calls the designated one with "Red".

2. Step 2: Analyze the object creation

   Creating myCar with Car() calls the convenience initializer, which sets color to "Red".

3. Final Answer:

   "Red" -> Option B

4. Quick Check:

   Convenience init sets color to "Red" [OK]

Hint: Convenience init calls designated init with default values [OK]

Common Mistakes:

• Assuming color is empty or nil without initialization
• Thinking convenience init does not call designated init
• Expecting compilation error due to missing default init

4. Identify the error in this Swift class using a convenience initializer:

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

medium

A. Convenience initializer must call a designated initializer, not assign properties directly

B. Designated initializer must be marked as convenience

C. Property 'name' must be optional

D. No error, code is correct

Solution

1. Step 1: Recall rules for convenience initializers

   A convenience initializer must call another initializer in the same class, not assign properties directly.

2. Step 2: Check the convenience init implementation

   Here, self.name = "Unknown" is assigned directly, which is not allowed.

3. Final Answer:

   Convenience initializer must call a designated initializer, not assign properties directly -> Option A

4. Quick Check:

   Convenience init must call designated init [OK]

Hint: Convenience init cannot set properties directly; must call designated init [OK]

Common Mistakes:

• Assigning properties directly in convenience initializer
• Marking designated initializer as convenience
• Making properties optional unnecessarily

5. You want to create a Swift class Rectangle with a designated initializer that takes width and height, and a convenience initializer that creates a square by taking only one side length. Which of the following implementations correctly uses a convenience initializer?

hard

A. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.width = side
self.height = side
}
}

B. class Rectangle {
var width: Double
var height: Double
convenience init(width: Double, height: Double) {
self.width = width
self.height = height
}
init(side: Double) {
self.width = side
self.height = side
}
}

C. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.init(width: side, height: side)
}
}

D. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
convenience init(side: Double) {
self.height = side
}
}

Solution

1. Step 1: Identify designated and convenience initializers

   The designated initializer takes width and height. The convenience initializer should call the designated one.

2. Step 2: Check each option for correct usage

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } convenience init(side: Double) { self.init(width: side, height: side) } } correctly defines the designated init and the convenience init calls it with equal sides. Others either misuse convenience keyword or assign properties directly.

3. Final Answer:

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } convenience init(side: Double) { self.init(width: side, height: side) } } -> Option C

4. Quick Check:

   Convenience init calls designated init with side values [OK]

Hint: Convenience init must call designated init with all required parameters [OK]

Common Mistakes:

• Marking designated initializer as convenience
• Assigning properties directly in convenience initializer
• Not calling designated initializer from convenience initializer