Extension syntax in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an extension for the String type.

Swift

extension String [1] {
    func greet() {
        print("Hello, \(self)!")
    }
}

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

A(

B{

C[

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or brackets instead of curly braces for the extension body.

In Swift, extensions are declared using the 'extension' keyword followed by the type name and then a pair of curly braces '{ }' to contain the new methods or properties.

2fill in blank

medium

Complete the code to add a computed property 'isLong' to String that returns true if the string length is greater than 10.

Swift

extension String {
    var isLong: Bool [1] {
        return self.count > 10
    }
}

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

Aget()

Bset

Cget

Dset()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'get' for a read-only computed property.

Adding parentheses after 'get'.

Computed properties in Swift use 'get' to define the getter block without parentheses.

3fill in blank

hard

Fix the error in the extension by completing the method signature correctly.

Swift

extension Int {
    func squared() -> Int [1] {
        return self * self
    }
}

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

A{

B(

C[

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or brackets instead of curly braces after the method signature.

Function bodies in Swift are enclosed in curly braces '{ }'. The method signature must be followed by '{' to start the body.

4fill in blank

hard

Fill both blanks to add a mutating method 'increment' to Int that increases its value by 1.

Swift

extension Int {
    mutating func increment() [1] [2] self += 1
}

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

A{

B}

C-> Void

Din

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting curly braces or using '-> Void' incorrectly.

Using 'in' which is for closures, not method bodies.

Mutating methods in Swift structs must have their body enclosed in curly braces '{' and '}'. The '-> Void' is optional and 'in' is used in closures, not here.

5fill in blank

hard

Fill all three blanks to add a static method 'description' to Double that returns a string describing the type.

Swift

extension Double {
    static func description() [1] String [2] {
        return "Double type"
    [3]

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

A->

B{

C}

Din

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the return arrow '->'.

Misplacing or missing curly braces.

Using 'in' which is for closures.

Function return types are indicated with '->'. The method body starts with '{' and ends with '}'. 'in' is not used here.

Practice

1. What is the main purpose of using extension in Swift?

easy

A. To add new functionality to an existing type without modifying its original code

B. To create a new subclass from an existing class

C. To replace the original implementation of a type

D. To delete methods from a type

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new features to existing types without changing their original source code.

2. Step 2: Compare options with this definition

   Only adding new functionality to an existing type without modifying its original code correctly describes this purpose. Replacing original implementation refers to overriding, creating subclasses is inheritance, and deleting methods is unrelated to extensions.

3. Final Answer:

   To add new functionality to an existing type without modifying its original code -> Option A

4. Quick Check:

   Extension purpose = Add features without changing original code [OK]

Hint: Extensions add features without changing original code [OK]

Common Mistakes:

• Confusing extensions with subclassing
• Thinking extensions replace original code
• Believing extensions can remove methods

2. Which of the following is the correct syntax to start an extension for the String type in Swift?

easy

A. extend String { }

B. extension String { }

C. extension String() { }

D. extension String[] { }

Solution

1. Step 1: Recall Swift extension syntax

   The correct syntax starts with the keyword extension followed by the type name without parentheses or brackets.

2. Step 2: Check each option

   extension String { } uses correct syntax. extend String { } uses invalid keyword. extension String() { } adds invalid parentheses. extension String[] { } uses invalid array notation.

3. Final Answer:

   extension String { } -> Option B

4. Quick Check:

   Extension syntax = extension TypeName { } [OK]

Hint: Use 'extension TypeName { }' without parentheses or brackets [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Adding parentheses after type name
• Using brackets after type name

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

extension Int {
    func squared() -> Int {
        return self * self
    }
}

let number = 4
print(number.squared())

medium

A. 16

B. 8

C. 4

D. Compile error

Solution

1. Step 1: Understand the extension method

   The extension adds a method squared() to Int that returns the number multiplied by itself.

2. Step 2: Calculate the output for number = 4

   Calling 4.squared() returns 4 * 4 = 16.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply the number by itself in the extension method [OK]

Common Mistakes:

• Confusing squared with doubling
• Expecting syntax error due to extension
• Forgetting to return the value

4. Identify the error in this Swift extension code:

extension Double {
    func half() -> Double {
        self / 2
    }
}

medium

A. Missing return keyword in the method

B. Extension cannot add methods to Double

C. Method name 'half' is invalid

D. No error, code is correct

Solution

1. Step 1: Check method syntax in extension

   In Swift, single-expression functions can omit the return keyword.

2. Step 2: Verify if extension on Double is allowed

   Extensions can add methods to Double. The method name 'half' is valid.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Single-expression methods can omit return [OK]

Hint: Single-expression methods can omit 'return' keyword [OK]

Common Mistakes:

• Assuming 'return' is always required
• Thinking extensions can't add methods to built-in types
• Believing method names have restrictions

5. You want to add a computed property isEven to Int using an extension that returns true if the number is even, otherwise false. Which code correctly implements this?

hard

A. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } }

B. extension Int { func isEven() -> Bool { return self % 2 == 0 } }

C. extension Int { var isEven: Bool { self % 2 == 0 } }

D. extension Int { var isEven: Bool { self / 2 == 0 } }

Solution

1. Step 1: Understand computed property syntax

   Computed properties use var with a getter. extension Int { var isEven: Bool { self % 2 == 0 } } uses correct concise syntax; the func version declares a method, the if version lacks an else branch causing compile error, the division version has wrong logic.

2. Step 2: Check logic correctness

   extension Int { var isEven: Bool { self % 2 == 0 } } uses concise expression self % 2 == 0 which returns true if even. extension Int { var isEven: Bool { if self % 2 == 0 { return true } } } omits else branch, causing compiler error. extension Int { var isEven: Bool { self / 2 == 0 } } incorrectly uses division instead of modulo.

3. Step 3: Verify distractors

   The method requires parentheses to call like num.isEven(). Incomplete if fails to compile. Division fails for even numbers like 2 (2/2 == 1 != 0).

4. Final Answer:

   extension Int { var isEven: Bool { self % 2 == 0 } } -> Option C

5. Quick Check:

   Computed property with modulo check = extension Int { var isEven: Bool { self % 2 == 0 } } [OK]

Hint: Use computed property with 'self % 2 == 0' for even check [OK]

Common Mistakes:

• Using division instead of modulo
• Confusing method with computed property
• Forgetting to return a Bool value