Why extensions add capability without modifying in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a method to the String type using an extension.

Swift

extension String {
    func shout() -> String {
        return self.[1]uppercased()
    }
}

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

A.

B->

C:

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '->' or ':' instead of '.' to call a method.

In Swift, to call a method on self, you use the dot . operator.

2fill in blank

medium

Complete the code to add a computed property to Int using an extension.

Swift

extension Int {
    var isEven: Bool {
        return self [1] 2 == 0
    }
}

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

A*

B/

C%

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' which does division, not remainder.

The modulo operator % gives the remainder. If remainder is 0 when divided by 2, the number is even.

3fill in blank

hard

Fix the error in the extension that adds a method to Double to square the value.

Swift

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

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' or '-' which add or subtract instead of multiply.

To square a number, multiply it by itself using the * operator.

4fill in blank

hard

Fill both blanks to create an extension that adds a method to Array of Int to sum only positive numbers.

Swift

extension Array where Element == Int {
    func sumPositive() -> Int {
        return self.filter { $0 [1] 0 }.reduce(0, [2])
    }
}

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

A>

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' or '*' in reduce which changes the sum incorrectly.

Filter selects elements greater than 0, then reduce sums them using '+'.

5fill in blank

hard

Fill all three blanks to create an extension that adds a method to Dictionary to get keys with values above a threshold.

Swift

extension Dictionary where Value: Comparable {
    func keysAbove(_ threshold: Value) -> [Key] {
        return self.filter { $0.value [1] threshold }.map { [2].[3] }
    }
}

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

A>

B$0

Ckey

Dkeys

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'keys' property which returns all keys, not filtered ones.

Filter keys where value is greater than threshold, then map to get the keys.

Practice

1. Why do Swift extensions add capability without modifying the original type?

easy

A. Because they require subclassing the original type

B. Because they add new features without changing existing code

C. Because they delete old methods before adding new ones

D. Because they replace the original type completely

Solution

1. Step 1: Understand what extensions do

   Extensions add new methods or properties to existing types without altering the original source code.

2. Step 2: Recognize the benefit

   This allows adding features safely without changing or breaking existing code.

3. Final Answer:

   Because they add new features without changing existing code -> Option B

4. Quick Check:

   Extensions add features without modification = D [OK]

Hint: Extensions add, not change, original code [OK]

Common Mistakes:

• Thinking extensions replace original types
• Believing extensions delete old methods
• Confusing extensions with subclassing

2. Which of the following is the correct syntax to add a method using an extension in Swift?

easy

A. extension MyType { func newMethod() { } }

B. extend MyType { func newMethod() { } }

C. extension MyType: { func newMethod() { } }

D. extension MyType func newMethod() { }

Solution

1. Step 1: Recall Swift extension syntax

   Extensions start with the keyword 'extension' followed by the type name and braces enclosing new members.

2. Step 2: Check each option

   extension MyType { func newMethod() { } } matches the correct syntax: 'extension MyType { func newMethod() { } }'. Others have syntax errors.

3. Final Answer:

   extension MyType { func newMethod() { } } -> Option A

4. Quick Check:

   Correct extension syntax = A [OK]

Hint: Remember: 'extension TypeName { ... }' [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Adding colon after type name
• Omitting braces around methods

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

struct Person { var name: String }

extension Person {
    func greet() -> String {
        return "Hello, \(name)!"
    }
}

let p = Person(name: "Anna")
print(p.greet())

medium

A. Hello, Anna!

B. Hello, !

C. Error: greet() not found

D. Hello, name!

Solution

1. Step 1: Understand the extension adds greet() method

   The extension adds a greet() method to Person that returns "Hello, \(name)!" using the instance's name.

2. Step 2: Check the instance and method call

   Creating Person(name: "Anna") and calling greet() returns "Hello, Anna!".

3. Final Answer:

   Hello, Anna! -> Option A

4. Quick Check:

   Extension method works, output = Hello, Anna! [OK]

Hint: Extension methods use instance data normally [OK]

Common Mistakes:

• Assuming extension methods can't access properties
• Expecting compile error for extension method
• Confusing string interpolation syntax

4. Identify the error in this Swift extension code:

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

let result = 5.squared
print(result)

medium

A. Cannot extend Int type

B. squared() must be static method

C. self keyword not allowed in extensions

D. Missing parentheses when calling squared()

Solution

1. Step 1: Check method call syntax

   The method squared() is defined correctly, but calling 5.squared misses parentheses.

2. Step 2: Understand method call requirements

   In Swift, methods must be called with parentheses even if no parameters, so 5.squared() is correct.

3. Final Answer:

   Missing parentheses when calling squared() -> Option D

4. Quick Check:

   Method call needs () = B [OK]

Hint: Call methods with () even if no parameters [OK]

Common Mistakes:

• Calling methods without parentheses
• Thinking Int cannot be extended
• Believing self is disallowed in extensions

5. You want to add a computed property isEven to Int using an extension that returns true if the number is even. Which code correctly achieves this without modifying the original Int type?

hard

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

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

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

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

Solution

1. Step 1: Understand computed property syntax

   Computed properties use 'var name: Type { expression }' without parentheses.

2. Step 2: Check each option

   extension Int { var isEven: Bool { self % 2 == 0 } } correctly defines a computed property isEven returning true if self % 2 == 0. extension Int { func isEven() -> Bool { self % 2 == 0 } } uses a method, not a property. extension Int { static var isEven: Bool { self % 2 == 0 } } incorrectly uses static. extension Int { var isEven: Bool { return self / 2 == 0 } } uses wrong logic (division instead of modulo).

3. Final Answer:

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

4. Quick Check:

   Computed property with modulo check = A [OK]

Hint: Computed properties use var without parentheses [OK]

Common Mistakes:

• Using func instead of var for computed property
• Making property static when it should be instance
• Using division instead of modulo for even check