Extending built-in types in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extended String method

What is the output of this Swift code that extends the String type with a method to reverse words?

Swift

extension String {
    func reversedWords() -> String {
        self.split(separator: " ").reversed().joined(separator: " ")
    }
}

let phrase = "hello world from Swift"
print(phrase.reversedWords())

A "Swift from world hello"

B "olleh dlrow morf tfiwS"

C "hello world from Swift"

D "Swift from world"

💡 Hint

Think about what the reversedWords method does to the words in the string.

The extension splits the string into words, reverses the order of the words, then joins them back with spaces. So the output reverses the word order, not the letters.

❓ Predict Output

intermediate

Output of extended Array property

What will this Swift code print when extending Array to add a property that returns the middle element?

Swift

extension Array {
    var middle: String? {
        guard !isEmpty else { return nil }
        return "\(self[count / 2])"
    }
}

let numbers = [10, 20, 30, 40, 50]
print(numbers.middle ?? "No middle")

A No middle

B 20

C 40

D 30

💡 Hint

Remember that integer division truncates the decimal part.

The array has 5 elements, so count is 5. count / 2 is 2, so the element at index 2 is 30.

🔧 Debug

advanced

Why does this extension cause a compile error?

This Swift code tries to add a method to Int to check if it is even. Why does it cause a compile error?

Swift

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

let number = 4
print(number.isEven)

A Because isEven is a method but called without parentheses

B Because Int cannot be extended

C Because the modulo operator % is invalid for Int

D Because the method must be static

💡 Hint

Check how methods are called in Swift.

The method isEven() requires parentheses when called. Using number.isEven without () causes a compile error.

🧠 Conceptual

advanced

Effect of extending built-in types with computed properties

What happens if you add a computed property with the same name as an existing property in a Swift built-in type extension?

A The original property is used, ignoring the extension

B The extension property overrides the original property

C The compiler throws an error about duplicate definitions

D The program crashes at runtime

💡 Hint

Think about how Swift handles naming conflicts in extensions.

Swift does not allow extensions to add properties or methods that conflict with existing ones. This causes a compile-time error.

❓ Predict Output

expert

Output of extended Dictionary method with generic constraint

What is the output of this Swift code that extends Dictionary only when its Value type is Int, adding a method to sum all values?

Swift

extension Dictionary where Value == Int {
    func sumValues() -> Int {
        self.values.reduce(0, +)
    }
}

let scores = ["Alice": 10, "Bob": 20, "Carol": 15]
print(scores.sumValues())

A nil

B 45

C 0

D Compilation error

💡 Hint

Check how reduce works on the values collection.

The extension applies only when Value is Int. The sumValues method sums all Int values in the dictionary, so 10 + 20 + 15 = 45.

Practice

1. What is the main purpose of extending built-in types in Swift?

easy

A. To change the stored properties of existing types

B. To delete existing methods from built-in types

C. To add new methods or computed properties to existing types

D. To create subclasses of built-in types

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new methods or computed properties to existing types without subclassing.

2. Step 2: Identify what extensions cannot do

   Extensions cannot add stored properties or delete existing methods.

3. Final Answer:

   To add new methods or computed properties to existing types -> Option C

4. Quick Check:

   Extensions add methods/properties = B [OK]

Hint: Remember: extensions add features, not stored data [OK]

Common Mistakes:

• Thinking extensions can add stored properties
• Confusing extensions with subclassing
• Believing extensions can remove methods

2. Which of the following is the correct syntax to extend the built-in Int type with a method isEven() that returns a Bool?

easy

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

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

C. extension Int { func isEven { return self % 2 == 0 } }

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

Solution

1. Step 1: Identify the correct keyword for extension

   Swift uses the keyword extension to add methods to existing types.

2. Step 2: Check method syntax

   The method must have parentheses and a return type: func isEven() -> Bool. The body uses self % 2 == 0.

3. Final Answer:

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

4. Quick Check:

   Correct extension syntax = D [OK]

Hint: Use 'extension' keyword and full method signature [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Omitting parentheses in method declaration
• Missing 'func' keyword before method name

3. Consider this Swift code:

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let word = "hello"
print(word.reversedString())

What is the output when this code runs?

medium

A. Error: reversed() not found

B. hello

C. nil

D. olleh

Solution

1. Step 1: Understand the extension method

   The method reversedString() returns a new String made by reversing the characters of the original string.

2. Step 2: Apply method to 'hello'

   Reversing "hello" results in "olleh".

3. Final Answer:

   olleh -> Option D

4. Quick Check:

   String reversed = olleh [OK]

Hint: Remember reversed() returns characters reversed [OK]

Common Mistakes:

• Expecting original string output
• Thinking reversed() returns an error
• Confusing reversed() with reverse()

4. What is wrong with this Swift extension code?

extension Double {
    var squared: Double {
        return self * self
    }

    var storedValue: Double = 0.0
}

medium

A. Extensions cannot add stored properties like 'storedValue'

B. Computed properties must use 'func' keyword

C. The 'squared' property must be a method, not a property

D. The extension must be declared as 'class extension'

Solution

1. Step 1: Review extension property rules

   Extensions can add computed properties but cannot add stored properties.

2. Step 2: Identify the stored property

   The line var storedValue: Double = 0.0 tries to add a stored property, which is not allowed.

3. Final Answer:

   Extensions cannot add stored properties like 'storedValue' -> Option A

4. Quick Check:

   No stored properties in extensions = A [OK]

Hint: Remember: only computed properties allowed in extensions [OK]

Common Mistakes:

• Trying to add stored properties in extensions
• Confusing computed properties with stored properties
• Thinking extensions require 'class' keyword

5. You want to extend the built-in Array type in Swift to add a method secondElement() that returns the second element if it exists, or nil otherwise. Which of the following extensions correctly implements this?

hard

A. extension Array {
func secondElement() -> Element {
return self[1]
}
}

B. extension Array {
func secondElement() -> Element? {
return self.count > 1 ? self[1] : nil
}
}

C. extension Array {
var secondElement: Element? {
if self.count >= 2 { return self[2] } else { return nil }
}
}

D. extension Array {
func secondElement() -> Element? {
if self.count > 0 { return self[1] } else { return nil }
}
}

Solution

1. Step 1: Check method signature and return type

   The method should return an optional Element (Element?) because the second element might not exist.

2. Step 2: Verify index and condition

   Arrays are zero-indexed, so the second element is at index 1. The condition should check if count > 1 to safely access index 1.

3. Step 3: Compare options

   The correct one uses ternary operator: return self.count > 1 ? self[1] : nil. The non-optional return crashes on short arrays. The computed property uses index 2 (third element). The if-else checks count > 0, allowing index 1 access when count == 1, causing runtime crash.

4. Final Answer:

   extension Array { func secondElement() -> Element? { return self.count > 1 ? self[1] : nil } } -> Option B

5. Quick Check:

   Safe optional return with index 1 = A [OK]

Hint: Check array count > 1 and return optional Element [OK]

Common Mistakes:

• Accessing wrong index (like 2 instead of 1)
• Returning non-optional Element causing crash
• Using computed property but with wrong index