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.