Extensions with constraints in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension with protocol constraint

What is the output of this Swift code?

Swift

protocol Describable {
    func describe() -> String
}

struct Item: Describable {
    var name: String
    func describe() -> String {
        return "Item: \(name)"
    }
}

extension Array where Element: Describable {
    func allDescriptions() -> [String] {
        self.map { $0.describe() }
    }
}

let items = [Item(name: "Pen"), Item(name: "Book")]
print(items.allDescriptions())

A Compilation error: 'describe()' not found

B ["Pen", "Book"]

C ["Item: Pen", "Item: Book"]

D Runtime error: Cannot convert value

💡 Hint

Check how the extension applies only when Element conforms to Describable.

The extension adds allDescriptions() only to arrays whose elements conform to Describable. Since Item conforms, calling allDescriptions() returns an array of descriptions.

❓ Predict Output

intermediate

Extension with class constraint output

What will this Swift code print?

Swift

class Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
}

class Car: Vehicle {}

extension Array where Element: Vehicle {
    func totalWheels() -> Int {
        self.reduce(0) { $0 + $1.wheels }
    }
}

let vehicles: [Vehicle] = [Car(wheels: 4), Vehicle(wheels: 2)]
print(vehicles.totalWheels())

A Compilation error: Cannot extend Array with class constraint

B 6

C Runtime error: Method not found

D 4

💡 Hint

Remember the extension applies to arrays of Vehicle or subclasses.

The extension adds totalWheels() to arrays of Vehicle or subclasses. It sums wheels of all elements: 4 + 2 = 6.

🔧 Debug

advanced

Why does this extension not compile?

Why does this Swift extension cause a compilation error?

Swift

extension Array where Element: Equatable & Comparable {
    func isSortedAndUnique() -> Bool {
        for i in 1..<self.count {
            if self[i] <= self[i-1] {
                return false
            }
        }
        return true
    }
}

A Swift does not allow multiple protocol constraints with '&' in extensions

B The syntax for multiple constraints should use a where clause with commas, not '&'

C The extension requires Element to be a class, but protocols are used

D The operator '<=' is not defined for Element

💡 Hint

Check if the operator used is guaranteed by the protocols.

The error is because Element conforms to Equatable and Comparable, but Comparable requires < and ==, not necessarily <=. The operator <= is not guaranteed.

📝 Syntax

advanced

Correct syntax for extension with generic constraint

Which option shows the correct syntax to extend Array only when its elements conform to Hashable?

A extension Array where Element: Hashable {}

B extension Array where Element == Hashable {}

C extension Array<Element> where Element: Hashable {}

D extension Array<Element: Hashable> {}

💡 Hint

Remember the syntax for constrained extensions uses 'where'.

The correct syntax is extension Array where Element: Hashable {}. Option A and C are invalid syntax. Option A uses '==' which means equality, not conformance.

🚀 Application

expert

Implement extension with multiple constraints

Given the following code, which option correctly implements an extension on Array where Element is both Numeric and Comparable, adding a method maxValue() that returns the maximum element?

Swift

extension Array where Element: Numeric & Comparable {
    func maxValue() -> Element? {
        guard var maxElem = self.first else { return nil }
        for elem in self {
            if elem > maxElem {
                maxElem = elem
            }
        }
        return maxElem
    }
}

A

extension Array where Element: Numeric & Comparable {
    func maxValue() -> Element? {
        self.max()
    }
}

B

extension Array where Element: Numeric, Comparable {
    func maxValue() -> Element? {
        self.max()
    }
}

C

extension Array<Element> where Element: Numeric & Comparable {
    func maxValue() -> Element? {
        self.max()
    }
}

D

extension Array where Element == Numeric & Comparable {
    func maxValue() -> Element? {
        self.max()
    }
}

💡 Hint

Check the correct syntax for multiple protocol constraints in extensions.

Option A correctly uses where Element: Numeric & Comparable to constrain the extension. Option A uses a comma which is invalid syntax. Option A incorrectly uses generic parameter syntax. Option A uses '==' which means equality, not conformance.