Associated types in protocols in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of protocol with associated type and conforming struct

What is the output of this Swift code when run?

Swift

protocol Container {
    associatedtype Item
    var items: [Item] { get }
    func firstItem() -> Item?
}

struct IntContainer: Container {
    var items: [Int]
    func firstItem() -> Int? {
        return items.first
    }
}

let container = IntContainer(items: [10, 20, 30])
print(container.firstItem() ?? 0)

A 20

B 10

C nil

D Compilation error

💡 Hint

Look at what the firstItem() function returns from the items array.

The IntContainer struct conforms to Container with Item as Int. The firstItem() returns the first element of items, which is 10.

❓ Predict Output

intermediate

Output of generic function using protocol with associated type

What will be printed by this Swift code?

Swift

protocol Summable {
    associatedtype Number
    var values: [Number] { get }
    func sum() -> Number
}

struct IntSum: Summable {
    var values: [Int]
    func sum() -> Int {
        values.reduce(0, +)
    }
}

func printSum<T: Summable>(_ container: T) {
    print(container.sum())
}

let intSum = IntSum(values: [1, 2, 3, 4])
printSum(intSum)

A 0

B 1234

C Compilation error due to associatedtype

D 10

💡 Hint

The sum() function adds all elements in values.

The sum() method adds 1+2+3+4 = 10, so printSum prints 10.

🔧 Debug

advanced

Identify the error in protocol conformance with associated type

Why does this Swift code fail to compile?

Swift

protocol Identifiable {
    associatedtype ID
    var id: ID { get }
}

struct User: Identifiable {
    var id: String
    var name: String
}

struct Product: Identifiable {
    var id: Int
    var price: Double
}

func printID<T: Identifiable>(_ item: T) {
    print(item.id)
}

let user = User(id: "abc123", name: "Alice")
let product = Product(id: 101, price: 9.99)
printID(user)
printID(product)

A No error, code runs and prints ids

B Compilation error because associatedtype ID is not constrained to a specific type

C Compilation error because protocol Identifiable requires a method, not a property

D Runtime error due to type mismatch in printID

💡 Hint

Check if the protocol requirements are met by the structs.

Both User and Product conform to Identifiable by providing an id property of their own type. The generic function printID works for both types.

📝 Syntax

advanced

Which option correctly declares a protocol with an associated type and a method using it?

Select the correct Swift syntax for a protocol with an associated type named Element and a method that returns an Element.

A

protocol Container {
    associatedtype Element
    func getElement() -> Element?
}

B

protocol Container {
    typealias Element
    func getElement() -> Element
}

C

protocol Container {
    associatedtype Element
    func getElement() -> Element
}

D

protocol Container {
    associatedtype Element
    func getElement() -> Optional<Element>
}

💡 Hint

associatedtype declares a placeholder type in protocols.

Option C correctly uses associatedtype and a method returning Element. Option C uses typealias which is invalid here. Options C and D change the return type to optional, which is not what the prompt asks.

🚀 Application

expert

How many items are in the resulting array after filtering with associated type constraint?

Given this Swift code, how many elements does the filtered array contain after applying the filter?

Swift

protocol Filterable {
    associatedtype Item
    var items: [Item] { get }
    func filterItems(_ predicate: (Item) -> Bool) -> [Item]
}

struct StringFilter: Filterable {
    var items: [String]
    func filterItems(_ predicate: (String) -> Bool) -> [String] {
        items.filter(predicate)
    }
}

let filter = StringFilter(items: ["apple", "banana", "cherry", "date"])
let result = filter.filterItems { $0.count > 5 }
print(result.count)

A 2

B 3

C 4

D 1

💡 Hint

Count the strings with more than 5 characters.

Only "banana" and "cherry" have more than 5 characters, so the filtered array has 2 elements.