Sequence protocol for custom iteration in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom sequence iteration

What is the output of this Swift code that uses a custom sequence conforming to the Sequence protocol?

Swift

struct Countdown: Sequence {
    let start: Int
    func makeIterator() -> some IteratorProtocol {
        var current = start
        return AnyIterator {
            if current < 0 { return nil }
            defer { current -= 1 }
            return current
        }
    }
}

let countdown = Countdown(start: 3)
for number in countdown {
    print(number, terminator: " ")
}

A 3 2 1

B 0 1 2 3

C 3 2 1 0

D 3 2 1 0 -1

💡 Hint

Look at how the iterator decreases the current value and stops when it goes below zero.

The iterator starts at 3 and yields values down to 0. When current becomes -1, it returns nil to stop iteration.

🧠 Conceptual

intermediate

Understanding Sequence protocol requirements

Which of the following is a required method to implement when conforming to the Sequence protocol in Swift?

A func sequence() -> [Element]

B func next() -> Element?

C func start() -> Element

D func makeIterator() -> some IteratorProtocol

💡 Hint

Sequence requires a method that returns an iterator.

The Sequence protocol requires the makeIterator() method that returns an iterator conforming to IteratorProtocol.

🔧 Debug

advanced

Identify the error in custom iterator implementation

What error will this Swift code produce when trying to iterate over CustomSequence?

Swift

struct CustomSequence: Sequence {
    let values: [Int]
    func makeIterator() -> some IteratorProtocol {
        var index = 0
        return AnyIterator {
            if index >= values.count {
                return nil
            }
            let val = values[index]
            index += 1
            return val
        }
    }
}

let seq = CustomSequence(values: [1, 2, 3])
for v in seq {
    print(v)
}

A No error, prints 1 2 3

B Runtime error: Index out of range

C Compile-time error: Cannot use 'values' inside closure because it is not captured correctly

D Compile-time error: Missing return type in makeIterator()

💡 Hint

The code compiles and runs without errors. Local mutable variables and struct properties can both be captured correctly by escaping closures.

There is no compile-time or runtime error. The closure captures the mutable local variable 'index' and accesses 'values' from a copy of 'self' (immutable since 'let'). It correctly prints 1 2 3.

📝 Syntax

advanced

Correct syntax for custom iterator

Which option shows the correct syntax for a custom iterator conforming to IteratorProtocol in Swift?

A

struct MyIterator: IteratorProtocol {
    var current = 0
    func next() -> Int {
        current += 1
        return current
    }
}

B

struct MyIterator: IteratorProtocol {
    var current = 0
    mutating func next() -> Int? {
        defer { current += 1 }
        return current < 3 ? current : nil
    }
}

C

struct MyIterator: IteratorProtocol {
    var current = 0
    mutating func next() -> Int {
        if current < 3 {
            current += 1
            return current
        }
        return nil
    }
}

D

struct MyIterator: IteratorProtocol {
    var current = 0
    mutating func next() -> Int? {
        if current > 3 {
            return nil
        }
        current += 1
        return current
    }
}

💡 Hint

Remember next() must be mutating and return an optional Element.

Option B correctly implements next() as mutating and returns Int? with proper logic. Others either miss optional return or have logic errors.

🚀 Application

expert

Number of elements iterated in custom sequence

Given this Swift custom sequence, how many elements will be printed when iterating over it?

Swift

struct EvenNumbers: Sequence {
    let max: Int
    func makeIterator() -> some IteratorProtocol {
        var current = 0
        return AnyIterator {
            if current > max {
                return nil
            }
            let result = current
            current += 2
            return result
        }
    }
}

let evens = EvenNumbers(max: 5)
var count = 0
for _ in evens {
    count += 1
}
print(count)

A 3

B 2

C 4

D 5

💡 Hint

Count the even numbers from 0 up to 5 inclusive.

The sequence yields 0, 2, 4 and stops before 6 because 6 > 5. So 3 elements are printed.