Sequence protocol for custom iteration in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the class conform to the Sequence protocol.

Swift

class Countdown: Sequence {
    let start: Int
    init(start: Int) {
        self.start = start
    }
    func makeIterator() -> [1] {
        return CountdownIterator(current: start)
    }
}

class CountdownIterator: IteratorProtocol {
    var current: Int
    init(current: Int) {
        self.current = current
    }
    func next() -> Int? {
        if current <= 0 {
            return nil
        } else {
            defer { current -= 1 }
            return current
        }
    }
}

Drag options to blanks, or click blank then click option'

AIteratorProtocol

BSequence

CCountdownIterator

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the wrong type from makeIterator(), like Int or Sequence.

Forgetting to implement makeIterator() method.

The makeIterator() method must return an instance of the iterator type, here CountdownIterator.

2fill in blank

medium

Complete the code to use the custom sequence in a for-in loop.

Swift

let countdown = Countdown(start: 3)
for number in [1] {
    print(number)
}

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Acountdown

BCountdownIterator()

CCountdown()

DCountdown.makeIterator()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to iterate over the iterator directly instead of the sequence.

Calling makeIterator() explicitly in the for-in loop.

You iterate over the instance of the Sequence, here 'countdown'.

3fill in blank

hard

Fix the error in the next() method to correctly stop iteration.

Swift

func next() -> Int? {
    if current [1] 0 {
        return nil
    } else {
        defer { current -= 1 }
        return current
    }
}

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A<=

B==

C>=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or != causes the loop to never end or behave incorrectly.

Using == only stops at exactly zero, missing negative values.

The iteration should stop when current is less than or equal to zero.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps numbers to their squares for numbers greater than 2.

Swift

let squares = Dictionary(uniqueKeysWithValues: (3...6).map { number in
    (number, number [1] number) 
}).filter { $0.key [2] 2 }

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A*

B>

C+

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * for squaring.

Using < instead of > for filtering.

Use * to square the number and > to filter numbers greater than 2.

5fill in blank

hard

Fill all three blanks to create a dictionary from a sequence where keys are strings uppercased and values are filtered to be greater than zero.

Swift

let data = ["a": 1, "b": -1, "c": 3]
let filtered = data.reduce(into: [:]) { result, pair in
    let (key, value) = pair
    if value [1] 0 {
        result[[2]] = [3]
    }
}

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A>

Bkey.uppercased()

Cvalue

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for filtering.

Using key instead of key.uppercased() for dictionary keys.

Assigning key instead of value to the dictionary.

Use > to filter positive values, uppercase the key for dictionary keys, and assign the value.