Why Sequence protocol for custom iteration in Swift? - Purpose & Use Cases

The Big Idea

What if you could make your own collections that just work with Swift's loops, no extra hassle?

The Scenario

Imagine you have a list of items and you want to go through each one, but the way you want to go through them is special -- maybe skipping some, or combining steps. Doing this by hand means writing lots of loops and checks everywhere.

The Problem

Manually writing loops for every special way to go through items is slow and tiring. It's easy to make mistakes, forget to update all loops if the rules change, and your code becomes messy and hard to read.

The Solution

The Sequence protocol lets you define exactly how to go through your items once, in one place. Then you can use simple loops everywhere else, and your special rules happen automatically and cleanly.

Before vs After

✗ Before

var i = 0
while i < items.count {
  if shouldInclude(items[i]) {
    print(items[i])
  }
  i += 1
}

✓ After

struct CustomSequence: Sequence {
  func makeIterator() -> some IteratorProtocol {
    // custom iterator logic here
    return CustomIterator()
  }
}

struct CustomIterator: IteratorProtocol {
  mutating func next() -> ItemType? {
    // custom iteration logic here
  }
}

for item in CustomSequence() {
  print(item)
}

What It Enables

You can create your own easy-to-use collections that work naturally with Swift's powerful loops and functions.

Real Life Example

Think of a music playlist app that skips songs you dislike automatically when you play through the list. Using Sequence, the app can handle this skipping inside the playlist itself, so the player just plays songs one by one without extra checks.

Key Takeaways

Manual loops for custom steps are slow and error-prone.

Sequence protocol lets you define iteration once, cleanly.

Use simple loops everywhere else with your custom rules built-in.

Practice

1. What does the Sequence protocol in Swift require you to provide for custom iteration?

easy

A. A property that stores all elements in an array

B. A method named start() that begins the iteration

C. A function that returns the count of elements

D. An iterator with a next() method that returns elements or nil

Solution

1. Step 1: Understand the Sequence protocol requirement

   The Sequence protocol requires an iterator that can provide elements one by one.

2. Step 2: Identify the iterator method

   This iterator must have a next() method that returns the next element or nil when done.

3. Final Answer:

   An iterator with a next() method that returns elements or nil -> Option D

4. Quick Check:

   Sequence needs next() iterator method = C [OK]

Hint: Sequence needs an iterator with next() returning nil at end [OK]

Common Mistakes:

• Thinking Sequence requires a start() method
• Assuming all elements must be stored in an array
• Confusing count with iteration method

2. Which of the following is the correct way to declare a custom iterator type conforming to IteratorProtocol in Swift?

easy

A. struct MyIterator: IteratorProtocol { func next() -> Int { return 0 } }

B. class MyIterator { func next() -> Int { return 0 } }

C. struct MyIterator: Sequence { func next() -> Int? { return nil } }

D. struct MyIterator: IteratorProtocol { func next() -> Int? { return nil } }

Solution

1. Step 1: Check protocol conformance and method signature

   IteratorProtocol requires a next() method returning an optional element type, here Int?.

2. Step 2: Validate each option

   struct MyIterator: IteratorProtocol { func next() -> Int? { return nil } } correctly declares a struct conforming to IteratorProtocol with next() -> Int?. class MyIterator { func next() -> Int { return 0 } } lacks conformance and returns non-optional. struct MyIterator: Sequence { func next() -> Int? { return nil } } wrongly conforms to Sequence, not IteratorProtocol. struct MyIterator: IteratorProtocol { func next() -> Int { return 0 } } returns non-optional Int, which is incorrect.

3. Final Answer:

   struct MyIterator: IteratorProtocol { func next() -> Int? { return nil } } -> Option A

4. Quick Check:

   IteratorProtocol requires next() returning optional = D [OK]

Hint: IteratorProtocol's next() must return optional element type [OK]

Common Mistakes:

• Returning non-optional from next()
• Confusing Sequence with IteratorProtocol
• Missing protocol conformance declaration

3. Consider this Swift code implementing a custom sequence:

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
    }
  }
}

for number in CountDown(start: 3) {
  print(number)
}

What is the output?

medium

A. 3 2 1 0

B. 3 2 1

C. 4 3 2 1

D. 1 2 3

Solution

1. Step 1: Understand the iterator logic

   The iterator starts at start (3). It returns current if > 0, then decreases current by 1 after returning.

2. Step 2: Trace the iteration values

   First call returns 3, then current becomes 2. Next returns 2, current becomes 1. Next returns 1, current becomes 0. Next call returns nil because current is 0, ending iteration.

3. Final Answer:

   3 2 1 -> Option B

4. Quick Check:

   CountDown from 3 to 1 = 3 2 1 [OK]

Hint: Check when next() returns nil to find iteration end [OK]

Common Mistakes:

• Including 0 in output though iteration stops before
• Assuming count goes up instead of down
• Misreading defer behavior

4. This code attempts to create a custom sequence but has an error:

struct MySequence: Sequence {
  func makeIterator() -> MyIterator {
    return MyIterator()
  }
}

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

What is the main problem here?

medium

A. next() is a func but modifies count, which is a struct property

B. makeIterator() should return IteratorProtocol instead of MyIterator

C. count should start at 0, not 3

D. next() must return non-optional Int

Solution

1. Step 1: Identify mutability issue in next()

   The next() method tries to change count, but it is declared in a struct and next() is not marked mutating.

2. Step 2: Understand Swift struct rules

   In structs, methods that modify properties must be marked mutating. Without it, the compiler will error.

3. Final Answer:

   next() is a func but modifies count, which is a struct property -> Option A

4. Quick Check:

   Mutating methods must be marked mutating = A [OK]

Hint: Mark methods mutating if they change struct properties [OK]

Common Mistakes:

• Forgetting mutating keyword on next()
• Thinking return type must be non-optional
• Confusing return type with protocol requirement

5. You want to create a custom sequence that iterates over only even numbers from 2 up to a given limit. Which implementation correctly uses the Sequence protocol for this?

hard

A. struct EvenNumbers: Sequence {
let limit: Int
func makeIterator() -> AnyIterator<Int> {
var current = 2
return AnyIterator {
while current <= limit {
let result = current
current += 2
return result
}
return nil
}
}
}

B. struct EvenNumbers: Sequence {
let limit: Int
func makeIterator() -> AnyIterator<Int> {
var current = 0
return AnyIterator {
current += 2
if current < limit {
return current
}
return nil
}
}
}

C. struct EvenNumbers: Sequence {
let limit: Int
func makeIterator() -> AnyIterator<Int> {
var current = 2
return AnyIterator {
if current <= limit {
let result = current
current += 2
return result
}
return nil
}
}
}

D. struct EvenNumbers: Sequence {
let limit: Int
func makeIterator() -> AnyIterator<Int> {
var current = 0
return AnyIterator {
if current < limit {
current += 2
return current
}
return nil
}
}
}

Solution

1. Step 1: Check iterator logic for even numbers

   The iterator should start at 2 and return current if <= limit, then add 2 for next call.

2. Step 2: Analyze each option's iteration behavior

   struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 2 return AnyIterator { while current <= limit { let result = current current += 2 return result } return nil } } } uses a while loop inside the closure, which causes it to return only once and then exit, so it won't iterate properly. struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 2 return AnyIterator { if current <= limit { let result = current current += 2 return result } return nil } } } uses an if check and returns current then increments, correctly producing even numbers up to limit. struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 0 return AnyIterator { current += 2 if current < limit { return current } return nil } } } increments before checking, so first returned number is 2, which is correct, but starts at 0 which is okay. struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 0 return AnyIterator { if current < limit { current += 2 return current } return nil } } } increments after checking current < limit, so it may skip the limit if limit is even. struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 2 return AnyIterator { if current <= limit { let result = current current += 2 return result } return nil } } } is the clearest and correct implementation.

3. Final Answer:

   struct EvenNumbers: Sequence { let limit: Int func makeIterator() -> AnyIterator<Int> { var current = 2 return AnyIterator { if current <= limit { let result = current current += 2 return result } return nil } } } -> Option C

4. Quick Check:

   Use if check and increment after return for correct iteration = B [OK]

Hint: Use if condition inside iterator closure, not while loop [OK]

Common Mistakes:

• Using while loop inside iterator closure causing single return
• Incrementing before returning causing off-by-one errors
• Incorrect start value causing wrong first element