Sequence protocol for custom iteration in Swift - Step-by-Step Execution

Concept Flow - Sequence protocol for custom iteration

Define Sequence struct/class

↓

Implement makeIterator() method

↓

Create Iterator struct/class

↓

Implement next() method

↓

Use for-in loop to iterate

↓

next() returns next element or nil

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Iteration ends when next() returns nil

This flow shows how to create a custom sequence by defining a type that returns an iterator, which produces elements one by one until none are left.

Execution Sample

Swift

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

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

This code defines a Countdown sequence that counts down from a start number to 1, printing each number.

Execution Table

Step	current (Iterator)	Condition (current <= 0)	Action	Output
1	3	false	return 3, current becomes 2	3
2	2	false	return 2, current becomes 1	2
3	1	false	return 1, current becomes 0	1
4	0	true	return nil (end iteration)	nil

💡 Iteration stops when next() returns nil because current is 0

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
current	3	2	1	0	0

Key Moments - 2 Insights

Why does the iteration stop when current reaches 0?

Because in the next() method, when current <= 0, it returns nil, signaling the end of iteration as shown in execution_table step 4.

Why do we use 'defer { current -= 1 }' instead of subtracting before returning?

Using defer ensures the current value is returned first, then decreased. This matches the output sequence seen in execution_table steps 1-3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'current' after step 2?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'current' column in execution_table row for step 2 and after 2.

At which step does the next() method return nil, ending the iteration?

A Step 1

B Step 4

C Step 3

D Step 2

💡 Hint

Look at the 'Output' column in execution_table where output is nil.

If we change 'start' to 5, how many times will next() return a number before nil?

A 5 times

B 3 times

C 4 times

D 6 times

💡 Hint

Refer to variable_tracker and execution_table pattern for how many numbers are returned from start down to 1.

Concept Snapshot

Sequence protocol requires a makeIterator() method.
IteratorProtocol requires a next() method returning next element or nil.
Use for-in loops to iterate custom sequences.
next() returning nil ends iteration.
Use defer to update state after returning current value.

Full Transcript

This example shows how to create a custom sequence in Swift by defining a struct that conforms to Sequence. The struct provides a makeIterator() method that returns an iterator. The iterator conforms to IteratorProtocol and implements next(), which returns the next element or nil when done. The countdown example starts from a number and counts down to 1. Each call to next() returns the current number and then decreases it. When current reaches zero, next() returns nil, ending the iteration. The for-in loop uses this to print each number. The execution table traces each call to next(), showing current value, condition check, action, and output. The variable tracker shows how current changes after each step. Key moments clarify why iteration stops and why defer is used. The quiz tests understanding of variable changes and iteration end. This teaches how to build and use custom sequences in Swift simply and clearly.