Task groups for parallel execution in Swift - Time & Space Complexity

Time Complexity: Task groups for parallel execution

O(T)

Understanding Time Complexity

When using task groups for parallel execution, we want to know how the total work grows as we add more tasks.

We ask: How does running many tasks at once affect the time it takes?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

func fetchAllData(n: Int) async {
  await withTaskGroup(of: String.self) { group in
    for i in 1...n {
      group.addTask {
        await fetchData(id: i)
      }
    }
    for await result in group {
      print(result)
    }
  }
}

func fetchData(id: Int) async -> String {
  // Simulate network call
  return "Data \(id)"
}
    

This code runs n tasks in parallel to fetch data, then waits for all to finish.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Adding and running n tasks in the task group.
• How many times: The loop runs n times, creating n parallel tasks.

How Execution Grows With Input

As n grows, the number of tasks grows linearly, but tasks run at the same time.

Input Size (n)	Approx. Operations
10	10 tasks run in parallel
100	100 tasks run in parallel
1000	1000 tasks run in parallel

Pattern observation: The number of tasks grows with n, but total time depends on the longest single task, not the sum.

Final Time Complexity

Time Complexity: O(T)

This means the total time to complete all tasks stays roughly the same as n grows, assuming tasks run truly in parallel and each task takes time T.

Common Mistake

[X] Wrong: "More tasks always mean more total time because they add up."

[OK] Correct: When tasks run in parallel, total time depends on the slowest task, not the number of tasks.

Interview Connect

Understanding how parallel tasks affect time helps you explain real-world app performance and shows you can think about efficient code.

Self-Check

"What if tasks run sequentially instead of in parallel? How would the time complexity change?"

Practice

1. What is the main purpose of using a TaskGroup in Swift concurrency?

easy

A. To run multiple tasks in parallel and wait for all to complete

B. To create a single long-running task

C. To pause the current task indefinitely

D. To handle errors in synchronous code

Solution

1. Step 1: Understand what TaskGroup does

   A TaskGroup lets you run many tasks at the same time, managing them together.

2. Step 2: Identify the main use

   It allows parallel execution and waits until all tasks finish before continuing.

3. Final Answer:

   To run multiple tasks in parallel and wait for all to complete -> Option A

4. Quick Check:

   TaskGroup = parallel tasks + wait [OK]

Hint: TaskGroup runs tasks together and waits for all [OK]

Common Mistakes:

• Thinking TaskGroup runs only one task
• Confusing TaskGroup with error handling
• Assuming TaskGroup pauses tasks

2. Which of the following is the correct way to add a task inside a TaskGroup in Swift?

easy

A. group.runTask(fetchData())

B. group.createTask(fetchData)

C. group.addTask { await fetchData() }

D. group.startTask { fetchData() }

Solution

1. Step 1: Recall the method to add tasks

   The correct method to add a task is addTask with a closure.

2. Step 2: Check syntax correctness

   group.addTask { await fetchData() } uses group.addTask { await fetchData() }, which is the right syntax.

3. Final Answer:

   group.addTask { await fetchData() } -> Option C

4. Quick Check:

   Use addTask with closure [OK]

Hint: Use addTask with closure to add work [OK]

Common Mistakes:

• Using wrong method names like runTask or createTask
• Not using closure syntax
• Omitting await inside the task

3. What will be the output of this Swift code using a TaskGroup?

func fetchNumber(_ n: Int) async -> Int {
    return n * 2
}

func runGroup() async {
    await withTaskGroup(of: Int.self) { group in
        for i in 1...3 {
            group.addTask { await fetchNumber(i) }
        }
        var results = [Int]()
        for await result in group {
            results.append(result)
        }
        print(results.sorted())
    }
}

Task {
    await runGroup()
}

medium

A. Compilation error

B. [1, 2, 3]

C. [6, 4, 2]

D. [2, 4, 6]

Solution

1. Step 1: Understand the task group adds tasks for numbers 1 to 3

   Each task returns the input number multiplied by 2: 2, 4, and 6.

2. Step 2: Collect results and sort them before printing

   Results are collected in any order but sorted before printing, so output is [2, 4, 6].

3. Final Answer:

   [2, 4, 6] -> Option D

4. Quick Check:

   Results doubled and sorted = [2,4,6] [OK]

Hint: Remember to sort results after parallel tasks [OK]

Common Mistakes:

• Assuming results print in order added
• Forgetting to await inside addTask
• Confusing output with original numbers

4. Identify the error in this Swift code using a TaskGroup:

await withTaskGroup(of: Int.self) { group in
    for i in 1...3 {
        group.addTask { fetchNumber(i) }
    }
    for await result in group {
        print(result)
    }
}

Assuming fetchNumber is an async function.

medium

A. Missing await inside addTask closure

B. Cannot use for await with TaskGroup

C. TaskGroup must be declared with var

D. fetchNumber must return Void

Solution

1. Step 1: Check how fetchNumber is called inside addTask

   fetchNumber is async, so it must be awaited inside the closure.

2. Step 2: Identify missing await keyword

   The closure calls fetchNumber(i) without await, causing a compile error.

3. Final Answer:

   Missing await inside addTask closure -> Option A

4. Quick Check:

   Async calls inside addTask need await [OK]

Hint: Always await async calls inside addTask closures [OK]

Common Mistakes:

• Forgetting await inside addTask
• Thinking for await can't be used with TaskGroup
• Declaring TaskGroup incorrectly

5. You want to fetch data from multiple URLs in parallel using a TaskGroup and collect only the successful results (non-nil). Which approach correctly filters out nil results while running tasks in parallel?

hard

A. Run tasks sequentially inside the group to avoid nil results

B. Add tasks returning optional data, then filter non-nil in for await loop

C. Use addTask with throwing function and catch errors outside group

D. Use withThrowingTaskGroup but ignore errors and collect all results

Solution

1. Step 1: Understand the goal to run tasks in parallel and filter nil

   Running tasks in parallel with TaskGroup allows concurrency; filtering nil after collecting results is straightforward.

2. Step 2: Evaluate options for filtering nil results

   Add tasks returning optional data, then filter non-nil in for await loop correctly adds tasks returning optional data and filters non-nil results in the for await loop, matching the goal.

3. Final Answer:

   Add tasks returning optional data, then filter non-nil in for await loop -> Option B

4. Quick Check:

   Filter nil after parallel tasks with for await [OK]

Hint: Filter nil results after collecting parallel task outputs [OK]

Common Mistakes:

• Trying to run tasks sequentially losing parallelism
• Ignoring errors without filtering nil
• Misusing throwing task groups for simple filtering