Await for calling async functions in Swift - Time & Space Complexity

Time Complexity: Await for calling async functions

O(n)

Understanding Time Complexity

When we use await to call async functions, we want to know how the waiting affects the program's speed.

We ask: How does the total time grow when we wait for multiple async calls?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

func fetchData(id: Int) async -> String {
    // Simulate network delay
    try? await Task.sleep(nanoseconds: 1_000_000_000) // 1 second
    return "Data \(id)"
}

func loadAllData(ids: [Int]) async -> [String] {
    var results = [String]()
    for id in ids {
        let data = await fetchData(id: id)
        results.append(data)
    }
    return results
}

This code fetches data for each id one by one, waiting for each async call to finish before starting the next.

Identify Repeating Operations

• Primary operation: The for loop calls fetchData for each id.
• How many times: Once for each element in the ids array.

How Execution Grows With Input

Each async call waits about 1 second before returning. Since calls happen one after another, total time adds up.

Input Size (n)	Approx. Operations (seconds)
10	~10 seconds
100	~100 seconds
1000	~1000 seconds

Pattern observation: Total time grows directly with the number of calls, because each waits for the previous to finish.

Final Time Complexity

Time Complexity: O(n)

This means the total waiting time grows linearly with the number of async calls.

Common Mistake

[X] Wrong: "Using await inside a loop runs all async calls at the same time, so total time stays the same no matter how many calls."

[OK] Correct: Await inside a loop waits for each call to finish before starting the next, so calls run one after another, increasing total time.

Interview Connect

Understanding how awaiting async calls affects time helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

What if we started all async calls without awaiting inside the loop, then awaited all results together? How would the time complexity change?

Practice

1. What does the await keyword do when calling an async function in Swift?

easy

A. It makes the function run faster by skipping some steps.

B. It pauses the current code until the async function finishes and returns a result.

C. It converts the async function into a synchronous one automatically.

D. It runs the async function in the background without waiting.

Solution

1. Step 1: Understand the role of await

   The await keyword tells Swift to pause the current code execution until the async function completes.

2. Step 2: Recognize the effect on program flow

   This means the program waits for the async task to finish and then continues with the result.

3. Final Answer:

   It pauses the current code until the async function finishes and returns a result. -> Option B

4. Quick Check:

   await pauses code = A [OK]

Hint: Remember: await waits for async tasks to finish before continuing [OK]

Common Mistakes:

• Thinking await makes code run faster
• Believing await runs code in background without waiting
• Confusing await with automatic sync conversion

2. Which of the following is the correct way to call an async function named fetchData() inside another async function in Swift?

easy

A. let result = fetchData()

B. let result = async fetchData()

C. await let result = fetchData()

D. let result = await fetchData()

Solution

1. Step 1: Recall async function call syntax

   When calling an async function, you must use await before the function call to wait for its result.

2. Step 2: Check the correct placement of await

   The correct syntax is let result = await fetchData(). The await keyword comes before the function call.

3. Final Answer:

   let result = await fetchData() -> Option D

4. Quick Check:

   Use await before async call = D [OK]

Hint: Put await right before the async function call [OK]

Common Mistakes:

• Omitting await before the async call
• Placing await after the assignment
• Using async keyword instead of await

3. What will be printed by the following Swift code?

func fetchNumber() async -> Int {
    return 42
}

func printNumber() async {
    let number = await fetchNumber()
    print(number)
}

Task {
    await printNumber()
}

medium

A. No output, code will not run

B. Optional(42)

C. 42

D. Compilation error due to missing await

Solution

1. Step 1: Understand async function behavior

   The function fetchNumber() returns 42 asynchronously. Using await waits for this value.

2. Step 2: Trace the print statement

   The printNumber() function awaits fetchNumber() and prints the returned integer 42.

3. Final Answer:

   42 -> Option C

4. Quick Check:

   Await returns 42 printed = A [OK]

Hint: Await returns the actual value, so print shows 42 [OK]

Common Mistakes:

• Expecting Optional(42) instead of 42
• Thinking code won't run without main async
• Missing await causing compile error

4. Identify the error in this Swift code snippet:

func getData() async -> String {
    return "Hello"
}

func showData() {
    let message = await getData()
    print(message)
}

medium

A. showData() must be marked async to use await inside it

B. Missing await keyword before getData()

C. getData() should not be async if awaited

D. print(message) cannot print async results

Solution

1. Step 1: Check where await is used

   The await keyword is used inside showData(), but this function is not marked async.

2. Step 2: Understand async function rules

   In Swift, you can only use await inside an async function or a Task closure. So showData() must be declared async.

3. Final Answer:

   showData() must be marked async to use await inside it -> Option A

4. Quick Check:

   Await requires async function = B [OK]

Hint: Use await only inside async functions or Task closures [OK]

Common Mistakes:

• Forgetting to mark function async when using await
• Thinking await can be used in normal functions
• Confusing async function declaration with await usage

5. You want to fetch user data and then fetch user posts, both are async functions: fetchUser() and fetchPosts(userId:). How do you correctly wait for both results in sequence inside an async function?

hard

A. let user = await fetchUser()
let posts = await fetchPosts(userId: user.id)

B. let user = fetchUser()
let posts = fetchPosts(userId: user.id)

C. await let user = fetchUser()
await let posts = fetchPosts(userId: user.id)

D. let user = await fetchUser()
let posts = fetchPosts(userId: await user.id)

Solution

1. Step 1: Await the first async call to get user

   You must use await before fetchUser() to get the user data before using it.

2. Step 2: Await the second async call using user.id

   After getting the user, call fetchPosts(userId: user.id) with await to get posts.

3. Final Answer:

   let user = await fetchUser() let posts = await fetchPosts(userId: user.id) -> Option A

4. Quick Check:

   Await each async call in order = C [OK]

Hint: Await each async call separately in sequence [OK]

Common Mistakes:

• Not awaiting the second async call
• Trying to await variable declarations incorrectly
• Using await inside function arguments wrongly