Closures as function parameters in Swift - Time & Space Complexity

Time Complexity: Closures as function parameters

O(n)

Understanding Time Complexity

When we use closures as function parameters, we want to know how the time to run the code changes as the input grows.

We ask: How many times does the closure run as input size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

func repeatTask(times: Int, task: () -> Void) {
    for _ in 0..

This code runs a closure called task a certain number of times given by times.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The closure task() is called inside a loop.
• How many times: It runs exactly times times, once per loop iteration.

How Execution Grows With Input

As the times value grows, the number of times the closure runs grows the same way.

Input Size (times)	Approx. Operations (closure calls)
10	10
100	100
1000	1000

Pattern observation: The number of operations grows directly with the input size.

Final Time Complexity

Time Complexity: O(n)

This means if you double the input size, the closure runs twice as many times.

Common Mistake

[X] Wrong: "The closure runs only once no matter the input size."

[OK] Correct: The closure is inside a loop that runs times times, so it runs as many times as the input number.

Interview Connect

Understanding how closures run inside loops helps you explain how your code scales and shows you can think about performance clearly.

Self-Check

"What if the closure itself contains a loop that runs n times? How would the time complexity change?"

Practice

1.

What is the main purpose of passing a closure as a function parameter in Swift?

easy

A. To allow the function to execute custom code provided by the caller

B. To make the function run faster

C. To prevent the function from returning a value

D. To declare a global variable inside the function

Solution

1. Step 1: Understand what a closure is

   A closure is a block of code that can be passed around and executed later.

2. Step 2: Understand why closures are passed to functions

   Passing a closure lets the function run custom code without changing its own code.

3. Final Answer:

   To allow the function to execute custom code provided by the caller -> Option A

4. Quick Check:

   Closures customize function behavior = A [OK]

Hint: Closures let functions run your code inside them [OK]

Common Mistakes:

• Thinking closures speed up the function
• Confusing closures with variables
• Believing closures stop function returns

2.

Which of the following is the correct syntax to declare a function that takes a closure parameter with no input and no return value?

func performAction(____) { }

easy

A. action: () => Void

B. action: Void -> ()

C. action: () -> () -> Void

D. action: () -> Void

Solution

1. Step 1: Recall closure syntax in Swift

   A closure with no input and no return is written as () -> Void.

2. Step 2: Match syntax to function parameter

   The parameter should be named and typed as action: () -> Void.

3. Final Answer:

   action: () -> Void -> Option D

4. Quick Check:

   Closure type syntax = D [OK]

Hint: Closure type: inputs in (), arrow, then return type [OK]

Common Mistakes:

• Using 'Void -> ()' which is invalid
• Adding extra arrows like '() -> () -> Void'
• Using '=>' which is not Swift syntax

3.

What will be printed when the following Swift code runs?

func greet(action: () -> String) {
    print("Greeting: " + action())
}

greet { "Hello, Swift!" }

medium

A. Hello, Swift!

B. Greeting: action

C. Greeting: Hello, Swift!

D. Error: Missing return type

Solution

1. Step 1: Understand the function call

   The function greet takes a closure that returns a String and prints "Greeting: " plus the closure's result.

2. Step 2: Evaluate the closure passed

   The closure returns "Hello, Swift!", so the print statement outputs "Greeting: Hello, Swift!".

3. Final Answer:

   Greeting: Hello, Swift! -> Option C

4. Quick Check:

   Closure returns string printed with prefix = A [OK]

Hint: Closure returns string, function prints with prefix [OK]

Common Mistakes:

• Expecting just the closure string without prefix
• Confusing closure name with output
• Thinking code causes error

4.

Identify the error in this Swift function that takes a closure parameter:

func repeatTask(times: Int, task: () -> Void) {
    for _ in 1...times {
        task
    }
}

medium

A. The closure 'task' is not called with parentheses

B. The loop range should start from 0

C. The closure parameter must return a value

D. The function must return a value

Solution

1. Step 1: Check how the closure is used inside the loop

   The code uses 'task' without parentheses, so the closure is not executed.

2. Step 2: Correct closure call syntax

   To run the closure, it must be called as 'task()' with parentheses.

3. Final Answer:

   The closure 'task' is not called with parentheses -> Option A

4. Quick Check:

   Call closure with () to execute = C [OK]

Hint: Call closures with () to run them [OK]

Common Mistakes:

• Forgetting parentheses to call closure
• Changing loop range unnecessarily
• Expecting closure to return value when void

5.

Given this function that filters numbers using a closure, how would you call it to get only even numbers?

func filterNumbers(_ numbers: [Int], using condition: (Int) -> Bool) -> [Int] {
    var result = [Int]()
    for number in numbers {
        if condition(number) {
            result.append(number)
        }
    }
    return result
}

Choose the correct call:

hard

A. filterNumbers([1,2,3,4]) { number in number * 2 }

B. filterNumbers([1,2,3,4]) { $0 % 2 == 0 }

C. filterNumbers([1,2,3,4]) { _ in true }

D. filterNumbers([1,2,3,4]) { $0 > 5 }

Solution

1. Step 1: Understand the closure parameter

   The closure takes an Int and returns Bool to decide if number is included.

2. Step 2: Identify condition for even numbers

   Even numbers satisfy number % 2 == 0, so closure should return true for those.

3. Step 3: Match correct call

   filterNumbers([1,2,3,4]) { $0 % 2 == 0 } uses shorthand $0 % 2 == 0, which returns true for even numbers.

4. Final Answer:

   filterNumbers([1,2,3,4]) { $0 % 2 == 0 } -> Option B

5. Quick Check:

   Closure returns true for even numbers = B [OK]

Hint: Use $0 % 2 == 0 to filter even numbers [OK]

Common Mistakes:

• Returning number * 2 instead of Bool
• Returning always true to get all numbers
• Filtering numbers greater than 5 instead of even