Closures as function parameters in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the function with a closure that prints "Hello".

Swift

func greet(action: () -> Void) {
    action()
}

greet [1] {
    print("Hello")
}

Drag options to blanks, or click blank then click option'

A()

B(action: () -> Void)

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

D() -> Void

Attempts:

3 left

💡 Hint

Common Mistakes

Including the closure type in the call instead of just parentheses.

Omitting parentheses entirely when no parameters are passed.

The function greet is called with a trailing closure, so just empty parentheses () are needed before the closure block.

2fill in blank

medium

Complete the function parameter to accept a closure that takes an Int and returns an Int.

Swift

func modify(number: Int, with closure: [1]) -> Int {
    return closure(number)
}

Drag options to blanks, or click blank then click option'

A() -> Int

B(Int) -> Int

C(Int) -> Void

D(Int, Int) -> Int

Attempts:

3 left

💡 Hint

Common Mistakes

Using a closure type that returns Void instead of Int.

Using a closure type with no parameters.

The closure parameter must take an Int and return an Int, so the type is (Int) -> Int.

3fill in blank

hard

Fix the error in the closure parameter type to accept a closure that takes two Ints and returns Bool.

Swift

func compare(a: Int, b: Int, using predicate: [1]) -> Bool {
    return predicate(a, b)
}

Drag options to blanks, or click blank then click option'

A(Bool, Bool) -> Int

B(Int) -> Bool

C(Int, Int) -> Bool

D() -> Bool

Attempts:

3 left

💡 Hint

Common Mistakes

Using a closure type with only one parameter.

Using a closure type with wrong parameter or return types.

The closure must take two Int parameters and return a Bool, so the type is (Int, Int) -> Bool.

4fill in blank

hard

Fill both blanks to create a function that accepts a closure with no parameters and returns a String, then call it.

Swift

func fetchMessage(completion: [1]) {
    let message = completion()
    print(message)
}

fetchMessage [2] {
    return "Done"
}

Drag options to blanks, or click blank then click option'

A() -> String

B(String) -> Void

C()

D() -> Void

Attempts:

3 left

💡 Hint

Common Mistakes

Using closure types that return Void instead of String.

Omitting parentheses when calling the function.

The function parameter is a closure with no parameters returning String, so () -> String. The call uses empty parentheses () before the closure.

5fill in blank

hard

Complete the code to define and call a function that accepts a closure taking a String and returning an Int, then prints the result.

Swift

func process(text: String, with closure: [1]) {
    let length = closure(text)
    print("Length: \(length)")
}

process(text: "Swift") { text in
    return text.count
}

Drag options to blanks, or click blank then click option'

A(String) -> Int

B{

C}

Din

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting braces around the closure body.

Forgetting the 'in' keyword in the closure syntax.

The closure type is (String) -> Int. The closure syntax uses braces { } and the keyword in to separate parameters from the body.

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