Closures as function parameters in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using a closure parameter?

Consider this Swift function that takes a closure as a parameter and calls it:

func greetUser(name: String, action: (String) -> String) {
    let message = action(name)
    print(message)
}

greetUser(name: "Anna") { userName in
    return "Hello, \(userName)!"
}

What will be printed when this code runs?

Swift

func greetUser(name: String, action: (String) -> String) {
    let message = action(name)
    print(message)
}

greetUser(name: "Anna") { userName in
    return "Hello, \(userName)!"
}

A Hello, userName!

B Hello, Anna!

C greetUser(name: "Anna")

D Error: Missing return statement

💡 Hint

Look at how the closure uses the parameter userName inside the string.

The closure receives the string "Anna" as userName and returns "Hello, Anna!". This string is printed by the function.

❓ Predict Output

intermediate

What does this Swift code print when using a trailing closure?

Examine this Swift code that uses a trailing closure syntax:

func performOperation(_ a: Int, _ b: Int, operation: (Int, Int) -> Int) {
    let result = operation(a, b)
    print(result)
}

performOperation(4, 5) { x, y in
    x * y
}

What is the output?

Swift

func performOperation(_ a: Int, _ b: Int, operation: (Int, Int) -> Int) {
    let result = operation(a, b)
    print(result)
}

performOperation(4, 5) { x, y in
    x * y
}

A Error: Missing return in closure

B 9

C 45

D 20

💡 Hint

Remember that the closure multiplies the two numbers.

The closure multiplies 4 and 5, resulting in 20, which is printed.

🔧 Debug

advanced

What error does this Swift code produce?

Look at this Swift function call with a closure parameter:

func calculate(value: Int, operation: (Int) -> Int) {
    let result = operation(value)
    print(result)
}

calculate(value: 10) { number in
    if number > 5 {
        number * 2
    }
}

What error will this code cause when compiled?

Swift

func calculate(value: Int, operation: (Int) -> Int) {
    let result = operation(value)
    print(result)
}

calculate(value: 10) { number in
    if number > 5 {
        number * 2
    }
}

A Error: Missing return in closure

B Runtime error: nil value

C No error, prints 20

D Syntax error: unexpected if statement

💡 Hint

Check if the closure always returns a value.

The closure has an if statement but no else branch and no explicit return, so it may not return a value in all cases, causing a compile error.

🧠 Conceptual

advanced

Which option correctly describes escaping closures in Swift?

In Swift, what does it mean when a closure parameter is marked with @escaping?

A The closure can be stored and called after the function returns

B The closure must be called immediately inside the function

C The closure cannot capture variables from its surrounding context

D The closure is automatically executed on the main thread

💡 Hint

Think about when the closure might be used relative to the function call.

An escaping closure can be saved and called later, even after the function that received it has finished.

❓ Predict Output

expert

What is the output of this Swift code using nested closures?

Analyze this Swift code with nested closures passed as parameters:

func outerFunction(action: (Int) -> (Int) -> Int) {
    let innerClosure = action(3)
    let result = innerClosure(4)
    print(result)
}

outerFunction { x in
    return { y in
        x + y
    }
}

What will be printed?

Swift

func outerFunction(action: (Int) -> (Int) -> Int) {
    let innerClosure = action(3)
    let result = innerClosure(4)
    print(result)
}

outerFunction { x in
    return { y in
        x + y
    }
}

A 12

B Error: Cannot convert closure type

C 7

D 34

💡 Hint

Trace the values of x and y through the closures.

The outer closure receives 3 as x and returns an inner closure that adds x and y. Calling innerClosure(4) adds 3 + 4 = 7.