Why closures are fundamental in Swift - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift closure example?

Consider this Swift code that uses a closure to add two numbers. What will be printed?

Swift

let add: (Int, Int) -> Int = { (a, b) in a + b }
print(add(3, 5))

A 8

B Error: Missing return statement

C 35

D 0

💡 Hint

Closures can be assigned to variables and called like functions.

The closure adds 3 and 5, so the output is 8.

🧠 Conceptual

intermediate

Why are closures fundamental in Swift?

Which of these best explains why closures are fundamental in Swift?

A They are used only to create classes and structs.

B They are only used for asynchronous programming and have no other use.

C They replace all functions and methods in Swift.

D They allow you to write inline, self-contained blocks of code that can capture and store references to variables and constants from the surrounding context.

💡 Hint

Think about how closures can remember values from where they were created.

Closures can capture values from their surrounding scope, making them powerful for callbacks, event handling, and more.

🔧 Debug

advanced

What error does this closure code produce?

Look at this Swift code using a closure. What error will it cause?

Swift

var counter = 0
let incrementer = { counter += 1 }
incrementer()
print(counter)

A Error: Cannot assign to value: 'counter' is a 'let' constant

B No error, prints 1

C Error: Closure cannot capture mutable variable

D Error: Missing return in closure

💡 Hint

Check if the variable is mutable and if the closure modifies it.

The variable counter is mutable (var), so the closure can increment it. The output is 1.

❓ Predict Output

advanced

What is the output of this closure capturing example?

What will this Swift code print?

Swift

func makeIncrementer(amount: Int) -> () -> Int {
    var total = 0
    let incrementer: () -> Int = {
        total += amount
        return total
    }
    return incrementer
}

let incrementByTen = makeIncrementer(amount: 10)
print(incrementByTen())
print(incrementByTen())

A

0
10

B

10
10

C

10
20

D Error: Cannot capture 'total' in closure

💡 Hint

Closures can capture and modify variables from their creation context.

The closure captures 'total' and adds 'amount' each time it is called, so outputs are 10 then 20.

🚀 Application

expert

How many items are in the resulting array after using this closure?

What is the count of elements in the array 'results' after running this Swift code?

Swift

let numbers = [1, 2, 3, 4, 5]
let results = numbers.map { num in
    if num % 2 == 0 {
        return num * 2
    } else {
        return num
    }
}
print(results.count)

A 5

B 3

C 2

D 0

💡 Hint

The map function returns an array with the same number of elements as the original.

The map closure transforms each element but keeps the array size the same, so count is 5.