Closure expression syntax in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this closure expression?

Consider the following Swift code using a closure expression. What will be printed when this code runs?

Swift

let numbers = [1, 2, 3, 4]
let doubled = numbers.map({ (num: Int) -> Int in
    return num * 2
})
print(doubled)

A [4, 8, 12, 16]

B [1, 2, 3, 4]

C Compilation error

D [2, 4, 6, 8]

💡 Hint

Look at how the closure multiplies each number by 2.

The closure takes each number and returns it multiplied by 2, so the map function produces a new array with doubled values.

❓ Predict Output

intermediate

What does this shorthand closure print?

What is the output of this Swift code using shorthand argument names in a closure?

Swift

let names = ["Anna", "Alex", "Brian", "Jack"]
let sortedNames = names.sorted(by: { $0 > $1 })
print(sortedNames)

A ["Jack", "Brian", "Alex", "Anna"]

B Runtime error

C ["Alex", "Anna", "Brian", "Jack"]

D ["Anna", "Alex", "Brian", "Jack"]

💡 Hint

The closure sorts names in descending order.

The closure compares two names and returns true if the first is greater than the second, so the array is sorted in reverse alphabetical order.

🔧 Debug

advanced

Identify the error in this closure syntax

This Swift code tries to filter an array using a closure but causes a compile error. What is the error?

Swift

let numbers = [1, 2, 3, 4, 5]
let evens = numbers.filter { num in
    num % 2 == 0
}

print(evens)

A Missing closing brace '}' for the closure

B Missing return keyword inside the closure

C Incorrect use of parentheses in filter call

D Using 'num' instead of '$0' causes error

💡 Hint

Check if all braces are properly closed.

The closure is missing a closing brace '}', causing a syntax error.

❓ Predict Output

advanced

What is the output of this nested closure?

What will this Swift code print when run?

Swift

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

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

A 0\n10

B Compilation error

C 10\n20

D 10\n10

💡 Hint

The closure captures and updates 'total' each time it is called.

The closure remembers the 'total' variable and adds 'amount' each call, so first call returns 10, second returns 20.

🧠 Conceptual

expert

Which option correctly explains trailing closure syntax?

In Swift, which statement best describes the trailing closure syntax?

A Trailing closures must always have explicit parameter types declared.

B A closure passed as the last argument can be written outside the parentheses of the function call.

C Trailing closures cannot capture variables from the surrounding context.

D Trailing closures are only allowed if the function has exactly one parameter.

💡 Hint

Think about how you can write closures after function calls.

Trailing closure syntax lets you write a closure outside the parentheses if it is the last argument, making code cleaner.