Try? for optional result in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using try?

Consider this Swift code that uses try? to handle errors. What will be printed?

Swift

enum SampleError: Error {
    case fail
}

func mightThrow(_ shouldThrow: Bool) throws -> Int {
    if shouldThrow {
        throw SampleError.fail
    }
    return 42
}

let result = try? mightThrow(true)
print(result ?? "nil")

A 42

B Runtime error

C SampleError.fail

D nil

💡 Hint

Remember that try? converts errors into nil instead of throwing.

The function throws an error because shouldThrow is true. Using try? converts the thrown error into nil, so result is nil. The print statement prints "nil".

❓ Predict Output

intermediate

What does this code print when try? succeeds?

What will be printed by this Swift code?

Swift

func safeDivide(_ a: Int, _ b: Int) throws -> Int {
    if b == 0 { throw NSError(domain: "", code: 1) }
    return a / b
}

let result = try? safeDivide(10, 2)
print(result ?? "nil")

A 5

B nil

C 0

D Runtime error

💡 Hint

When no error is thrown, try? returns the value wrapped in an optional.

The division succeeds (10 / 2 = 5), so try? returns Optional(5). The print statement unwraps it and prints 5.

🔧 Debug

advanced

Why does this code print nil instead of the value?

Look at this Swift code snippet. It prints "nil" even though the function seems to return a value. Why?

Swift

enum MyError: Error {
    case error
}

func test() throws -> Int {
    return 10
}

let value = try? test()
print(value ?? "nil")

A The function throws an error, so try? returns nil

B The code has a syntax error and does not compile

C The function returns 10, so try? returns Optional(10)

D The print statement is incorrect and prints nil

💡 Hint

Check if the function actually throws an error or not.

The function test() does not throw an error; it returns 10. Using try? wraps the result in an optional. So value is Optional(10), and the print statement prints 10.

📝 Syntax

advanced

Which option causes a compile-time error with try??

Which of these Swift code snippets will cause a compile-time error?

A let result = try someThrowingFunction()

B let result = try?? someThrowingFunction()

C let result = try! someThrowingFunction()

D let result = try? someThrowingFunction()

💡 Hint

Check the correct syntax for using try with optional chaining.

Using try?? is invalid syntax in Swift. The correct forms are try?, try!, or try. Option B causes a compile-time error.

🚀 Application

expert

How many items are in the array after filtering with try??

Given this Swift code, how many elements are in the results array after filtering?

Swift

enum ParseError: Error {
    case invalid
}

func parse(_ s: String) throws -> Int {
    guard let value = Int(s) else { throw ParseError.invalid }
    return value
}

let inputs = ["1", "two", "3", "four"]
let results = inputs.compactMap { try? parse($0) }

A 2

B 3

C 4

D 0

💡 Hint

Count how many strings can be converted to Int without throwing.

Only "1" and "3" can be converted to Int without error. The others throw errors, so try? returns nil for them. compactMap removes nils, so the array has 2 elements.