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.

Practice

1. What does try? do when used with a throwing function in Swift?

easy

A. It runs the function and returns an optional result, nil if an error occurs.

B. It forces the function to crash if an error is thrown.

C. It catches the error and prints it automatically.

D. It converts the error into a string message.

Solution

1. Step 1: Understand try? behavior

   The try? keyword runs a throwing function and converts its result into an optional value.

2. Step 2: What happens on error?

   If the function throws an error, try? returns nil instead of crashing or propagating the error.

3. Final Answer:

   It runs the function and returns an optional result, nil if an error occurs. -> Option A

4. Quick Check:

   try? returns optional or nil on error [OK]

Hint: try? returns nil on error instead of crashing [OK]

Common Mistakes:

• Thinking try? crashes on error
• Assuming try? prints error messages
• Believing try? converts errors to strings

2. Which of the following is the correct syntax to use try? with a throwing function named loadData()?

easy

A. let data = try loadData()

B. let data = try! loadData()

C. let data = try?loadData

D. let data = try? loadData()

Solution

1. Step 1: Identify correct try? syntax

   The correct syntax requires try? followed by the function call with parentheses.

2. Step 2: Check each option

   let data = try loadData() uses try without ?, which doesn't return optional. let data = try! loadData() uses try! which forces crash on error. let data = try?loadData misses parentheses after function name.

3. Final Answer:

   let data = try? loadData() -> Option D

4. Quick Check:

   try? requires parentheses for function call [OK]

Hint: Use try? followed by function call with parentheses [OK]

Common Mistakes:

• Omitting parentheses after function name
• Using try without ? for optional
• Using try! which forces crash on error

3. Consider this Swift code:

func fetchNumber() throws -> Int {
  if Bool.random() { return 10 } else { throw NSError() }
}

let result = try? fetchNumber()
print(result)

What are possible outputs?

medium

A. Prints either Optional(10) or nil

B. Always prints Optional(10)

C. Always prints nil

D. Causes a runtime crash

Solution

1. Step 1: Understand fetchNumber behavior

   The function randomly returns 10 or throws an error.

2. Step 2: Effect of try? on fetchNumber

   Using try? converts the throwing call into an optional Int. If no error, it returns Optional(10). If error, it returns nil.

3. Final Answer:

   Prints either Optional(10) or nil -> Option A

4. Quick Check:

   try? returns optional or nil based on error [OK]

Hint: try? returns optional value or nil on error [OK]

Common Mistakes:

• Assuming always returns value
• Assuming always returns nil
• Expecting a crash on error

4. Identify the error in this Swift code snippet:

func readFile() throws -> String {
  throw NSError()
}

let content = try readFile()
print(content)

medium

A. print(content) is invalid syntax

B. Function readFile() should not throw error

C. Missing try? to handle error as optional

D. No error, code runs fine

Solution

1. Step 1: Check error handling in calling code

   The function readFile() throws an error, but the call uses plain try without handling errors.

2. Step 2: Fix error handling with try?

   Using try? converts the throwing call into an optional, preventing runtime crash and handling error gracefully.

3. Final Answer:

   Missing try? to handle error as optional -> Option C

4. Quick Check:

   Use try? to avoid crash on throwing function [OK]

Hint: Use try? to safely call throwing functions without crash [OK]

Common Mistakes:

• Using try without catch or try?
• Assuming throwing function never throws
• Thinking print causes error

5. You want to call a throwing function parseData() that returns a dictionary, but you want to safely get an empty dictionary if it fails. Which code correctly uses try? to achieve this?

hard

A. let dict = try parseData() ?? [:]

B. let dict = try? parseData() ?? [:]

C. let dict = try! parseData() ?? [:]

D. let dict = parseData() ?? [:]

Solution

1. Step 1: Understand requirement for safe call

   You want to call a throwing function and get an optional result, defaulting to empty dictionary if it fails.

2. Step 2: Use try? with nil coalescing

   Using try? converts the throwing call to optional. The ?? [:] provides empty dictionary if nil.

3. Final Answer:

   let dict = try? parseData() ?? [:] -> Option B

4. Quick Check:

   try? with ?? gives safe default on error [OK]

Hint: Combine try? with ?? to provide default value [OK]

Common Mistakes:

• Using try without ? causes crash on error
• Using try! forces crash on error
• Calling throwing function without try