Error handling (try, catch, throw) in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

What error does this Swift code produce?

Consider this Swift code snippet that tries to throw an error without marking the function as throwing. What error will the compiler show?

iOS Swift

func testError() {
  throw NSError(domain: "Test", code: 1, userInfo: nil)
}

A Compile-time error: missing catch block

B Runtime error: unhandled exception

C No error, code runs fine

D Compile-time error: 'throw' used in non-throwing function

💡 Hint

In Swift, only functions marked with 'throws' can throw errors.

The function 'testError' is not marked with 'throws', but it tries to throw an error. Swift requires functions that throw errors to be explicitly marked with 'throws'.

📝 Syntax

intermediate

What error does this Swift code produce?

Consider this Swift code snippet that calls a throwing function with 'try' without handling the error. What will the compiler show?

iOS Swift

func mightFail() throws {
  throw NSError(domain: "Fail", code: 1, userInfo: nil)
}

try mightFail()

A Program crashes with a runtime error

B Compiler forces you to handle the error

C Error is ignored and program continues

D Program logs the error but continues

💡 Hint

Swift requires explicit error handling ('do-catch', 'try?', or 'try!') for calls to throwing functions.

The code uses 'try mightFail()' without handling the potential error (no 'do-catch', 'try?', or 'try!'). Swift compiler enforces error handling, resulting in a compile-time error like 'Errors thrown from here are not handled'.

🧠 Conceptual

advanced

Which option correctly uses 'try?' to handle errors?

Given a throwing function, which code snippet safely calls it and returns nil if an error occurs?

iOS Swift

func fetchData() throws -> String {
  throw NSError(domain: "Data", code: 404, userInfo: nil)
}

A let result = try? fetchData()

B let result = try fetchData() catch { return nil }

C let result = fetchData()

D let result = try! fetchData()

💡 Hint

'try?' converts errors to nil instead of throwing.

'try?' calls a throwing function and returns an optional. If an error occurs, it returns nil instead of throwing.

❓ lifecycle

advanced

What is the output of this Swift error handling code?

Analyze the output printed by this code snippet:

iOS Swift

enum SampleError: Error {
  case failed
}

func test() throws {
  throw SampleError.failed
}

do {
  try test()
  print("Success")
} catch {
  print("Caught error")
}
print("Done")

A

Caught error
Done

B

Success
Done

C

Done
Caught error

D Caught error

💡 Hint

The error thrown is caught in the catch block.

The function 'test' throws an error. The 'do' block tries to call it, but the error is caught, so 'Caught error' is printed. Then 'Done' is printed after the do-catch.

🔧 Debug

expert

Why does this Swift code cause a runtime crash?

Examine this code and identify why it crashes at runtime:

iOS Swift

func risky() throws -> String {
  throw NSError(domain: "Crash", code: 1, userInfo: nil)
}

let result = try! risky()
print(result)

A The function is not marked with 'throws'

B Missing 'do-catch' block causes syntax error

C Using 'try!' forces a crash if an error is thrown

D The error is caught but ignored

💡 Hint

'try!' assumes no error will occur and crashes if one does.

'try!' unwraps the result forcefully. If the function throws an error, the app crashes at runtime.