Do-try-catch execution flow in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of do-try-catch with successful try

What is the output of this Swift code snippet?

Swift

enum SampleError: Error {
    case fail
}

func testFunc() throws -> String {
    return "Success"
}

do {
    let result = try testFunc()
    print(result)
} catch {
    print("Error caught")
}

A Error caught

B Success

C No output

D Compilation error

💡 Hint

The function does not throw an error, so the catch block is skipped.

Since testFunc() returns "Success" without throwing, the try succeeds and prints "Success".

❓ Predict Output

intermediate

Output when try throws an error

What will this Swift code print?

Swift

enum SampleError: Error {
    case fail
}

func testFunc() throws -> String {
    throw SampleError.fail
}

do {
    let result = try testFunc()
    print(result)
} catch {
    print("Error caught")
}

A Success

B No output

C Runtime crash

D Error caught

💡 Hint

The function throws an error, so the catch block runs.

Since testFunc() throws SampleError.fail, the try fails and the catch block prints "Error caught".

🔧 Debug

advanced

Identify the error in do-try-catch usage

What error does this Swift code produce when compiled?

Swift

func testFunc() throws -> String {
    return "Hello"
}

let result = try testFunc()
print(result)

A Compile-time error: 'try' must be used inside a 'do' block or marked with 'try?' or 'try!'

B Runtime error: uncaught error

C No error, prints 'Hello'

D Compile-time error: missing catch block

💡 Hint

In Swift, 'try' must be inside a do-catch or handled with try? or try! outside.

Using 'try' outside a do-catch without try? or try! causes a compile-time error.

🧠 Conceptual

advanced

Behavior of multiple catch blocks

Given this Swift code, which catch block will execute?

Swift

enum SampleError: Error {
    case fail
    case anotherFail
}

func testFunc() throws {
    throw SampleError.anotherFail
}

do {
    try testFunc()
} catch SampleError.fail {
    print("Caught fail")
} catch SampleError.anotherFail {
    print("Caught anotherFail")
} catch {
    print("Caught unknown error")
}

A Caught anotherFail

B Caught fail

C Caught unknown error

D No output

💡 Hint

The thrown error matches the second catch pattern.

The thrown error is SampleError.anotherFail, so the matching catch block prints "Caught anotherFail".

❓ Predict Output

expert

Output of nested do-try-catch blocks

What is the output of this Swift code?

Swift

enum SampleError: Error {
    case fail
}

func innerFunc() throws {
    throw SampleError.fail
}

do {
    do {
        try innerFunc()
    } catch {
        print("Inner catch")
        throw error
    }
} catch {
    print("Outer catch")
}

A Inner catch

B Outer catch

C Inner catch\nOuter catch

D No output

💡 Hint

The inner catch rethrows the error, which is caught by the outer catch.

The inner do-catch catches the error and prints "Inner catch", then rethrows it. The outer catch catches it and prints "Outer catch".