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".

Practice

1. What is the main purpose of using do-try-catch in Swift?

easy

A. To handle errors safely without crashing the program

B. To create loops that run multiple times

C. To define new functions

D. To declare variables with default values

Solution

1. Step 1: Understand the role of do-try-catch

   This structure is used to run code that might throw errors and handle those errors gracefully.

2. Step 2: Identify the purpose

   It prevents the program from crashing by catching errors and allowing alternative actions.

3. Final Answer:

   To handle errors safely without crashing the program -> Option A

4. Quick Check:

   Error handling = A [OK]

Hint: Remember: try means risky code, catch means safe backup [OK]

Common Mistakes:

• Confusing error handling with loops
• Thinking try-catch defines functions
• Assuming it declares variables

2. Which of the following is the correct syntax to start a do-try-catch block in Swift?

easy

A. try someFunction() do { catch handleError() }

B. try { do someFunction() } catch { handleError() }

C. catch { try someFunction() } do { handleError() }

D. do { try someFunction() } catch { handleError() }

Solution

1. Step 1: Recall correct do-try-catch syntax

   The correct syntax starts with do {}, inside which try is used before the function that might throw.

2. Step 2: Match syntax with options

   do { try someFunction() } catch { handleError() } correctly places do block, uses try inside, and follows with catch block.

3. Final Answer:

   do { try someFunction() } catch { handleError() } -> Option D

4. Quick Check:

   Correct syntax = D [OK]

Hint: Remember: do { try ... } catch { ... } is the pattern [OK]

Common Mistakes:

• Placing try outside do block
• Swapping catch and do blocks
• Missing curly braces

3. What will be the output of the following Swift code?

enum MyError: Error { case fail }

func risky() throws {
  throw MyError.fail
}

do {
  try risky()
  print("Success")
} catch {
  print("Caught an error")
}

medium

A. Success

B. Caught an error

C. No output

D. Compile-time error

Solution

1. Step 1: Analyze the function risky()

   This function always throws an error of type MyError.fail.

2. Step 2: Trace the do-try-catch block

   When try risky() runs, it throws an error, so the code jumps to the catch block and prints "Caught an error".

3. Final Answer:

   Caught an error -> Option B

4. Quick Check:

   Error thrown = catch block runs = B [OK]

Hint: If try throws, catch block runs, not code after try [OK]

Common Mistakes:

• Assuming code after try runs despite error
• Thinking no output occurs
• Confusing compile error with runtime error

4. Identify the error in this Swift code snippet:

do {
  try someFunction()
} catch error {
  print("Error: \(error)")
}

func someFunction() {
  print("Running")
}

medium

A. Function someFunction() must be marked with throws

B. Catch block syntax is incorrect

C. Missing do keyword

D. Print statement inside catch is invalid

Solution

1. Step 1: Check function declaration

   The function someFunction() is called with try, so it must be declared with throws to indicate it can throw errors.

2. Step 2: Identify mismatch causing error

   Since someFunction() is not marked throws, the compiler will raise an error.

3. Final Answer:

   Function someFunction() must be marked with throws -> Option A

4. Quick Check:

   try requires throws function = C [OK]

Hint: Only functions marked throws can be called with try [OK]

Common Mistakes:

• Ignoring throws keyword on function
• Thinking catch syntax is wrong
• Assuming print inside catch is invalid

5. Given this Swift code, what will be printed?

enum FileError: Error { case notFound, unreadable }

func readFile(_ name: String) throws -> String {
  if name == "missing.txt" { throw FileError.notFound }
  if name == "bad.txt" { throw FileError.unreadable }
  return "File content"
}

do {
  let content = try readFile("bad.txt")
  print(content)
} catch FileError.notFound {
  print("File not found")
} catch {
  print("Other error")
}

hard

A. File content

B. File not found

C. Other error

D. No output

Solution

1. Step 1: Analyze the function call

   Calling readFile("bad.txt") throws FileError.unreadable.

2. Step 2: Check catch blocks for matching error

   The first catch handles FileError.notFound, which does not match. The second catch is a general catch that handles all other errors, so it runs and prints "Other error".

3. Final Answer:

   Other error -> Option C

4. Quick Check:

   Specific catch misses, general catch runs = A [OK]

Hint: Specific catch runs only if error matches exactly [OK]

Common Mistakes:

• Assuming first catch runs for all errors
• Thinking no output occurs
• Confusing error types