Do-try-catch execution flow in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the function that might throw an error.

Swift

do {
    try [1]()
} catch {
    print("Error caught")
}

Drag options to blanks, or click blank then click option'

AriskyFunction

BsafeFunction

CprintError

DhandleError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that does not throw with try.

Forgetting to use try before the function call.

The try keyword is used before calling a function that can throw an error. Here, riskyFunction() is the function that might throw.

2fill in blank

medium

Complete the code to catch a specific error type.

Swift

do {
    try riskyFunction()
} catch [1] {
    print("Specific error caught")
}

Drag options to blanks, or click blank then click option'

Alet error as NSError

Blet error as MyError

Clet error as String

Dlet error as Int

Attempts:

3 left

💡 Hint

Common Mistakes

Catching errors as the wrong type.

Using types that are not error types.

To catch a specific error type, you use catch let error as MyError. This matches errors of type MyError.

3fill in blank

hard

Fix the error in the catch block to properly handle the error.

Swift

do {
    try riskyFunction()
} catch {
    print([1])
}

Drag options to blanks, or click blank then click option'

Aerror.message

Berror

Cerror.description

Derror.localizedDescription

Attempts:

3 left

💡 Hint

Common Mistakes

Using properties that do not exist on the error object.

Printing the error object directly without description.

Inside a catch block, error.localizedDescription gives a readable message about the error.

4fill in blank

hard

Fill both blanks to create a do-try-catch block that handles two different error types.

Swift

do {
    try riskyFunction()
} catch [1] {
    print("First error caught")
} catch [2] {
    print("Second error caught")
}

Drag options to blanks, or click blank then click option'

Alet error as MyError

Blet error as NSError

Clet error as String

Dlet error as Int

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-error types in catch blocks.

Mixing up the order of catch blocks.

The first catch handles MyError type errors, the second handles NSError type errors.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps error codes to messages inside a do-try-catch block.

Swift

do {
    try riskyFunction()
} catch let error as [1] {
    let errorCode = error.code
    let errorMessages = [errorCode: [2]]
    print(errorMessages[[3]] ?? "Unknown error")
}

Drag options to blanks, or click blank then click option'

ANSError

Berror.localizedDescription

CerrorCode

DMyError

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong error types that don't have a code property.

Mixing keys and values in the dictionary.

Accessing the dictionary with wrong keys.

The catch block catches NSError. The dictionary maps errorCode to error.localizedDescription. The print statement accesses the message by errorCode.

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