Why error handling is explicit in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that can throw an error.

Swift

func readFile() [1] { }

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

Athrows

Bthrow

Ctry

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' instead of 'throws' to declare the function.

Confusing 'try' with the function declaration keyword.

The throws keyword declares that the function can throw an error.

2fill in blank

medium

Complete the code to call a throwing function with error handling.

Swift

do {
    try readFile()
} [1] {
    print("Error occurred")
}

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

Atry

Bthrow

Cthrows

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throws' instead of 'catch' after the do block.

Placing 'try' outside the do block.

The catch block handles errors thrown by the try call.

3fill in blank

hard

Fix the error in the code to correctly handle errors when calling a throwing function.

Swift

let result = [1] readFile()

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

AreadFile()

Bthrows

Ctry

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the function without 'try' causes a compile error.

Using 'throws' instead of 'try' when calling the function.

The try keyword is required before calling a function that can throw an error.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters keys with values greater than 10.

Swift

let filtered = dictionary.filter { $0.value [1] 10 }.map { ($0.key, $0.value) }.reduce(into: [:]) { $0[$1.[2]] = $1.1 }

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

A>

B<

Ckey

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the filter condition.

Using 'value' instead of 'key' when accessing dictionary keys.

The filter uses > to select values greater than 10, and key extracts the dictionary key.

5fill in blank

hard

Fill all three blanks to create a throwing function that reads a file and returns its content length if successful.

Swift

func readFile() [1] -> Int {{
    let content = try String(contentsOfFile: "file.txt")
    return content.[2]
}}

let length = try? readFile() [3] nil

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

Athrows

Bcount

C??

Dtry

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting 'throws' in the function declaration.

Using 'length' instead of 'count' for string length.

Not using '??' to handle the optional result.

The function declares throws, returns the count of the content string, and uses ?? to provide a default value if the call fails.

Practice

1. Why does Swift require error handling to be explicit using throws and try?

easy

A. To make sure programmers handle errors clearly and avoid unexpected crashes

B. To make the code run faster by skipping error checks

C. To hide errors from users automatically

D. To allow errors to be ignored silently

Solution

1. Step 1: Understand Swift's error handling design

   Swift forces explicit error handling to make sure errors are noticed and managed properly.

2. Step 2: Recognize the benefit of explicit handling

   This avoids unexpected crashes and makes programs more reliable and easier to maintain.

3. Final Answer:

   To make sure programmers handle errors clearly and avoid unexpected crashes -> Option A

4. Quick Check:

   Explicit error handling = clear, safe code [OK]

Hint: Explicit means you must handle errors clearly every time [OK]

Common Mistakes:

• Thinking errors are handled automatically
• Believing explicit handling slows down code
• Confusing explicit with optional error handling

2. Which of the following is the correct way to call a function that can throw an error in Swift?

easy

A. let result = someFunction()

B. let result = try someFunction()

C. let result = throws someFunction()

D. let result = catch someFunction()

Solution

1. Step 1: Recall Swift syntax for calling throwing functions

   Functions that can throw errors must be called with the try keyword.

2. Step 2: Identify the correct syntax

   Only let result = try someFunction() uses the correct keyword try before the function call.

3. Final Answer:

   let result = try someFunction() -> Option B

4. Quick Check:

   Use try before throwing functions [OK]

Hint: Use 'try' before calling throwing functions [OK]

Common Mistakes:

• Omitting 'try' when calling throwing functions
• Using 'throws' instead of 'try' when calling
• Using 'catch' incorrectly in function calls

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

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

func test() {
  do {
    let result = try canThrow()
    print(result)
  } catch {
    print("Error caught")
  }
}
test()

medium

A. Success

B. Error caught

C. No output

D. Compile error

Solution

1. Step 1: Analyze the throwing function behavior

   The function canThrow() is marked as throwing but always returns "Success" without throwing an error.

2. Step 2: Understand the do-catch block execution

   The try succeeds, so print(result) prints "Success" and the catch block is skipped.

3. Final Answer:

   Success -> Option A

4. Quick Check:

   try succeeds, prints result [OK]

Hint: If no error thrown, do-catch prints success path [OK]

Common Mistakes:

• Assuming catch runs even if no error
• Thinking throws always cause errors
• Expecting compile error without catch

4. Identify the error in this Swift code snippet:

func risky() throws -> Int {
  return 42
}

let value = risky()

medium

A. Cannot assign function result to a constant

B. Function must return a String, not Int

C. Missing 'throws' keyword in function declaration

D. Missing 'try' keyword when calling a throwing function

Solution

1. Step 1: Check function declaration and call

   The function risky() is declared with throws, so it can throw errors.

2. Step 2: Verify calling syntax

   Calling a throwing function requires the try keyword, which is missing here.

3. Final Answer:

   Missing 'try' keyword when calling a throwing function -> Option D

4. Quick Check:

   Call throwing functions with 'try' [OK]

Hint: Always use 'try' when calling throwing functions [OK]

Common Mistakes:

• Calling throwing functions without 'try'
• Confusing return types
• Ignoring function's throws declaration

5. You want to write a Swift function that reads a file and returns its content as a String. The file reading can fail. How should you design this function to follow Swift's explicit error handling?

hard

A. Use do-catch inside the function but do not mark it with throws

B. Return an optional String and ignore errors silently

C. Declare the function with throws and use try when calling file reading methods

D. Catch errors inside the function and print them, then return an empty String

Solution

1. Step 1: Understand Swift's explicit error handling philosophy

   Functions that can fail should be marked with throws to force callers to handle errors explicitly.

2. Step 2: Apply correct design for file reading

   The function should declare throws and use try when calling file reading methods that can throw errors.

3. Final Answer:

   Declare the function with throws and use try when calling file reading methods -> Option C

4. Quick Check:

   Use throws and try for error propagation [OK]

Hint: Mark throwing functions with 'throws' and call with 'try' [OK]

Common Mistakes:

• Ignoring errors by returning optionals
• Catching errors inside without propagating
• Returning empty strings on failure silently