Try? for optional result in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the string to an integer.

Swift

let number = Int("123")
print([1])

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

Anumber

BInt

Ctry

D123

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int instead of the variable name.

Trying to print try keyword.

Printing the string literal instead of the variable.

The variable number holds the optional integer result from Int("123"). Printing number shows the optional value.

2fill in blank

medium

Complete the code to safely unwrap the optional integer using if let.

Swift

if let value = [1] {
    print("Value is \(value)")
} else {
    print("Conversion failed")
}

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

AInt("456")

BInt("abc")

Ctry Int("456")

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using try Int("456") causes a compile error.

Using Int("abc") which returns nil.

Using an undefined variable number.

Using Int("456") returns an optional integer. The if let safely unwraps it to value.

3fill in blank

hard

Fix the error in the code by completing the try? expression.

Swift

func parseNumber(_ text: String) throws -> Int {
    if let number = Int(text) {
        return number
    } else {
        throw NSError(domain: "ParseError", code: 1)
    }
}

let result = [1] parseNumber("789")
print(result.map(String.init) ?? "No number")

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

Atry!

Btry??

Ctry?

Dtry

Attempts:

3 left

💡 Hint

Common Mistakes

Using try without handling errors causes a compile error.

Using try! which crashes if an error is thrown.

Using invalid syntax like try??.

The try? keyword converts the throwing function result into an optional, returning nil if an error occurs.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths only for words longer than 3 characters.

Swift

let words = ["apple", "cat", "banana", "dog"]
let lengths = [word: [1] for word in words if word.[2] 3]

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

Aword.count

Bcount

Ccount >

Dcount <

Attempts:

3 left

💡 Hint

Common Mistakes

Using just count without word. prefix.

Using count < 3 which filters shorter words.

Using word.count but wrong comparison operator.

word.count gets the length of each word. The condition count > 3 filters words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values only for positive numbers.

Swift

let data = ["a": 1, "b": -2, "c": 3]
let filtered = [[1]: [2] for (key, value) in data where value [3] 0]

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

Akey.uppercased()

Bvalue

C>

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using key instead of key.uppercased().

Using value < 0 which filters negative numbers.

Swapping keys and values in the dictionary comprehension.

key.uppercased() converts keys to uppercase. The dictionary includes only entries where value > 0.

Practice

1. What does try? do when used with a throwing function in Swift?

easy

A. It runs the function and returns an optional result, nil if an error occurs.

B. It forces the function to crash if an error is thrown.

C. It catches the error and prints it automatically.

D. It converts the error into a string message.

Solution

1. Step 1: Understand try? behavior

   The try? keyword runs a throwing function and converts its result into an optional value.

2. Step 2: What happens on error?

   If the function throws an error, try? returns nil instead of crashing or propagating the error.

3. Final Answer:

   It runs the function and returns an optional result, nil if an error occurs. -> Option A

4. Quick Check:

   try? returns optional or nil on error [OK]

Hint: try? returns nil on error instead of crashing [OK]

Common Mistakes:

• Thinking try? crashes on error
• Assuming try? prints error messages
• Believing try? converts errors to strings

2. Which of the following is the correct syntax to use try? with a throwing function named loadData()?

easy

A. let data = try loadData()

B. let data = try! loadData()

C. let data = try?loadData

D. let data = try? loadData()

Solution

1. Step 1: Identify correct try? syntax

   The correct syntax requires try? followed by the function call with parentheses.

2. Step 2: Check each option

   let data = try loadData() uses try without ?, which doesn't return optional. let data = try! loadData() uses try! which forces crash on error. let data = try?loadData misses parentheses after function name.

3. Final Answer:

   let data = try? loadData() -> Option D

4. Quick Check:

   try? requires parentheses for function call [OK]

Hint: Use try? followed by function call with parentheses [OK]

Common Mistakes:

• Omitting parentheses after function name
• Using try without ? for optional
• Using try! which forces crash on error

3. Consider this Swift code:

func fetchNumber() throws -> Int {
  if Bool.random() { return 10 } else { throw NSError() }
}

let result = try? fetchNumber()
print(result)

What are possible outputs?

medium

A. Prints either Optional(10) or nil

B. Always prints Optional(10)

C. Always prints nil

D. Causes a runtime crash

Solution

1. Step 1: Understand fetchNumber behavior

   The function randomly returns 10 or throws an error.

2. Step 2: Effect of try? on fetchNumber

   Using try? converts the throwing call into an optional Int. If no error, it returns Optional(10). If error, it returns nil.

3. Final Answer:

   Prints either Optional(10) or nil -> Option A

4. Quick Check:

   try? returns optional or nil based on error [OK]

Hint: try? returns optional value or nil on error [OK]

Common Mistakes:

• Assuming always returns value
• Assuming always returns nil
• Expecting a crash on error

4. Identify the error in this Swift code snippet:

func readFile() throws -> String {
  throw NSError()
}

let content = try readFile()
print(content)

medium

A. print(content) is invalid syntax

B. Function readFile() should not throw error

C. Missing try? to handle error as optional

D. No error, code runs fine

Solution

1. Step 1: Check error handling in calling code

   The function readFile() throws an error, but the call uses plain try without handling errors.

2. Step 2: Fix error handling with try?

   Using try? converts the throwing call into an optional, preventing runtime crash and handling error gracefully.

3. Final Answer:

   Missing try? to handle error as optional -> Option C

4. Quick Check:

   Use try? to avoid crash on throwing function [OK]

Hint: Use try? to safely call throwing functions without crash [OK]

Common Mistakes:

• Using try without catch or try?
• Assuming throwing function never throws
• Thinking print causes error

5. You want to call a throwing function parseData() that returns a dictionary, but you want to safely get an empty dictionary if it fails. Which code correctly uses try? to achieve this?

hard

A. let dict = try parseData() ?? [:]

B. let dict = try? parseData() ?? [:]

C. let dict = try! parseData() ?? [:]

D. let dict = parseData() ?? [:]

Solution

1. Step 1: Understand requirement for safe call

   You want to call a throwing function and get an optional result, defaulting to empty dictionary if it fails.

2. Step 2: Use try? with nil coalescing

   Using try? converts the throwing call to optional. The ?? [:] provides empty dictionary if nil.

3. Final Answer:

   let dict = try? parseData() ?? [:] -> Option B

4. Quick Check:

   try? with ?? gives safe default on error [OK]

Hint: Combine try? with ?? to provide default value [OK]

Common Mistakes:

• Using try without ? causes crash on error
• Using try! forces crash on error
• Calling throwing function without try