Try? for optional result in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does try? do in Swift?

try? attempts to run a function that can throw an error. If the function succeeds, it returns an optional with the result. If it throws an error, it returns nil instead of crashing.

beginner

How is try? different from try!?

try? returns an optional and safely handles errors by returning nil. try! forces the call and crashes if an error is thrown.

beginner

What type does a function call with try? return?

It returns an optional type wrapping the function's return type. For example, if the function returns Int, try? returns Int?.

beginner

Example: What is the output of <code>let result = try? Int("123")</code>?

The output is Optional(123) because the string "123" can be converted to an integer successfully.

beginner

Example: What happens with <code>let result = try? Int("abc")</code>?

The output is nil because "abc" cannot be converted to an integer, so the error is caught and nil is returned.

What does try? return if the function throws an error?

A The function's normal return value

B The error itself

C It crashes the program

D nil

try? returns nil when an error is thrown, preventing a crash.

Which of these is true about try? in Swift?

A It converts errors into optional nil values

B It forces the program to crash on error

C It ignores errors silently and returns a default value

D It throws errors without catching them

try? converts thrown errors into nil optionals safely.

If a function returns String, what type does try? return?

A String?

B Optional<String>

C Both A and C

D String

In Swift, String? is shorthand for Optional<String>. So both are correct.

What happens if you use try! and the function throws an error?

A The program crashes

B It returns nil

C It returns an optional

D It ignores the error

try! forces the call and crashes if an error is thrown.

Which keyword safely handles errors by returning an optional?

A catch

B try?

C try

D try!

try? safely handles errors by returning an optional result.

Explain how try? works in Swift and when you might use it.

Think about how to avoid crashes but still get a result or nil.

Compare try? and try! in Swift. What are the risks and benefits of each?

Consider what happens when an error is thrown.

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