Throwing functions with throws in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a throwing function call

What is the output of this Swift code when calling checkNumber(5)?

Swift

enum NumberError: Error {
    case tooSmall
}

func checkNumber(_ num: Int) throws -> String {
    if num < 10 {
        throw NumberError.tooSmall
    }
    return "Number is \(num)"
}

var result: String

do {
    result = try checkNumber(5)
} catch {
    result = "Error caught"
}
print(result)

A Error caught

B Number is 5

C Compilation error

D Runtime crash

💡 Hint

Remember that the function throws an error if the number is less than 10.

The function throws an error because 5 is less than 10. The catch block catches the error and sets result to "Error caught".

❓ Predict Output

intermediate

Value of variable after try? usage

What is the value of output after running this Swift code?

Swift

enum MyError: Error {
    case fail
}

func riskyFunction(_ flag: Bool) throws -> String {
    if flag {
        return "Success"
    } else {
        throw MyError.fail
    }
}

let output = try? riskyFunction(false)
print(output ?? "nil")

A nil

B Success

C Compilation error

D Runtime crash

💡 Hint

The try? converts errors to nil.

Since riskyFunction throws an error when flag is false, try? returns nil instead of throwing, so output is nil.

🔧 Debug

advanced

Identify the error in throwing function usage

What error does this Swift code produce when compiled?

Swift

enum SampleError: Error {
    case errorCase
}

func test() throws {
    throw SampleError.errorCase
}

func callTest() {
    try test()
    print("Done")
}

A No error, prints "Done"

B Error: Call can throw but is not marked with try

C Error: Missing catch block for try

D Error: Function callTest must be marked with throws

💡 Hint

Check the function call inside callTest and its error handling.

The function callTest calls a throwing function but is not marked with throws or handling the error, so compiler requires callTest to be marked throws.

📝 Syntax

advanced

Correct syntax for throwing function declaration

Which option shows the correct way to declare a throwing function in Swift?

A

func fetchData() throw -> String {
    return "Data"
}

B

func fetchData() -> throws String {
    return "Data"
}

C

func fetchData() throws -> String {
    return "Data"
}

D

func fetchData() throws String {
    return "Data"
}

💡 Hint

The keyword 'throws' comes after the parentheses and before the return arrow.

The correct syntax places 'throws' after the parameter list and before the return type arrow.

🚀 Application

expert

Number of items in dictionary after throwing function filter

Given this Swift code, how many items are in the resulting dictionary filteredDict?

Swift

enum FilterError: Error {
    case invalidValue
}

func filterDict(_ dict: [Int: Int]) throws -> [Int: Int] {
    var result = [Int: Int]()
    for (key, value) in dict {
        if value < 0 {
            throw FilterError.invalidValue
        }
        if value % 2 == 0 {
            result[key] = value
        }
    }
    return result
}

let inputDict = [1: 2, 2: -1, 3: 4, 4: 5]

var filteredDict = [Int: Int]()
do {
    filteredDict = try filterDict(inputDict)
} catch {
    filteredDict = [:]
}
print(filteredDict.count)

A 2

B 0

C 1

D 3

💡 Hint

The function throws an error if any value is negative, causing the catch block to run.

Since the dictionary contains a negative value (-1), the function throws an error and the catch block sets filteredDict to empty, so count is 0.