CompactMap for optional unwrapping in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using compactMap?

Consider the following Swift code snippet that uses compactMap to unwrap optionals. What will be printed?

Swift

let numbers: [String?] = ["1", nil, "3", "five", "7"]
let validInts = numbers.compactMap { Int($0 ?? "") }
print(validInts)

A [1, 3, 0, 7]

B [1, 3, 7]

C [1, 3, nil, 7]

D Compilation error

💡 Hint

Remember that compactMap removes nil values after transformation.

The compactMap tries to convert each optional string to an Int. If conversion fails or the value is nil, it is skipped. "five" cannot be converted, so it is removed. nil is also removed. The result is [1, 3, 7].

❓ Predict Output

intermediate

What does this compactMap expression produce?

Given this Swift code, what is the value of result?

Swift

let array: [String?] = ["10", "20", nil, "abc", "30"]
let result = array.compactMap { $0 }.compactMap { Int($0) }
print(result)

A [10, 20, 30]

B ["10", "20", "30"]

C [10, 20, nil, 30]

D Runtime error

💡 Hint

Two compactMaps are used: first to remove nils, second to convert strings to Ints.

The first compactMap removes nil values, resulting in ["10", "20", "abc", "30"]. The second compactMap converts strings to Ints, skipping "abc" which fails conversion. Final result is [10, 20, 30].

🔧 Debug

advanced

Why does this compactMap code cause a runtime error?

Examine this Swift code snippet. It compiles but crashes at runtime. What causes the crash?

Swift

let strings: [String?] = ["5", nil, "15"]
let numbers = strings.compactMap { Int($0!) }
print(numbers)

A The array contains invalid strings causing crash.

B compactMap cannot be used with optionals.

C Int initializer throws an error for nil strings.

D Force unwrapping nil causes a runtime crash.

💡 Hint

Look at the use of force unwrap (!) inside the closure.

The code force unwraps $0 with $0!, but some elements are nil. Force unwrapping nil causes a runtime crash (fatal error).

📝 Syntax

advanced

Which option correctly uses compactMap to unwrap optionals and convert to Int?

Choose the Swift code snippet that compiles and produces [2, 4, 6] from the array ["2", nil, "4", "six", "6"] using compactMap.

A

let arr: [String?] = ["2", nil, "4", "six", "6"]
let nums = arr.compactMap { $0 != nil ? Int($0!) : nil }
print(nums)

B

let arr: [String?] = ["2", nil, "4", "six", "6"]
let nums = arr.compactMap { Int($0) }
print(nums)

C

let arr: [String?] = ["2", nil, "4", "six", "6"]
let nums = arr.compactMap { Int($0 ?? "") }
print(nums)

D

let arr: [String?] = ["2", nil, "4", "six", "6"]
let nums = arr.compactMap { Int($0!) }
print(nums)

💡 Hint

Avoid force unwrapping nil values to prevent crashes.

Option C safely unwraps optionals with $0 ?? "" and tries to convert to Int. Invalid strings convert to nil and are removed. Option C force unwraps nil causing a crash. Option C tries to convert optional String? directly to Int which is invalid.

🚀 Application

expert

How many elements are in the resulting array after this compactMap chain?

Given this Swift code, how many elements does finalArray contain?

Swift

let data: [String?] = ["100", "abc", nil, "200", "300", "xyz", nil]
let finalArray = data.compactMap { $0 }.compactMap { Int($0) }

A 3

B 4

C 5

D 6

💡 Hint

Count how many strings convert successfully to Int after removing nils.

First compactMap removes nils: ["100", "abc", "200", "300", "xyz"]. Second compactMap converts to Int, skipping "abc" and "xyz". Valid ints are 100, 200, 300, so 3 elements.