FlatMap for nested collections in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of flatMap on nested arrays

What is the output of this Swift code using flatMap on nested arrays?

Swift

let nested = [[1, 2], [3, 4], [5]]
let flat = nested.flatMap { $0 }
print(flat)

A [1, 2, 3, 4, 5]

B [[1, 2], [3, 4], [5]]

C [[1], [2], [3], [4], [5]]

D [1, 2, [3, 4], 5]

💡 Hint

flatMap flattens one level of nested arrays into a single array.

flatMap takes each inner array and combines their elements into one flat array. So [[1,2],[3,4],[5]] becomes [1,2,3,4,5].

❓ Predict Output

intermediate

flatMap with optional values in nested arrays

What is the output of this Swift code using flatMap with optional values inside nested arrays?

Swift

let nested: [[Int?]] = [[1, nil], [3, 4], [nil]]
let flat = nested.flatMap { $0.compactMap { $0 } }
print(flat)

A [1, 3, 4, nil]

B [1, nil, 3, 4, nil]

C [Optional(1), Optional(3), Optional(4)]

D [1, 3, 4]

💡 Hint

compactMap removes nil values while flatMap flattens the arrays.

compactMap removes nils from each inner array, then flatMap merges all inner arrays into one array without nils.

🔧 Debug

advanced

Output of flatMap with map transformation

What is the output of this Swift code using flatMap with map?

Swift

let nested = [[1, 2], [3, 4], [5]]
let flat = nested.flatMap { $0.map { $0 * 2 } }
print(flat)

A TypeError: Cannot convert value of type '[[Int]]' to expected argument type '[Int]'

B [2, 4, 6, 8, 10]

C [[2, 4], [6, 8], [10]]

D Runtime error: Index out of range

💡 Hint

flatMap flattens after applying the map transformation.

flatMap applies the map to each inner array doubling values, then flattens the result into a single array.

📝 Syntax

advanced

Syntax error in flatMap usage

Which option contains a syntax error when trying to flatten nested arrays with flatMap in Swift?

A let flat = nested.flatMap { $0 }

B let flat = nested.flatMap { $0.map { $0 * 2 } }

C let flat = nested.flatMap { $0 * 2 }

D let flat = nested.flatMap { $0.flatMap { [$0] } }

💡 Hint

Inside flatMap closure, you must return a sequence, not a single value.

Option C tries to multiply an array by 2 directly, which is invalid syntax in Swift.

🚀 Application

expert

Count unique words from nested sentences using flatMap

Given an array of sentences, which Swift code correctly uses flatMap to count the number of unique words across all sentences?

Swift

let sentences = ["hello world", "hello swift", "swift programming"]

A

let uniqueCount = Set(sentences.flatMap { $0.split(separator: " ") }).count
print(uniqueCount)

B

let uniqueCount = Set(sentences.map { $0.split(separator: " ") }).count
print(uniqueCount)

C

let uniqueCount = sentences.flatMap { $0.split(separator: " ") }.count
print(uniqueCount)

D

let uniqueCount = sentences.map { $0.split(separator: " ") }.count
print(uniqueCount)

💡 Hint

flatMap flattens the arrays of words before creating a Set to count unique words.

Option A splits each sentence into words, flattens all words into one array, then creates a Set to count unique words.