FlatMap for nested collections in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to flatten the nested arrays into a single array.

Swift

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

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

Amap

BflatMap

Cfilter

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of flatMap results in a nested array.

Using filter or reduce does not flatten the array.

The flatMap function flattens nested collections by applying a closure and then flattening the result. Here, it flattens the nested arrays into one array.

2fill in blank

medium

Complete the code to flatten and then double each number in the nested arrays.

Swift

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

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

AflatMap

Breduce

Cfilter

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using map only results in a nested array of doubled numbers.

Using filter or reduce does not flatten the array.

flatMap is used here to flatten the nested arrays after doubling each element inside them.

3fill in blank

hard

Fix the error in the code to flatten the nested arrays correctly.

Swift

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

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

Afilter

Bmap

CflatMap

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of flatMap causes nested arrays in the output.

Using flatMap correctly flattens the nested arrays. Using map would keep the nested structure.

4fill in blank

hard

Fill both blanks to create a dictionary with words as keys and their lengths as values, only for words longer than 3 letters.

Swift

let words = ["apple", "bat", "car", "dolphin"]
let lengths = Dictionary(uniqueKeysWithValues: words.filter { word in word.count > 3 }.map { word in ([1], [2]) })

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

Aword

Bword.count

Clen(word)

Dword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using Python syntax like len(word) instead of Swift's word.count.

Using word.length which is not valid in Swift.

The map closure uses word as the key and word.count as the value to create the dictionary for words longer than 3 letters.

5fill in blank

hard

Fill all three blanks to flatten a nested array of strings, convert each to uppercase, and filter only those starting with 'A'.

Swift

let nested = [["apple", "banana"], ["avocado", "berry"], ["apricot"]]
let result = nested.[1] { $0.map { $0.[2]() }.filter { $0.hasPrefix([3]) } }
print(result)

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

AflatMap

Bmap

C"A"

Duppercased

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of flatMap causes nested arrays in the result.

Using lowercase 'a' in hasPrefix causes no matches.

Forgetting to call uppercased() as a method.

flatMap flattens the nested arrays, uppercased() converts strings to uppercase, and filtering with hasPrefix("A") keeps only strings starting with 'A'.