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'.

Practice

1. What does the flatMap function do when applied to a nested array in Swift?

easy

A. It flattens the nested arrays into a single array by combining all elements.

B. It sorts the nested arrays in ascending order.

C. It removes duplicate elements from the nested arrays.

D. It converts the nested arrays into a dictionary.

Solution

1. Step 1: Understand the purpose of flatMap

   The flatMap function is designed to take a collection of collections and combine them into one single collection by flattening the nested structure.

2. Step 2: Identify the effect on nested arrays

   When applied to nested arrays, flatMap merges all inner arrays into one continuous array containing all elements.

3. Final Answer:

   It flattens the nested arrays into a single array by combining all elements. -> Option A

4. Quick Check:

   flatMap flattens nested arrays = A [OK]

Hint: Remember: flatMap flattens nested arrays into one array [OK]

Common Mistakes:

• Thinking flatMap sorts the array
• Confusing flatMap with removing duplicates
• Assuming flatMap converts arrays to dictionaries

2. Which of the following is the correct syntax to flatten a nested array let nested = [[1, 2], [3, 4]] using flatMap in Swift?

easy

A. let flat = nested.flatten()

B. let flat = nested.map { $0 }

C. let flat = nested.flatMap()

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

Solution

1. Step 1: Recall flatMap syntax

   The correct syntax for flattening nested arrays uses flatMap with a closure that returns each inner array: nested.flatMap { $0 }.

2. Step 2: Evaluate each option

   let flat = nested.flatMap { $0 } uses the correct closure syntax. let flat = nested.map { $0 } uses map which does not flatten. let flat = nested.flatMap() is invalid syntax. let flat = nested.flatten() uses a non-existent method flatten().

3. Final Answer:

   let flat = nested.flatMap { $0 } -> Option D

4. Quick Check:

   flatMap with closure { $0 } flattens nested arrays [OK]

Hint: Use flatMap with closure { $0 } to flatten nested arrays [OK]

Common Mistakes:

• Using map instead of flatMap
• Calling flatMap without closure
• Using non-existent flatten() method

3. What is the output of the following Swift code?

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

medium

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

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

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

D. [1, 4, 9, 16, 25]

Solution

1. Step 1: Understand the inner map operation

   Each inner array is mapped to a new array where each element is multiplied by 2, so [1,2] becomes [2,4], [3,4] becomes [6,8], and [5] becomes [10].

2. Step 2: Apply flatMap to flatten the mapped arrays

   The flatMap then flattens these arrays into a single array: [2, 4, 6, 8, 10].

3. Final Answer:

   [2, 4, 6, 8, 10] -> Option B

4. Quick Check:

   flatMap + map doubles and flattens = [2,4,6,8,10] [OK]

Hint: flatMap flattens after map transforms inner elements [OK]

Common Mistakes:

• Confusing flatMap with map output
• Expecting nested arrays instead of flattened
• Multiplying elements incorrectly

4. Identify the error in the following Swift code that attempts to flatten a nested array:

let nested = [["a", "b"], ["c"]]
let flat = nested.flatMap()
print(flat)

medium

A. flatMap requires a closure to specify how to flatten the nested arrays.

B. flatMap cannot be used on arrays of strings.

C. The variable 'flat' is immutable and cannot be assigned.

D. The print statement syntax is incorrect.

Solution

1. Step 1: Check flatMap usage

   The flatMap function requires a closure to specify how to flatten the nested arrays, such as { $0 }. Calling flatMap() without arguments is invalid.

2. Step 2: Verify other parts of the code

   Arrays of strings can be flattened with flatMap, variables can be assigned with let, and print syntax is correct.

3. Final Answer:

   flatMap requires a closure to specify how to flatten the nested arrays. -> Option A

4. Quick Check:

   flatMap needs closure { $0 } to flatten [OK]

Hint: Always provide closure { $0 } when using flatMap on nested arrays [OK]

Common Mistakes:

• Calling flatMap without closure
• Thinking flatMap can't flatten strings
• Misunderstanding let variable assignment
• Assuming print syntax is wrong

5. Given a nested array of optional integers let nested: [[Int?]] = [[1, nil], [3, 4], [nil]], which Swift code using flatMap correctly produces a flat array of non-nil integers?

hard

A. let flat = nested.flatMap { $0.filter { $0 != nil } }

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

C. let flat = nested.flatMap { $0.compactMap { $0 } }

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

Solution

1. Step 1: Understand the data and goal

   The nested array contains optional integers, some are nil. The goal is to flatten and remove nil values, keeping only non-nil integers.

2. Step 2: Analyze each option

   let flat = nested.flatMap { $0.compactMap { $0 } } uses compactMap inside flatMap to unwrap and remove nils, then flatten. let flat = nested.flatMap { $0.map { $0! } } force unwraps which can crash if nil exists. let flat = nested.flatMap { $0.filter { $0 != nil } } filters nil but keeps optionals, so result is nested optionals. let flat = nested.flatMap { $0 } just flattens but keeps nils.

3. Final Answer:

   let flat = nested.flatMap { $0.compactMap { $0 } } -> Option C

4. Quick Check:

   flatMap + compactMap unwraps and flattens non-nil values [OK]

Hint: Use compactMap inside flatMap to remove nils and flatten [OK]

Common Mistakes:

• Force unwrapping optionals causing runtime errors
• Filtering nil but not unwrapping optionals
• Flattening without removing nil values