Map, filter, reduce patterns in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new array with each number doubled using map.

Swift

let numbers = [1, 2, 3, 4]
let doubled = numbers.[1] { $0 * 2 }

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

Afilter

Breduce

Cmap

DcompactMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter instead of map, which only selects elements but does not transform them.

The map function applies the given closure to each element and returns a new array with the results.

2fill in blank

medium

Complete the code to filter out numbers less than 3.

Swift

let numbers = [1, 2, 3, 4, 5]
let filtered = numbers.[1] { $0 >= 3 }

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

Afilter

Breduce

Cmap

DflatMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of filter, which transforms elements but does not remove any.

The filter function returns a new array containing only elements that satisfy the condition.

3fill in blank

hard

Fix the error in the code to sum all numbers using reduce.

Swift

let numbers = [1, 2, 3, 4]
let sum = numbers.reduce([1]) { $0 + $1 }

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

A1

B0

C[]

D""

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as initial value, which adds an extra 1 to the sum.

Using an empty array or string, which causes a type error.

The initial value for summing numbers should be 0, so the sum starts from zero.

4fill in blank

hard

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

Swift

let words = ["apple", "bat", "car", "door"]
let lengths = Dictionary(uniqueKeysWithValues: words.filter { $0.count [1] 3 }.map { [2] in ([2], [2].count) })

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

A>

B<

Cword

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in the filter condition.

Using 'count' instead of 'word' as the key in the tuple.

We filter words with length greater than 3, then map each word to a tuple of (word, length).

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase words as keys, their counts as values, filtering words with count greater than 1.

Swift

let words = ["apple", "bat", "apple", "car", "bat", "bat"]
let counts = words.reduce(into: [:]) { counts, [1] in
    counts[[2]] = (counts[[3]] ?? 0) + 1
}
let filteredCounts = counts.filter { $0.value > 1 }

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

Aword

Bword.uppercased()

Attempts:

3 left

💡 Hint

Common Mistakes

Using the lowercase word as key instead of uppercase.

Using inconsistent keys for reading and writing counts.

We count occurrences of uppercase words. The key in the dictionary is the uppercase word, so we use word.uppercased() for keys and lookups.

Practice

1. What does the map function do in Swift when applied to an array?

easy

A. It changes each item in the array based on a rule.

B. It removes items that don't match a condition.

C. It combines all items into a single value.

D. It sorts the array in ascending order.

Solution

1. Step 1: Understand the purpose of map

   The map function applies a transformation to each element in the array, creating a new array with the changed items.

2. Step 2: Compare with other functions

   Unlike filter which removes items, or reduce which combines items, map changes each item individually.

3. Final Answer:

   It changes each item in the array based on a rule. -> Option A

4. Quick Check:

   map = change each item [OK]

Hint: Map transforms every item, filter picks, reduce combines [OK]

Common Mistakes:

• Confusing map with filter or reduce
• Thinking map removes items
• Assuming map combines items into one

2. Which of the following is the correct syntax to use filter on an array numbers to keep only even numbers?

easy

A. let evens = numbers.map { $0 % 2 == 0 }

B. let evens = numbers.filter($0 % 2 == 0)

C. let evens = numbers.reduce(0) { $0 % 2 == 0 }

D. let evens = numbers.filter { $0 % 2 == 0 }

Solution

1. Step 1: Identify correct filter syntax

   The filter method uses a closure with a condition inside curly braces, like { $0 % 2 == 0 }, to keep items matching the condition.

2. Step 2: Check each option

   let evens = numbers.filter { $0 % 2 == 0 } correctly uses filter with the closure. let evens = numbers.map { $0 % 2 == 0 } uses map which changes items, not filters. let evens = numbers.reduce(0) { $0 % 2 == 0 } misuses reduce. let evens = numbers.filter($0 % 2 == 0) misses the closure braces.

3. Final Answer:

   let evens = numbers.filter { $0 % 2 == 0 } -> Option D

4. Quick Check:

   filter syntax = filter { condition } [OK]

Hint: Filter needs braces and a condition inside { } [OK]

Common Mistakes:

• Using map instead of filter
• Omitting braces in closure
• Using reduce incorrectly for filtering

3. What is the output of this Swift code?

let numbers = [1, 2, 3, 4]
let result = numbers.reduce(0) { $0 + $1 }
print(result)

medium

A. 0

B. [1, 2, 3, 4]

C. 10

D. Error

Solution

1. Step 1: Understand reduce with initial value 0

   The reduce starts with 0 and adds each number in the array: 0 + 1 + 2 + 3 + 4.

2. Step 2: Calculate the sum

   Adding all numbers: 0 + 1 = 1, 1 + 2 = 3, 3 + 3 = 6, 6 + 4 = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Sum of [1,2,3,4] = 10 [OK]

Hint: Reduce with + sums all items starting from initial value [OK]

Common Mistakes:

• Confusing reduce with map or filter output
• Forgetting initial value in reduce
• Expecting array output instead of single value

4. Identify the error in this Swift code snippet:

let words = ["apple", "banana", "cherry"]
let filtered = words.filter { word in
    word.count > 5
}
print(filtered)

medium

A. No error, code runs correctly

B. Incorrect use of filter instead of map

C. Using word.count instead of word.length

D. Missing return keyword inside filter closure

Solution

1. Step 1: Check filter closure syntax

   The closure uses implicit return because it is a single expression, so no explicit return is needed.

2. Step 2: Verify property usage

   word.count is the correct way to get string length in Swift, so no error there.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Single-expression closures return implicitly [OK]

Hint: Single-expression closures return automatically, no return needed [OK]

Common Mistakes:

• Adding unnecessary return keyword
• Confusing count with length property
• Thinking filter needs map syntax

5. Given an array of integers, how would you use map, filter, and reduce together to find the sum of squares of only the odd numbers? Choose the correct Swift code.

hard

A. let sum = numbers.map { $0 * $0 }.filter { $0 % 2 != 0 }.reduce(0, +)

B. let sum = numbers.filter { $0 % 2 != 0 }.map { $0 * $0 }.reduce(0, +)

C. let sum = numbers.reduce(0) { $0 + $1 * $1 }

D. let sum = numbers.filter { $0 % 2 == 0 }.map { $0 * $0 }.reduce(0, +)

Solution

1. Step 1: Filter odd numbers first

   We want only odd numbers, so use filter { $0 % 2 != 0 } to keep them.

2. Step 2: Map to square each odd number

   Use map { $0 * $0 } to get squares of the filtered odd numbers.

3. Step 3: Reduce to sum all squares

   Use reduce(0, +) to add all squared values into one sum.

4. Final Answer:

   let sum = numbers.filter { $0 % 2 != 0 }.map { $0 * $0 }.reduce(0, +) -> Option B

5. Quick Check:

   Filter odds, map squares, reduce sum [OK]

Hint: Filter odds first, then map squares, finally reduce sum [OK]

Common Mistakes:

• Mapping before filtering odd numbers
• Filtering even numbers by mistake
• Using reduce without mapping squares