Map, filter, reduce patterns in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the map function do in Swift?

The map function takes a collection and applies a transformation to each item, returning a new collection with the transformed items.

beginner

Explain the purpose of the filter function.

The filter function selects elements from a collection that meet a condition, returning a new collection with only those elements.

intermediate

What is the role of reduce in Swift collections?

reduce combines all elements of a collection into a single value by repeatedly applying a combining function, starting from an initial value.

beginner

Given an array [1, 2, 3], what is the result of map { $0 * 2 }?

The result is [2, 4, 6] because each number is multiplied by 2.

beginner

How would you use filter to get only even numbers from [1, 2, 3, 4]?

Use filter { $0 % 2 == 0 } to get [2, 4], selecting only numbers divisible by 2.

What does the map function return?

A The original collection unchanged

B A single combined value

C A new collection with transformed elements

D Only elements that meet a condition

map transforms each element and returns a new collection with those transformed elements.

Which function would you use to select elements based on a condition?

A filter

B map

C reduce

D sort

filter selects elements that satisfy a condition.

What is the initial value used for in reduce?

A To start combining elements

B To filter elements

C To transform elements

D To sort elements

The initial value is the starting point for combining elements in reduce.

What will [1, 2, 3].filter { $0 > 1 } return?

A []

B [1]

C [1, 2, 3]

D [2, 3]

It returns elements greater than 1, which are 2 and 3.

Which pattern combines all elements into one value?

A map

B reduce

C filter

D forEach

reduce combines all elements into a single value.

Describe how you would use map, filter, and reduce to process a list of numbers.

Think about changing, selecting, and summarizing data.

Explain the difference between map and filter with examples.

One changes all items, the other picks some items.

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