Map, filter, reduce patterns in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map and filter combination

What is the output of the following Swift code?

Swift

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

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

B [2, 4]

C [6, 8, 10]

D [10]

💡 Hint

First multiply each number by 2, then keep only those greater than 5.

The map doubles each number: [2, 4, 6, 8, 10]. The filter keeps numbers > 5: [6, 8, 10].

❓ Predict Output

intermediate

Reduce to sum even numbers

What is the output of this Swift code?

Swift

let numbers = [1, 2, 3, 4, 5, 6]
let sumEven = numbers.filter { $0 % 2 == 0 }.reduce(0) { $0 + $1 }
print(sumEven)

A 21

B 12

C 0

D 6

💡 Hint

Filter even numbers, then add them all together.

Even numbers are [2, 4, 6]. Their sum is 2 + 4 + 6 = 12.

🔧 Debug

advanced

Identify the error in reduce usage

What error does this Swift code produce?

Swift

let words = ["apple", "banana", "cherry"]
let sentence = words.reduce { $0 + ", " + $1 }
print(sentence)

A Runtime error: index out of range

B Missing initial result in reduce causes a compile error

C Type mismatch error

D No error, prints "apple, banana, cherry"

💡 Hint

Reduce without an initial value uses the first element as the initial accumulator if the array is non-empty.

No error. Swift's `reduce` without an initial value uses the first element ("apple") as the starting accumulator and applies the closure to combine with subsequent elements: "apple, banana" then "apple, banana, cherry".

❓ Predict Output

advanced

Chained map, filter, reduce output

What is the output of this Swift code?

Swift

let values = [3, 6, 9, 12]
let result = values.map { $0 / 3 }.filter { $0 % 2 == 0 }.reduce(1) { $0 * $1 }
print(result)

A 8

B 24

C 6

D 1

💡 Hint

Divide each by 3, keep even numbers, then multiply all starting from 1.

Map: [1, 2, 3, 4]. Filter even: [2, 4]. Reduce multiply: 1 * 2 * 4 = 8.

🧠 Conceptual

expert

Understanding reduce initial value impact

Given the array [2, 4, 6], what is the value of 'result' after running this Swift code? let result = [2, 4, 6].reduce(10) { acc, val in acc - val }

A −2

B −18

C 12

D 10

💡 Hint

Start from 10, subtract each element in order.

Calculation: 10 - 2 = 8; 8 - 4 = 4; 4 - 6 = -2.