FlatMap for nested collections in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of flatMap on nested lists

What is the output of this Kotlin code using flatMap on nested lists?

Kotlin

val nested = listOf(listOf(1, 2), listOf(3, 4))
val result = nested.flatMap { it.map { it * 2 } }
println(result)

A [1, 2, 3, 4]

B [4, 8]

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

D [2, 4, 6, 8]

💡 Hint

Remember that flatMap flattens one level after mapping.

The inner map doubles each number, producing [[2,4],[6,8]]. flatMap then flattens this to a single list [2,4,6,8].

❓ Predict Output

intermediate

Result size after flatMap

Given this Kotlin code, how many elements does result contain?

Kotlin

val nested = listOf(listOf("a", "b"), listOf("c"))
val result = nested.flatMap { it }
println(result.size)

A 3

B 2

C 1

D 0

💡 Hint

flatMap flattens one level of nested lists.

The nested list has 2 elements in the first list and 1 in the second, total 3 elements after flattening.

🔧 Debug

advanced

Identify the error in flatMap usage

What error does this Kotlin code produce?

Kotlin

val nested = listOf(listOf(1, 2), listOf(3, 4))
val result = nested.flatMap { it * 2 }
println(result)

A NullPointerException at runtime

B No error, output: [2, 4, 6, 8]

C Type mismatch: 'it' is List<Int>, cannot multiply by Int

D IndexOutOfBoundsException

💡 Hint

Check the operation inside the lambda for flatMap.

The lambda tries to multiply a list by 2, which is invalid in Kotlin, causing a type mismatch error.

❓ Predict Output

advanced

Output of flatMap with filtering

What is the output of this Kotlin code using flatMap with a filter?

Kotlin

val nested = listOf(listOf(1, 2, 3), listOf(4, 5))
val result = nested.flatMap { it.filter { it % 2 == 0 } }
println(result)

A [1, 3, 5]

B [2, 4]

C [2, 3, 4, 5]

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

💡 Hint

Filter keeps only even numbers before flattening.

The filter keeps only even numbers: 2 from first list, 4 from second. flatMap flattens to [2, 4].

🧠 Conceptual

expert

Why use flatMap instead of map for nested collections?

Which statement best explains why flatMap is preferred over map when working with nested collections in Kotlin?

A flatMap applies a transformation and flattens the result into a single list, avoiding nested lists.

B flatMap only works with lists of integers, while map works with any type.

C flatMap is faster because it uses parallel processing automatically.

D flatMap changes the original list, while map creates a new list.

💡 Hint

Think about the shape of the output collections.

flatMap combines mapping and flattening in one step, producing a single-level list instead of nested lists.