FlatMap for nested collections in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the flatMap function do in Kotlin?

It transforms each element of a collection into another collection and then flattens all those collections into a single list.

beginner

Given a list of lists, how does flatMap help compared to using map?

map returns a list of lists, while flatMap returns a single flattened list combining all inner lists.

beginner

Example: What is the output of listOf(listOf(1, 2), listOf(3, 4)).flatMap { it }?

The output is [1, 2, 3, 4] because flatMap flattens the nested lists into one list.

intermediate

Why is flatMap useful when working with nested collections?

It simplifies code by combining mapping and flattening steps, making it easier to work with nested data structures.

intermediate

How would you use flatMap to get all characters from a list of words in Kotlin?

Use words.flatMap { it.toList() } to convert each word to a list of characters and flatten them into one list.

What does flatMap return when applied to a list of lists?

A A set of unique elements

B A single flattened list containing all elements

C A map of keys to values

D A list of lists

flatMap flattens nested collections into one list by combining mapping and flattening.

Which Kotlin function would you use to transform and flatten a nested collection in one step?

A reduce

B filter

C fold

D flatMap

flatMap is designed to transform and flatten nested collections in one operation.

What is the difference between map and flatMap?

A <code>map</code> returns nested collections, <code>flatMap</code> flattens them

B <code>map</code> flattens collections, <code>flatMap</code> does not

C Both do the same thing

D <code>flatMap</code> only works on strings

map transforms elements but keeps nested structure; flatMap flattens the result.

What will listOf("hi", "ok").flatMap { it.toList() } return?

A [h, i]

B [hi, ok]

C [h, i, o, k]

D [o, k]

Each word is converted to a list of characters, then all characters are combined into one list.

If you want to get a list of all numbers from a list of lists of numbers, which function is best?

A flatMap

B filter

C map

D sorted

flatMap combines all inner lists into one list of numbers.

Explain how flatMap works with nested collections in Kotlin.

Think about turning many small lists into one big list.

Describe a real-life example where flatMap would simplify working with nested data.

Imagine a list of classrooms each with a list of students.