FlatMap for nested collections in Kotlin - Mini Project: Build & Apply

FlatMap for nested collections

📖 Scenario: You work at a bookstore that organizes books by genre. Each genre has a list of book titles. You want to create a single list of all book titles from all genres.

🎯 Goal: Build a Kotlin program that uses flatMap to combine nested lists of book titles into one flat list.

📋 What You'll Learn

Create a map called booksByGenre with genres as keys and lists of book titles as values

Create a variable called allBooks that uses flatMap on booksByGenre.values

Print the allBooks list

💡 Why This Matters

🌍 Real World

Bookstores and libraries often organize books by categories. Combining all books into one list helps with searching and displaying all available titles.

💼 Career

Understanding how to flatten nested collections is useful in data processing, backend development, and anywhere you handle grouped data.

1

Create the nested data structure

Create a map called booksByGenre with these exact entries: "Fiction" to listOf("The Alchemist", "1984"), "Science" to listOf("A Brief History of Time", "The Selfish Gene"), and "History" to listOf("Sapiens", "Guns, Germs, and Steel").

Kotlin

# Create the map booksByGenre with genres and book lists
# Your code here

Need a hint?

Use mapOf to create a map with string keys and list values.

2

Prepare to flatten the nested lists

Create a variable called allBooks and set it to booksByGenre.values. This will get all the lists of books from the map.

Kotlin

val booksByGenre = mapOf(
    "Fiction" to listOf("The Alchemist", "1984"),
    "Science" to listOf("A Brief History of Time", "The Selfish Gene"),
    "History" to listOf("Sapiens", "Guns, Germs, and Steel")
)
# Create a variable allBooks and assign booksByGenre.values
# Your code here

Need a hint?

Use booksByGenre.values to get all the lists of books.

3

Use flatMap to combine all book lists

Change the allBooks variable to use flatMap on booksByGenre.values to create a single list of all book titles. Use flatMap { it } to flatten the lists.

Kotlin

val booksByGenre = mapOf(
    "Fiction" to listOf("The Alchemist", "1984"),
    "Science" to listOf("A Brief History of Time", "The Selfish Gene"),
    "History" to listOf("Sapiens", "Guns, Germs, and Steel")
)

// Use flatMap on booksByGenre.values to create a flat list allBooks
# Your code here

Need a hint?

Use flatMap { it } to flatten the nested lists into one list.

4

Print the combined list of books

Write a println statement to print the allBooks list.

Kotlin

val booksByGenre = mapOf(
    "Fiction" to listOf("The Alchemist", "1984"),
    "Science" to listOf("A Brief History of Time", "The Selfish Gene"),
    "History" to listOf("Sapiens", "Guns, Germs, and Steel")
)

val allBooks = booksByGenre.values.flatMap { it }

// Print the allBooks list
# Your code here

Need a hint?

Use println(allBooks) to show the combined list.