Map creation (mapOf, mutableMapOf) in Kotlin - Mini Project: Build & Apply

Create and Use Maps with mapOf and mutableMapOf in Kotlin

📖 Scenario: You are organizing a small library. You want to keep track of book titles and their authors using a map.

🎯 Goal: Build a Kotlin program that creates a map of books and authors, adds a new book, and then prints all entries.

📋 What You'll Learn

Create an immutable map using mapOf with given book-author pairs

Create a mutable map using mutableMapOf from the immutable map

Add a new book and author to the mutable map

Print all book titles and authors from the mutable map

💡 Why This Matters

🌍 Real World

Maps are useful to store pairs of related data, like book titles and authors, product names and prices, or usernames and emails.

💼 Career

Understanding how to create and manipulate maps is important for many programming jobs, especially when working with data collections and key-value pairs.

1

Create an immutable map of books and authors

Create an immutable map called books using mapOf with these exact entries: "1984" to "George Orwell", "Brave New World" to "Aldous Huxley", and "Fahrenheit 451" to "Ray Bradbury".

Kotlin

# Create an immutable map called books with the given book-author pairs
# Your code here

Hint

Use mapOf with pairs like "key" to "value" inside parentheses.

2

Create a mutable map from the immutable map

Create a mutable map called mutableBooks using mutableMapOf and initialize it with the entries from the books map.

Kotlin

val books = mapOf("1984" to "George Orwell", "Brave New World" to "Aldous Huxley", "Fahrenheit 451" to "Ray Bradbury")
# Create a mutable map called mutableBooks from books
# Your code here

Hint

Create an empty mutable map with mutableMapOf<String, String>() and then add all entries from books using putAll.

3

Add a new book and author to the mutable map

Add a new entry to mutableBooks with the key "The Hobbit" and the value "J.R.R. Tolkien".

Kotlin

val books = mapOf("1984" to "George Orwell", "Brave New World" to "Aldous Huxley", "Fahrenheit 451" to "Ray Bradbury")
val mutableBooks = mutableMapOf()
mutableBooks.putAll(books)
# Add the book "The Hobbit" by "J.R.R. Tolkien" to mutableBooks
# Your code here

Hint

Use the bracket notation mutableBooks["The Hobbit"] = "J.R.R. Tolkien" to add a new entry.

4

Print all book titles and authors

Use a for loop with variables title and author to iterate over mutableBooks.entries and print each book and author in the format: "Title: [title], Author: [author]".

Kotlin

val books = mapOf("1984" to "George Orwell", "Brave New World" to "Aldous Huxley", "Fahrenheit 451" to "Ray Bradbury")
val mutableBooks = mutableMapOf()
mutableBooks.putAll(books)
mutableBooks["The Hobbit"] = "J.R.R. Tolkien"
# Print all books and authors in the specified format
# Your code here

Hint

Use a for loop with for ((title, author) in mutableBooks.entries) and print with println("Title: $title, Author: $author").

Practice

1. What does mapOf create in Kotlin?

easy

A. A list of values without keys

B. A mutable map that can be changed later

C. A fixed map that cannot be changed after creation

D. An empty map with no keys or values

Solution

1. Step 1: Understand mapOf purpose

   mapOf creates a map with key-value pairs that cannot be modified after creation.

2. Step 2: Compare with other map types

   mutableMapOf allows changes, but mapOf does not.

3. Final Answer:

   A fixed map that cannot be changed after creation -> Option C

4. Quick Check:

   mapOf = fixed map [OK]

Hint: Remember: mapOf = fixed map, mutableMapOf = changeable map [OK]

Common Mistakes:

• Confusing mapOf with mutableMapOf
• Thinking mapOf creates an empty map
• Assuming mapOf allows adding new entries

2. Which of the following is the correct syntax to create a mutable map with keys "a" and "b" and values 1 and 2 respectively?

easy

A. val map = mutableMapOf("a" to 1, "b" to 2)

B. val map = mapOf("a" -> 1, "b" -> 2)

C. val map = mutableMapOf("a" = 1, "b" = 2)

D. val map = mapOf("a" to 1, "b" to 2)

Solution

1. Step 1: Identify correct syntax for mutableMapOf

   The correct syntax uses mutableMapOf with pairs using to keyword.

2. Step 2: Check each option

   val map = mutableMapOf("a" to 1, "b" to 2) uses mutableMapOf and to correctly. val map = mapOf("a" -> 1, "b" -> 2) uses wrong arrow ->. val map = mutableMapOf("a" = 1, "b" = 2) uses = which is invalid here. val map = mapOf("a" to 1, "b" to 2) uses mapOf which creates an immutable map.

3. Final Answer:

   val map = mutableMapOf("a" to 1, "b" to 2) -> Option A

4. Quick Check:

   mutableMapOf + to = correct syntax [OK]

Hint: Use mutableMapOf with "key" to value pairs [OK]

Common Mistakes:

• Using -> instead of to for pairs
• Using = inside mapOf or mutableMapOf
• Confusing mapOf and mutableMapOf

3. What is the output of this Kotlin code?

val map = mapOf("x" to 10, "y" to 20)
println(map["x"])
println(map["z"])

medium

A. 20
null

B. 10
null

C. null
null

D. 10
0

Solution

1. Step 1: Access existing key "x"

   map["x"] returns 10 because "x" is a key with value 10.

2. Step 2: Access non-existing key "z"

   map["z"] returns null because "z" is not in the map.

3. Final Answer:

   10\nnull -> Option B

4. Quick Check:

   Existing key returns value, missing key returns null [OK]

Hint: map[key] returns value or null if key missing [OK]

Common Mistakes:

• Expecting 0 instead of null for missing key
• Confusing values of different keys
• Assuming map is mutable and can add keys

4. What is wrong with this Kotlin code?

val map = mapOf("a" to 1)
map["b"] = 2

medium

A. mapOf creates an immutable map, so you cannot add new entries

B. The syntax for adding entries is incorrect; should use put()

C. Keys must be integers, not strings

D. mapOf requires mutableMapOf to add entries

Solution

1. Step 1: Understand mapOf immutability

   mapOf creates an immutable map that cannot be changed after creation.

2. Step 2: Adding entries to immutable map causes error

   Trying to assign a new key-value pair with map["b"] = 2 causes a compile-time error because the map is immutable.

3. Final Answer:

   mapOf creates an immutable map, so you cannot add new entries -> Option A

4. Quick Check:

   Immutable map cannot be changed [OK]

Hint: Use mutableMapOf to add or change entries [OK]

Common Mistakes:

• Trying to add entries to mapOf
• Using wrong syntax for adding entries
• Confusing mutableMapOf and mapOf

5. You want to create a map that stores student names as keys and their scores as values. You also want to update scores later. Which Kotlin code snippet correctly achieves this?

hard

A. val scores = mutableMapOf("Alice" to 90, "Bob" to 85)
scores.add("Alice", 95)

B. val scores = mapOf("Alice" to 90, "Bob" to 85)
scores["Alice"] = 95

C. val scores = mapOf("Alice" to 90, "Bob" to 85)
scores.put("Alice", 95)

D. val scores = mutableMapOf("Alice" to 90, "Bob" to 85)
scores["Alice"] = 95

Solution

1. Step 1: Choose a mutable map to allow updates

   Since scores need to be updated, mutableMapOf is required.

2. Step 2: Use correct method to update value

   Assigning with scores["Alice"] = 95 correctly updates the value in a mutable map.

3. Step 3: Check other options for errors

   val scores = mapOf("Alice" to 90, "Bob" to 85) scores["Alice"] = 95 uses immutable map and tries to update, causing error. val scores = mapOf("Alice" to 90, "Bob" to 85) scores.put("Alice", 95) tries to use put on immutable map, error. val scores = mutableMapOf("Alice" to 90, "Bob" to 85) scores.add("Alice", 95) uses non-existent add method.

4. Final Answer:

   val scores = mutableMapOf("Alice" to 90, "Bob" to 85) scores["Alice"] = 95 -> Option D

5. Quick Check:

   mutableMapOf + assignment updates map [OK]

Hint: Use mutableMapOf and bracket assignment to update entries [OK]

Common Mistakes:

• Using mapOf when updates are needed
• Trying to use put on immutable map
• Using add method which does not exist