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Exercises: Module 01 — Introduction to Kotlin

Instructions

Complete each exercise in order. Exercises increase in difficulty. Use IntelliJ IDEA or the Kotlin Playground to run your code. Submit your answers by editing this file or committing a solutions file.

Easy Exercises (1–3)

Exercise 1

Difficulty: Easy Objective: Understand val vs var and type inference.

Write a Kotlin program with the following variables: - An immutable variable pi with the value 3.14159 - A mutable variable counter starting at 0 - An immutable variable greeting containing the string "Hello, Kotlin!"

Print each variable. Then increment counter to 5 and print it again. Try to reassign pi — observe and explain the compiler error.

fun main() {
    // Write your code here
}

Exercise 2

Difficulty: Easy Objective: Practice string templates and when expressions.

Write a function gradeMessage(score: Int): String that: - Returns "Excellent! Score: $score" for scores 90–100 - Returns "Good job! Score: $score" for scores 70–89 - Returns "Keep practicing. Score: $score" for scores 50–69 - Returns "Please review the material. Score: $score" for anything below 50

Test it with: 95, 82, 61, 45.

fun gradeMessage(score: Int): String {
    // Write your code here
}

fun main() {
    // Test your function here
}

Exercise 3

Difficulty: Easy Objective: Practice nullable types and safe calls.

Write a function getUserCity(user: Map<String, String>): String that: - Retrieves the "city" key from the map - Returns the city name if present - Returns "Unknown city" if the key is absent or the value is blank

Test with: 

val user1 = mapOf("name" to "Alice", "city" to "Prague")
val user2 = mapOf("name" to "Bob")
val user3 = mapOf("name" to "Carol", "city" to "  ")


fun getUserCity(user: Map<String, String>): String {
    // Write your code here
}

Medium Exercises (4–6)

Exercise 4

Difficulty: Medium Objective: Design and use a data class.

Create a data class Product with properties: id: Int, name: String, price: Double, inStock: Boolean.

Write a function formatProduct(product: Product): String that returns a formatted string like: "[101] Kotlin in Action - $39.99 (In Stock)" or "[102] Old Book - $5.00 (Out of Stock)".

Use the copy function to create a discounted version of a product (reduce price by 10%) and print both.

// Write your data class and functions here

Exercise 5

Difficulty: Medium Objective: Practice extension functions and the standard library.

Write the following extension functions on List<Int>: 1. fun List<Int>.secondLargest(): Int? — returns the second-largest unique value, or null if fewer than 2 unique values 2. fun List<Int>.isMonotonicallyIncreasing(): Boolean — returns true if each element is strictly greater than the previous

Test with: 

listOf(3, 1, 4, 1, 5, 9, 2, 6)   // secondLargest → 6, isMonotonically → false
listOf(1, 2, 3, 4, 5)              // secondLargest → 4, isMonotonically → true
listOf(5)                           // secondLargest → null, isMonotonically → true

Exercise 6

Difficulty: Medium Objective: Model states with a sealed class.

Create a sealed class hierarchy ParseResult<T> with: - data class ParseOk<T>(val value: T) — successful parse - data class ParseErr(val input: String, val reason: String) — failed parse

Write a function parsePositiveInt(input: String): ParseResult<Int> that: - Returns ParseOk(n) if input is a valid integer greater than zero - Returns ParseErr(input, "not a number") if it can't be parsed - Returns ParseErr(input, "must be positive") if it parses but is ≤ 0

Write a displayResult function that uses when (exhaustively) to print the result.

Hard Exercises (7–8)

Exercise 7

Difficulty: Hard Objective: Chain null safety operators in a realistic scenario.

You have the following data structure (simulating a JSON response):

data class Address(val street: String?, val city: String?, val country: String?)
data class Contact(val email: String?, val phone: String?, val address: Address?)
data class UserProfile(val name: String, val contact: Contact?)

Write a function getDisplayAddress(profile: UserProfile): String that: - Returns "city, country" if both city and country are non-null and non-blank - Returns just the city if only city is available - Returns just the country if only country is available - Returns "No address on file" otherwise

Use only null safety operators (?., ?:, let) — no explicit if (x != null) checks.

Exercise 8

Difficulty: Hard Objective: Write a mini text analysis program.

Write a program that takes a multiline string and produces a TextStats data class containing: - wordCount: Int — total word count - uniqueWords: Int — count of distinct words (case-insensitive) - longestWord: String — the longest word (if tie, take first) - mostFrequent: Pair<String, Int> — the most frequent word and its count

val text = """
    To be or not to be that is the question
    Whether tis nobler in the mind to suffer
    The slings and arrows of outrageous fortune
""".trimIndent()

Expected: wordCount=31, uniqueWords=27, longestWord="outrageous", mostFrequent=("to", 3)

Expert Exercise (9)

Exercise 9

Difficulty: Expert Objective: Build a type-safe command dispatch system.

Design a mini command-line interpreter using Kotlin idioms. Implement:

  1. A sealed class Command with subclasses: Help, Quit, Echo(text: String), Calculate(expression: String)
  2. A fun parse(input: String): Command? function that parses:
  3. "help" → Help
  4. "quit" or "exit" → Quit
  5. "echo <text>" → Echo(text)
  6. "calc <expr>" → Calculate(expr) where expr is like "3 + 4" or "10 * 5"
  7. anything else → null
  8. A fun execute(command: Command): String that runs the command:
  9. Help → a help message listing available commands
  10. Quit → "Goodbye!" (your main loop should stop)
  11. Echo → the text back
  12. Calculate → the arithmetic result (support +, -, *, / on integers)
  13. A main() that reads lines from stdin in a loop and runs the interpreter

This exercise requires no libraries beyond Kotlin's standard library.