fun main() {
    var count = 0
    var isDark = false

    fun updateUI() {
        println("Counter: $count")
        println("Theme: ${if (isDark) "Dark" else "Light"}")
    }

    fun increment() { count++; updateUI() }
    fun decrement() { count--; updateUI() }
    fun reset() { count = 0; updateUI() }
    fun toggleTheme() { isDark = !isDark; updateUI() }

    // Simulate some actions
    updateUI()
    increment()
    increment()
    toggleTheme()
    decrement()
    reset()
}

Demonstrates a simple counter with theme toggling using Kotlin, runnable in a JVM or Kotlin/JS environment.

class Person(val name: String, var age: Int) {
    fun introduce() {
        println("Hi, I'm $name and I'm $age years old.")
    }
}

fun main() {
    val alice = Person("Alice", 25)
    alice.introduce()
    alice.age += 1
    alice.introduce()
}

Defines a simple class with properties and methods, demonstrating object creation and usage.

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    val doubled = numbers.map { it * 2 }
    val even = numbers.filter { it % 2 == 0 }

    println("Original: $numbers")
    println("Doubled: $doubled")
    println("Even: $even")
}

Shows iterating over a list and using functional operations like map and filter.

fun main() {
    val name: String? = null
    println(name?.length ?: "Name is null")
    val greeting = name?.let { "Hello, $it" } ?: "Hello, Guest"
    println(greeting)
}

Demonstrates handling nullable types and safe calls in Kotlin.

data class Rectangle(val width: Int, val height: Int)

fun main() {
    val rect1 = Rectangle(10, 20)
    val rect2 = Rectangle(10, 20)
    println(rect1)
    println("Equal: ${rect1 == rect2}")
}

Uses a data class to store immutable data with automatic equals, hashCode, and toString.

fun main() {
    val x = 2
    val result = when(x) {
        1 -> "One"
        2 -> "Two"
        else -> "Other"
    }
    println(result)
}

Demonstrates the when expression for branching logic, similar to switch-case.

fun String.shout() = this.uppercase() + "!"

fun main() {
    val msg = "hello"
    println(msg.shout())
}

Adds a custom function to an existing class using extension functions.

fun main() {
    val numbers = listOf(1,2,3,4,5)
    numbers.forEach { println(it * 2) }
}

Demonstrates lambdas and higher-order functions in Kotlin.

fun main() {
    var time = 5
    while(time >= 0) {
        println("Time left: $time")
        Thread.sleep(1000)
        time--
    }
    println("Done!")
}

Simulates a countdown timer using a loop and Thread.sleep for demonstration purposes.

fun main() {
    val ages = mapOf("Alice" to 25, "Bob" to 30)
    for ((name, age) in ages) {
        println("$name is $age years old")
    }
}

Demonstrates map creation, iteration, and access in Kotlin.