Kotlin — Functions

Basic functions

fun add(a: Int, b: Int): Int {
    return a + b
}

// Expression body
fun multiply(a: Int, b: Int) = a * b

fun main() {
    println(add(3, 4))       // 7
    println(multiply(3, 4))  // 12
}

Default and named parameters

fun greet(name: String, greeting: String = "Hello") {
    println("$greeting, $name!")
}

fun main() {
    greet("Alice")           // Hello, Alice!
    greet("Bob", "Hi")      // Hi, Bob!
    greet(greeting = "Hey", name = "Charlie")
}

Lambda expressions

fun main() {
    val add = { a: Int, b: Int -> a + b }
    println(add(3, 4)) // 7

    val numbers = listOf(1, 2, 3, 4, 5)
    val doubled = numbers.map { it * 2 }
    println(doubled) // [2, 4, 6, 8, 10]

    val evens = numbers.filter { it % 2 == 0 }
    println(evens) // [2, 4]

    val sum = numbers.reduce { acc, i -> acc + i }
    println(sum) // 15
}

Higher-order functions

fun applyTwice(f: (Int) -> Int, x: Int): Int {
    return f(f(x))
}

fun main() {
    val double = { x: Int -> x * 2 }
    println(applyTwice(double, 3)) // 12
}

Extension functions

fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}

fun main() {
    println("racecar".isPalindrome()) // true
    println("hello".isPalindrome())   // false
}

Inline functions

inline fun measureTime(block: () -> Unit) {
    val start = System.currentTimeMillis()
    block()
    val end = System.currentTimeMillis()
    println("Time: ${end - start}ms")
}

fun main() {
    measureTime {
        Thread.sleep(100)
    }
}

Mini Practice

Write Kotlin code that:

1. Creates a lambda and passes it to a higher-order function
2. Uses map, filter, and reduce on a list
3. Creates an extension function for String
4. Uses an inline function with a lambda

Up Next

In the next lesson, you'll learn about Lambdas — functional programming in Kotlin.