Kotlin — Lambdas

Basic lambda

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

    val greet = { name: String -> println("Hello, $name!") }
    greet("Alice")
}

Trailing lambda

fun main() {
    val nums = listOf(1, 2, 3, 4, 5)

    // Trailing lambda syntax
    val evens = nums.filter { it % 2 == 0 }
    println(evens) // [2, 4]

    nums.forEach { println(it) }
}

It keyword

fun main() {
    val nums = listOf(1, 2, 3, 4, 5)

    nums.filter { it > 2 }.forEach { println(it) }
    // 3 4 5
}

Higher-order functions

fun applyOp(a: Int, b: Int, op: (Int, Int) -> Int): Int {
    return op(a, b)
}

fun main() {
    println(applyOp(3, 4) { a, b -> a + b }) // 7
    println(applyOp(3, 4) { a, b -> a * b }) // 12
}

Collection operations

fun main() {
    val nums = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

    val result = nums
        .filter { it % 2 == 0 }
        .map { it * it }
        .sorted()
        .take(3)
        .sum()

    println(result) // 20 (4 + 16 + 36)
}

Function references

fun isEven(n: Int) = n % 2 == 0

fun main() {
    val nums = listOf(1, 2, 3, 4, 5)
    val evens = nums.filter(::isEven)
    println(evens) // [2, 4]
}

Mini Practice

Write Kotlin code that:

1. Creates a lambda and passes it as an argument
2. Chains filter, map, and reduce with lambdas
3. Uses function references with ::
4. Creates a higher-order function

Up Next

In the next lesson, you'll learn about Extensions — extending existing classes.