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Kotlin Lambdas

A lambda is a small unnamed block of code held in a value. A program can run that code later by using the value name.

A lambda stored in a value

The braces hold a small block that adds its two values. The name add runs that block at each call.

fun main() {
    val add = { a: Int, b: Int -> a + b }
    println(add(3, 4))
    println(add(10, 5))
}

The short name it

This lambda writes no parameter name of its own. it stands for the value the list hands over on each pass.

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

forEach walks a list

forEach runs its lambda one time for each value in the list. The name before the arrow receives the current value.

fun main() {
    val names = listOf("Ana", "Bo", "Cy")
    names.forEach { name -> println("Hello, $name") }
}

Example

fun main() {
    val addTax = { price: Int -> price + 5 }
    val prices = listOf(10, 20, 30)
    prices.forEach { println(addTax(it)) }
}

Key points

  • Braces hold the code of a lambda.
  • Lambda parameters sit before the -> arrow inside the braces.
  • it: the short name for the single parameter when no name is written.
  • Use forEach to run a lambda once for each value in a list.

In this example, addTax is a lambda that adds 5 to its value. The list holds three prices. forEach hands each price to the lambda through it, one price at a time. The prints show 15, 25, and 35.

Note

A lambda has no name of its own, so its value name does the calling.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

What does it stand for inside a one parameter lambda?