Kotlin Training Program

Transform

To transform all elements of a list to something new, we have the map() function.

Following program computes the average mass of all planets using the map() function. Firstly List<Planet> is mapped to their masses to get List<Double>, then average() function is used :

 class Planet(
    val name: String,
    val mass: Double
)

fun main() {
    val planets = listOf(
        Planet("Earth", 5.9724e24),
        Planet("Jupiter", 1.8982e27),
        Planet("Mars", 6.4171e23),
        Planet("Mercury", 3.3011e23),
        Planet("Neptune", 1.0243e26),
        Planet("Pluto", 1.3030e22),
        Planet("Saturn", 5.6834e26),
        Planet("Uranus", 8.6810e25),
        Planet("Venus", 4.8675e24)
    )

    val averageMass = planets.map { it.mass }.average()

    println("Average mass = $averageMass kg")
}
 

To transform List<A> to List<B>, map() function takes a lambda transform: (A) → B as an argument. This transform operation is then performed on all items in the list and added to a new list.

 fun <A, B> List<A>.map(
    transform: (A) -> B
): List<B> {
    return buildList {
        this@map.forEach {
            add(transform(it))
        }
    }
}