Iterable<T>.map vs Iterable<T>.forEach

main.kt

import kotlin.time.Duration
import kotlin.time.measureTime

fun main() {
    val mapResults = mutableListOf<Duration>()
    val forEachResults = mutableListOf<Duration>()
    repeat(10) {
        mapResults.add(measureTime { (0..1_000_000).map { it + 1 } })
        forEachResults.add(measureTime { (0..1_000_000).forEach { it + 1 } })
    }
    println(mapResults)
    // [61.9ms, 81.5ms, 55.6ms, 15.5ms, 56.9ms, 18.6ms, 81.2ms, 80.9ms, 52.1ms, 11.5ms]
    println(forEachResults)
    // [7.09ms, 332us, 491us, 546us, 331us, 372us, 548us, 470us, 326us, 518us]
}