Slow range forEach in Kotlin

Question

I used the following code to measure performance of different syntax constructions in Kotlin

fun time(what: String, body: () -> Int) {
    val start =          


        

Answer 1

From documentation:

A for loop over a range or an array is compiled to an index-based loop that does not create an iterator object.

forEach as you can see at https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/for-each.html is special-cased for arrays, but has a single implementations for all Iterables, so it needs to create an iterator.