mapIndexed

Returns a sequence containing the results of applying the given transform function to each element and its index in the original sequence.

fun <T, R> Sequence<T>.mapIndexed(transform: (index: Int, T) -> R): Sequence<R>(source)

val numbers = sequenceOf(10, 20, 30, 40)

val indexedStrings = numbers
    .mapIndexed { index, number -> "Item #$index is $number" }
    .toList()

println(indexedStrings)
// Output: [Item #0 is 10, Item #1 is 20, Item #2 is 30, Item #3 is 40]

Source