reduceIndexed

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element with its index in the original sequence.

inline fun <S, T : S> Sequence<T>.reduceIndexed(operation: (index: Int, acc: S, T) -> S): S(source)

val numbers = sequenceOf(1, 2, 3, 4)

val result = numbers.reduceIndexed { index, acc, element ->
    // start with the first element, then add each element multiplied by its index
    acc + element * index
}

println(result)   // prints 21

Source