foldRightIndexed

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

inline fun <R> CharSequence.foldRightIndexed(initial: R, operation: (index: Int, Char, acc: R) -> R): R(source)

fun main() {
    val text = "abc"
    val result = text.foldRightIndexed("") { index, char, acc ->
        "$acc$index:$char "
    }
    println(result)   // prints: 0:a 1:b 2:c 
}

Source