foldIndexed

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

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

val input = "a1b2c3"

val sumIndices = input.foldIndexed(0) { index, acc, ch ->
    if (ch.isDigit()) acc + index else acc
}

println(sumIndices)  // Prints: 5

Source