mapIndexedNotNullTo

Applies the given transform function to each character and its index in the original char sequence and appends only the non-null results to the given destination.

@IgnorableReturnValueinline fun <R : Any, C : MutableCollection<in R>> CharSequence.mapIndexedNotNullTo(destination: C, transform: (index: Int, Char) -> R?): C(source)

val input = "a1b2c3"
val digitIndices = mutableListOf<Int>()

input.mapIndexedNotNullTo(digitIndices) { index, ch ->
    if (ch.isDigit()) index else null
}

println(digitIndices) // prints: [1, 3, 5]

Source