how to get index list in arrayList filter

Question

I want to get index list as follows.

val a = booleanArrayOf(true,false,true,false)

above code, True number is two. -> indexList = {0, 2} how to get indexList in Kotlin.

Answer 1

You could use mapIndexed() to get the index and the value of each element, convert to either the index or null, and then remove the nulls...

val b: List<Int> = a.mapIndexed { i, b -> if (b) i else null }.filterNotNull().toList()

Another way would be to use the withIndex() function, filter the values that are true, and map the resulting pairs to the index value. This might be a bit clearer.

val c: List<Int> = a.withIndex().filter { it.value }.map { it.index }