I need to iterate part of an array backwards. I'd like to do that "functionally" as it's more comprehensible, like that
for (b in buf.sliceArray(0 until bufLimit).reversedArray()) {}
But both sliceArray and reversedArray are not lazy. Is there a lazy version or should I probably fall back to
for (bIdx in bufLimit - 1 downTo 0) {
val b = buf[bIdx]
}
which is more confusing and verbose?
If you use a list instead of an array, then you can reverse it and then convert to a Sequence:
val buf: List = listOf(1, 2, 3, 4, 5)
val bufLimit = 3
for (b in buf.asReversed().asSequence().drop(buf.size - bufLimit)) {
println(b)
}
Functions with the as prefix only wrap objects without copying, so the code above does not copy the buf content.
Note that you shouldn't loose any performance compared to Array if you use an ArrayList.
However this solution does involve several iterators, so it is somewhat less efficient than the index code you have suggested in the question:
for (bIdx in bufLimit - 1 downTo 0) {
val b = buf[bIdx]
}
I suggest creating an extension function to handle your specific use case. e.g.:
/**
* Performs the given [action] on each element at the specified [indices].
*/
inline fun ByteArray.forEachAt(indices: Iterable<Int>, action: (Byte) -> Unit): Unit {
indices.forEach { index -> action(this[index]) }
}
Usage:
buf.forEachAt((0 until bufLimit).reversed)) {}
// or
buf.forEachAt(bufLimit - 1 downTo 0) {}
buf.take(bufLimit).reversed().forEach { ... }ByteArray.takeas well isn't lazy, it creates anArrayList