Kotlin Arrays: Lookahead without going out of bounds?-CodePudding

(Kotlin newbie here)

I have a long array of Booleans and I want to extract from it the index positions of TRUE followed by FALSE patterns. For example, this array

arrayOf(true, false, true, false, true, false)

should produce

[0, 2, 4]

but this one

arrayOf(true, false, false, false, true, false)

should produce

[0, 4]

Here's a fun for that except, of course, the 3rd example will crash Java:

fun main() {
    println(dontCrash(arrayOf(true, false, true, false, true, false)))
    println(dontCrash(arrayOf(true, false, false, false, true, false)))
    println(dontCrash(arrayOf(true, false, false, false, true, true)))
}
fun dontCrash(a: Array<Boolean>): List<Int> {
    val goodGuys = mutableListOf<Int>()
    for ((ind, element) in a.withIndex())
        if (element && !a[ind 1]) goodGuys.add(ind)
    return goodGuys
}

[0, 2, 4]
[0, 4]
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 6 out of bounds for length 6

My question: Does Kotlin have some cute trick to deal this (step??) or do I need to add a .size() check?

Thank you!

CodePudding user response：

windowed(2) or zipWithNext() lets you slide along pairs of adjacent values without going past the bounds, but it's only available for Iterables and Sequences, not arrays:

fun dontCrash(a: Array<Boolean>): List<Int> {
    a.asSequence()
        .zipWithNext()
        .mapIndexedNotNull { i, (a, b) -> i.takeIf { a && !b } }
        .toList()
}