Hi, I have a list of Booleans, and I need to know ...

# getting-started

[Nat Strangerweather]

Hi, I have a list of Booleans, and I need to know the index of all the ones that are false, so a list of indexes. What is the best way to achieve this?

[ephemient]

not sure what you've tried already, but most direct way is just fine

list.indices.filter { !list[it] }

[Nat Strangerweather]

Thanks, I had tried .indexOf() but was getting nowhere.

[Starr]

I think filterIndexed exists

[ephemient]

@Starr that will return a list of the original booleans, which isn't very useful

[ephemient]

list.mapIndexedNotNull { i, bool ->
    if (bool) null else i
}

would work but is not necessary when first one suffices

[Klitos Kyriacou]

An alternative:

list.withIndex().filter { !it.value }.map { it.index }

This may be more performant than ephemient's first solution if the list is large and not random-access.

[Nat Strangerweather]

Thanks, the list has 28 Booleans always. I assume that is a small list! 🙂

[Klitos Kyriacou]

But is it random access? If so, you might as well use ephemient's solution.

[Nat Strangerweather]

what does random-access mean?

[Klitos Kyriacou]

It means that retrieving the value of list[n] takes constant time, no matter what value n is. Examples of random-access lists include ArrayList and the type of list you get by calling listOf(). Examples of non-random-access lists include LinkedList and generally any list where finding the value of list[n] requires you to start from index 0 and iterate the list through each element until you get to element n.

[Nat Strangerweather]

Thanks for this, then it is random-access.

[ephemient]

Klitos, that's less performance than my second solution.
Nat, almost all lists are RandomAccess. IMO it was a design mistake to put non-random access lists in the same interface, but that's how the Java API was designed

[Anouar di Kali]

@ephemient Hi, is there a better way to check performance in intellij of a given solution than this code ? :

val list:List<Boolean> =BooleanArray(100){Random.nextBoolean()}.toList()
val start: Long = System.nanoTime()
list.mapIndexedNotNull { i, bool ->
    if (bool) null else i
}
val end: Long = System.nanoTime();
println(end - start)

[ephemient]

don't benchmark like that, use JMH (or kotlinx.benchmark which wraps JMH on JVM)

[Anouar di Kali]

Thanks!

[nkiesel]

Another advantage of the mapIndexedNotNull approach is that you do not need a named list. Of course, this does not matter if you anyway wrap it into a function.