General question for someone with greater wisdom. ...

# announcements

[Asq]

General question for someone with greater wisdom. This code (run with kotlin 1.5)

1:    val aut = setOf(2, 3, 1, 4, 0)
2:    print("$aut")
3:    val aux2a: List<Int> = aut.dropWhile{ 2 < it }
4:    a2.containsAll(aux2a) shouldBe true
5:    (aux2a.size < a2.size) shouldBe true

fails (i.e. (aux2a.size < a2.size) is false ) at line 5. the documentation for dropWhile says :

Returns a list containing all elements except first elements that satisfy the given predicate.

I would have expected that at least item 3 (from line 1) xor item 4 would have been dropped. Perhaps both under certain circumstances (ordering is not a property of Set). What is the expected behavior of dropWhile for a Set?

[robstoll]

already the first item 2 does not meed your predicate (2 < 2), so it stops dropping.

[Asq]

So, since ordering is not a property of Set, dropWhile may drop none or any arbitrary number of items matching a predicate. Is that impression correct?

[Paul Griffith]

yes, it's non-deterministic

[Paul Griffith]

on a given JVM with a given collection size it'll probably give you the same results, so tests and such could inadvertently pass

[Asq]

It follows that dropWhile on a Set is a good operation to avoid, then.

[Paul Griffith]

Indeed. dropWhile is an extension on Iterable, so that it works on Set is more of a consequence of type limitations than any deliberate decisions I'd say

[Asq]

I hear you. I suspected as much, but it's nice to have a second opinion. Thanks.

[ephemient]

Kotlin guarantees that `setOf()`/`mutableSetOf()`/`mapOf()`/`mutableMapOf()` will be iterated in insertion order

[ephemient]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/set-of.html "Elements of the set are iterated in the order they were specified."
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/mutable-set-of.html "The returned set preserves the element iteration order."

[ephemient]

you don't know the iteration order of an arbitrary Set, but you may know it for particular instances

[Asq]

expediency is the mother of bugs 😬. The above clarification is gratefully appreciated.