does `minBy` actually give a reference to original...

# announcements

[warriorprincess]

does minBy actually give a reference to original object? I do lowest = lowest?.next in my code above, does it actually move the head of the list stored in lists?

[diesieben07]

Objects work by references, you are never actually working on the object, only a reference to it.

[warriorprincess]

Okay so its like pointers in c++? If I manipulate lowest here it will change in the original array too?

[diesieben07]

No, it's not like pointers.

val x = MyObject() // x holds reference to that object
val y = x // y is set to the same value as x, i.e. the reference to the object

[diesieben07]

Everything is pass-by-value, but you are not passing objects, you are passing references.

[warriorprincess]

going with your example, val x = 9; var y = x; y = 8

[warriorprincess]

obviously x doesn't become 8

[diesieben07]

Yes.

Everything is pass-by-value

[warriorprincess]

okay yeah that makes sense i was just wondering how this LL works in such a scenario:

[warriorprincess]

class ListNode(var `val`: Int = 0) {
        var next: ListNode? = null
    }

[diesieben07]

What do you mean by LL?

[warriorprincess]

linkedlist* sorry

[warriorprincess]

var head: ListNode?
        head = head?.next

[warriorprincess]

I guess this works because head is simply being overwritten?

[diesieben07]

You are storing a new value in head, yes. I am not sure why you think this would not work?

[warriorprincess]

yeah sorry i was just confused i guess

[warriorprincess]

` var lowest = lists.minBy { it?.val ?: Int.MAX_VALUE }
`

[warriorprincess]

var lowest = lists.minBy {  it?.`val` ?: Int.MAX_VALUE }

[warriorprincess]

lowest = lowest?.next

[warriorprincess]

this is what im trying to do

[warriorprincess]

but of course the original list stored in lists isn't affected by lowest = lowest?.next

[diesieben07]

No, of course not.

[warriorprincess]

whats a good kotlin-y way to to do this in your opinion?

[diesieben07]

Depends on your exactl use-case, really.
What is your end goal?

[warriorprincess]

so we have a list of single linked lists

[warriorprincess]

the goal is to find the linked list with the smallest current HEAD value and then advance the head of said list to point to head.next

[warriorprincess]

makes sense?

[diesieben07]

You are representing your "linked list" by just a pointer to the head, which is a bit bad, since thereby your linked list does not really have an identity.
You would have to get the index of the linked list with the lowest value and then update that index of the main list.

[diesieben07]

For that you can do val (lowest, lowestIndex) = lists.withIndex().minBy { (value, index) -> value.whatever }

[diesieben07]

And then lists[lowestIndex] = lists[lowestIndex].next

[diesieben07]

But a much simpler and cleaner approach would be to write a class LinkedList(var head: Node?)

[diesieben07]

Then you could do

val listWithLowest = lists.minBy { it.head?.val ?: Int.MAX_VALUE }
if (listWithLowest != null) listWithLowest.head = listWithLowest.head?.next

[warriorprincess]

interesting

[warriorprincess]

val (lowest, lowestIndex) = lists.withIndex().minBy { (value, index) -> value.whatever }

[warriorprincess]

whats happening in this line?

[warriorprincess]

the docs for withIndex say 'Returns a lazy Iterable of IndexedValue for each element of the original array.'

[warriorprincess]

does it return some special version of List? I mean minBy still works on it

[diesieben07]

list.withIndex() turns a List<T> into a Iterable<IndexedValue<T>>. The (value, index) is a destructuring operation, grabbing the actual value and it's index in the list out of the IndexedValue.

[warriorprincess]

or is it some special collection with index too?

[diesieben07]

Yes.

[warriorprincess]

alright. so minBy works on Iterable<IndexedValue<T>> too?

[diesieben07]

Yes, minBy is defined for Iterable.

[warriorprincess]

Got it

[warriorprincess]

val (lowest, lowestIndex) = lists.withIndex().minBy { (value, index) -> value.`val` ?: Int.MAX_VALUE }

[warriorprincess]

apparently not nullable values are needed to destructure

[warriorprincess]

so i guess il need to check for nullness first then destructure it

[diesieben07]

Oh, right... minBy is nullable. Yes

[warriorprincess]

also, it says val is not defined for value

[diesieben07]

Yeah, it's (index, value), sorry.

[diesieben07]

You can also do minBy { it.value.val ?: Int.MAX_VALUE }, if you don't like the destructuring.

[warriorprincess]

got it!