Hi, How can i fill `prevItems` and `nextItems` of...

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[Kulwinder Singh]

Hi,
How can i fill prevItems and nextItems of below data class , i have tried recursion but it caused infinite loop because we have to find next and previous both but if i try with either previous or next then there is no infinite loop.
data class Root(val item: Int, val prevItems: List<Root>, val nextItems: List<Root>)

Suppose i have data for item is => 1,2,3,4,5 then result i wanted is =>

Root(item: 1, prevItems: [], nextItems: [2,3,4,5])
Root(item: 2, prevItems: [1], nextItems: [3,4,5])
Root(item: 3, prevItems: [1,2], nextItems: [4,5])
Root(item: 4, prevItems: [1,2,3], nextItems: [5])
Root(item: 5, prevItems: [1,2,3,4], nextItems: [])

*for making it short i am using 1,2,3… in prevItems/nextItems instead of Root object

what approach should i use to calculate above? also is there any kotlin collection function can help here ?

[CLOVIS]

I tried to, but honestly I don't think it's possible with immutable data, because if you copy() the node to edit the previous node, it has a different identity than previously so it breaks the chain.

[CLOVIS]

If you accept mutable data, or if you accept a one-way tree, it becomes trivial.

[Big Chungus]

Builder pattern that uses mutable structures and converts them to immutable ones on build

[Big Chungus]

Or you can have it as doubly linked list node and convert prev and next to dynamically computed properties

[CLOVIS]

@Big Chungus I don't think it's possible. Because you can build neither the previous nor the next.

[Kulwinder Singh]

@CLOVIS I have no issues in using mutable structure, can you help me to achieve it

[Roukanken]

My first question would be first: why you want to do this? Do you really need the whole list in each element of list?

It really looks more like double linked list as Marty said above - you would use much less memory, but still be able to easily compute those lists properties on demand

[Sourabh Rawat]

@Roukanken this is probably for Zipper Data structure (https://en.wikipedia.org/wiki/Zipper_%28data_structure%29)
It is useful in pure functional programming.

[Roukanken]

It's similar yes, but he wants to have all the "zippeers" at all times - eg, a list of lenght N, would be represented by at least N^2 amount of data... which is an obscenely large amount. (At least that's how I understood his request)

[bsimmons]

Yeah, I imagine that it's possible to do this sort of thing with a calculated field and a tricky implementation of a DoubleLinkedList.subList(i, j) to avoid N^2 memory usage.

[nkiesel]

I am still a bit unclear on the actual question. Do you want to have a function that creates a list of <n> Root objects out of a list of <n> items? Or do you want a function that creates a single Root object out of a list of items and a given item?