is there a neat way to group by value and respect adjacency

Question

is there a neat way to group by value and respect adjacency e g aa aa a aa adjacentGroupBy String length should return aa aa a aa

thanksforallthefish · Answer

I can think of <https kotlinlang org api latest jvm stdlib kotlin collections windowed html>

Bernhard · Answer

< thanksforallthefish> that requires a size

Bernhard · Answer

i thought about going from aa aa a aa to aa aa a aa and then use fold to manipulate the array

Bernhard · Answer

but that seems like a bad idea with lots of useless allocation

hho · Answer

<https pl kotl in 2TXgCoR3q> But yeah that s not efficient

Bernhard · Answer

or something like

Bernhard · Answer

``` fun lt T V gt adjacentGroupBy values Iterable lt T gt criterium T gt V List lt List lt T gt gt val result = ArrayList lt ArrayList lt T gt gt for value in values if result isEmpty result add arrayListOf value else if criterium result last last == criterium value result last add value else result add arrayListOf value return result ```

Bernhard · Answer

but that s ugly

thanksforallthefish · Answer

I think the way `windowed` works you can set a window size of 2 and a step of 1 so each element is processed along with the next one

thanksforallthefish · Answer

let me see if I can come up with some code

thanksforallthefish · Answer

yeah no it gets ugly

Bernhard · Answer

funny thing is that thats the default implementation in Haskell for groupBy

Bernhard · Answer

and I can t find it in the stdlib

hho · Answer

In Kotlin `groupBy` always means assign to some common key

Bernhard · Answer

yeah turns it into a map

Bernhard · Answer

afaik its similar to java s Stream groupingBy

Alowaniak · Answer

This question is quite similar to <https kotlinlang slack com archives C0922A726 p1555991397085900> I would probably write something like ``` fun lt T U gt List lt T gt adjacentGroupBy selector T gt U List lt List lt T gt gt return fold mutableListOf lt MutableList lt T gt gt acc e gt acc apply when isEmpty gt add mutableListOf e selector e == selector last last gt last add e else gt add mutableListOf e ```

karelpeeters · Answer

I m curious what s the use case This gets asked about once a week but I ve never come across anything like this before

Bernhard · Answer

< karelpeeters> grouping together opening hours you start out with a list of MONDAY 08 00 12 00 TUESDAY 08 00 12 00 WEDNESDAY 09 00 12 00 THURSDAY 08 00 12 00 and you want to end up with Mo Tu 08 00 12 00 We 09 00 12 00 Th 08 00 12 00