Hey there, is there a stdlib function to chunk a l...

# stdlib

[holgerbrandl]

Hey there, is there a stdlib function to chunk a list by an attribute which is changing? Something along

fun <T, K> List<T>.chunkBy(selector: (T) -> K): List<List<T>> {
    val result = mutableListOf<MutableList<T>>()
    var lastKey: K? = null
    for (item in this) {
        if (selector(item) != lastKey) result.add(mutableListOf(item)).also { lastKey = selector(item) }
        else result.last().add(item)
    }
    return result
}

 data class Person(val age: Int)

val persons = listOf(Person(23), Person(12), Person(23), Person(23), Person(22))
val chunkBy = persons.chunkBy { it.age }

but maybe its already part of stdlib?

[jw]

what is the expected output there? a list containing five, single-item lists?

[jw]

oh the wrapping confused me

[jw]

so it would be a list of one, one, two, one

[jw]

as far as i can there's no really good way to do this with built-in operators that isn't wildly more complex and opaque than your dead-simple imperative loop

[jw]

I'm not sure it should be named chunkedBy. It's almost like a runningGroupBy or something.

[holgerbrandl]

Thanks for your input @jw

[Adam S]

There's a ticket requesting .chunked(), but with a predicate https://youtrack.jetbrains.com/issue/KT-41648

[kevin.cianfarini]

How is this different than using groupBy and then grabbing the values from the resulting map?

val peopleByAge = persons.groupBy { it.age }.values

[jw]

The original example

listOf(Person(23), Person(12), Person(23), Person(23), Person(22))

should produce

[
  [Person(23)],
  [Person(12)],
  [Person(23), Person(23)],
  [Person(22)],
]

[jw]

your method would combine the two separate groupings of the "23"s