How to flatten a list that deep layer is not same: ex1: ```val deepList = listOf( listOf( ...

# getting-started

[Simon Lin]

How to flatten a list that deep layer is not same:

ex1:

val deepList = listOf(
    listOf(
        listOf(-1, 0),
        listOf(1)
    ),
    listOf(2, 3),
    listOf(4, 5, 6)
)
println(deepList.flatten()) // [-1, 0, 1, 2, 3, 4, 5, 6]

ex2:

val deepList = listOf(
    -1,
    listOf(listOf(listOf(0))),
    listOf(2, 3),
    listOf(4, 5, 6)
)
println(deepList.flatten()) // [-1, 0, 1, 2, 3, 4, 5, 6]

[Tobias Suchalla]

Flatten the list recursively:

fun main() {
    val deepList = listOf(
        listOf(
            listOf(-1, 0),
            listOf(1)
        ),
        listOf(2, 3),
        listOf(4, 5, 6)
    )

    println(deepList.deepFlatten())
}

fun Iterable<*>.deepFlatten(): Iterable<*> = flatMap {
    when (it) {
        is Iterable<*> -> it.deepFlatten()
        else -> listOf(it)
    }
}

(Quickly cobbled together, not necessarily the best approch. YMMV)

[Ulrik Rasmussen]

It's a bit subtle though, because anything that happens to implement Iterable will silently get flattened. It also loses the type argument. I'd recommend not having heterogeneous lists like that if that can be avoided.

[Ulrik Rasmussen]

For example, maps and sets are also Iterable, but I doubt you would want listOf(mapOf("foo" to 42)).deepFlatten() to give you listOf(Pair("foo", 42)).

[Tobias Suchalla]

Regarding type, the list already is List<Any>, so there is no type to begin with.
Regarding the use of Iterable, it depends on one's needs. You are right, however. To be safe, one might want seperate functions for list and set flattening.

However, Map does not implement Iterable 🤨

val deepList = listOf(mapOf("foo" to 42)).deepFlatten()
println(deepList.deepFlatten())
// [{foo=42}]

[Ulrik Rasmussen]

Oh! I thought it did. My bad!

[Ulrik Rasmussen]

That was just meant as an example though. My point is that Iterable is implemented by a lot of things.

[Tobias Suchalla]

Its a fair point! I just thought "broader = better!" without giving much thought to the consequences.