Is there an easy way to create a view of multiple ...

# announcements

[ribesg]

Is there an easy way to create a view of multiple lists? Like, I would like to be able to iterate over "list" A to iterate over lists B, C and D, without it being an actual separate collection, just a view. Without building my own iterators and stuff, I just want to know if there's another cool thing in the stdlib I don't know about

[Shawn]

what exactly are you trying to do

[Shawn]

oh, I see, you want to iterate over multiple lists using one view

[ribesg]

Trying to hide/abstract the fact that I have multiple lists of slightly different items being a single list of interface-implmenting items basically

[ribesg]

I don't want to pass lists of lists around for no reason, and I just wanted to check that there was no dead simple way to do what I want before doing it myself

[robin]

This should do the job:

val b = listOf("b")
val c = listOf("c")
val d = listOf("d")
val a = b.asSequence() + c + d

[robin]

The resulting sequence will iterate all the backing lists in order when iterated.

[robin]

...without creating any additional lists.

[ribesg]

Right, a Sequence, like an Iterator, is basically a view

[kevinherron]

@ribesg plain ol’ Java has Stream.concat

[kevinherron]

Kotlin has sequenceOf(...)

[kevinherron]

scratch that, not what you want. Robin was onto it though.

[Andreas Sinz]

you can use buildSequence (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines.experimental/build-sequence.html):

val sequence = buildSequence {
        for (subList in lists) {
            yieldAll(subList)
        }
    }

[robin]

That works, too, but in my opinion is less easy to understand when reading.

[karelpeeters]

sequenceOf(listA, listB, listC).flatMap { it.asSequence() }