<#C0B8MA7FA|getting-started> - I am new to Kotlin and trying to absorb as much as possible. I have a...

# getting-started

[Ankush]

#getting-started - I am new to Kotlin and trying to absorb as much as possible. I have a requirement where I have a Multi-Level List that I need to parse up to 4 levels. List<List<List<List<String>>>>.
The typical way would be using 4 for loops and keep iterating, but I am looking for a smarter Kotlin way of doing it. Could you guys please help me get there?

for(a in As){
    for(b in Bs){
        for(c in Cs){
            for(d in Ds){
                
            }
        }
    }
}

Another train of thought I have is to use flatMap() to flatten the last loop and drop the loops from 4 to 3. But there too, am getting beaten up by Kotlin syntax.

Any help is appreciated! TIA 🙂

EDIT: Sturcture is of complex type in nature:

data class A(
    val b: List<B>
)
data class B(
    val c: List<C>
)
data class C (
    val d: List<D>
        )
data class D(
    val string1: List<String>,
    val string2: String
)

[Casey Brooks]

Definitely a gross data structure, but you can call .flatten() multiple times to reduce the list down to a single List<String>

val deeplyNestedList: List<List<List<List<String>>>> = TODO()
val flatList: List<String> = deeplyNestedList
    .flatten()
    .flatten()
    .flatten()

[ephemient]

I'd go a bit further and say that if you're iterating, you should do so over the sequence,

for (x in deeplyNestedList.asSequence().flatten().flatten().flatten()) {
    println(x)
}

which avoids constructing all the partially-flattened lists along the way

[Ankush]

@Casey Brooks, can't agree more! Worst DS till date.

My Bad! @Casey Brooks & @ephemient
Structure is of complex nature - data class object. Parsed via JSON API call:

data class A(
    val b: List<B>
)
data class B(
    val c: List<C>
)
data class C (
    val d: List<D>
        )
data class D(
    val string1: List<String>,
    val string2: String
)

Will update the query too. Any suggestion for this type of structure?

[ephemient]

for ((string1, string2) in listOfA.asSequence().flatMap { it.b }.flatMap { it.c }.flatMap { it.d }) // ...

[Ankush]

@ephemient, tried it but getting the following error:

Type inference failed:

fun <T, R> Sequence<T>.flatMap
            (
    transform: (T) → Sequence<R>
)
: Sequence<R>
cannot be applied to

        receiver: Sequence<A>?
arguments:
(
        (A) → List<B>?
)

do you think there is a way out of here? If not, could you point me to more relevant documentation?

[ephemient]

where is the nullable coming from, you don't have that in your question

[ephemient]

but Kotlin has an .orEmpty() extension on collections

[Ankush]

data classes definitions are defined with type ?

[Ankush]

Yes, in vanilla for() I used: for(a in As.orEmpty)

[ephemient]

well you can do that in flatMap too

[Ankush]

Yes, still got the same error.

[ephemient]

make a real example, then. because what I said does work. https://pl.kotl.in/DQ9EDl_js

[Ankush]

do you mind checking the URL you shared, I added a piece right below it.

[ephemient]

changes are not shared. you need to create another URL

[Ankush]

@ephemient, shared a link. Let me know if it makes sense.