Kotlin: How to map a field from list to a new list and group it

Question

I have two lists

list1 = [ { city: 'Los Angeles', population: '4 million ' } ... ]
list2 = [ { nameOfCity: 'New York', language: 'English' }, {nameOfCity: 'Berlin', language: 'German' ]

Now I need to create a new list and filter the two lists by the name of city and then group it by the same language

newClass(val nameOfCities: List<String>, language: String)

So I need to check if city names of list2 are inside list 2 (list2.filter { it.nameOfCity != list1.city } and store all the names inside the nameOfCities list grouped by language

so the end result should be something like this:

[
{ nameOfCities: [New York, Chicago]
  language: Engilsh
},
{ nameOfCities: [Berlin]
  language: German
 }
]

So I get a new list of all the cities that don't exists in list1 and grouped by the same language.

This is the closest I got:

    for (item in list1) {
            reducedList = (list2.filter { it.nameOfCity != item.city }.map { newClass(it.nameOfCity, it.language) })
                .toList().groupBy { it.language }
        }

But the format is like this:

'english': [
    { nameOfCity: 'Los Angeles', language: 'English' },
    { nameOfCity: 'New York', language: 'English' },
],
'german': [ { nameOfCity: 'Berlin ', language: 'German' } ]

Answer 1

I'm not sure I understood what you meant but i try to help you:

I am assuming that you have 3 classes like as follow:

class newClass(val nameOfCities: List<String>, val language: String)
class ListElement1(val city: String, val population: String)
class ListElement2(val nameOfCity: String, val language: String)

And 2 lists:

val list1 = listOf(ListElement1("Los Angeles", "4 million"))
val list2 = listOf(ListElement2("New York", "English"), ListElement2("Berlin", "German"))

Filter list2 in order to delete all cities that are in list1 and map all to a list of newClass:

var reducedList: List<NewClass> = listOf()
list1.forEach{ listElement1 ->
    = list2.filter { listElement1.city != it.nameOfCity }
           .groupBy { it.language }.map { newClass(it.value.map { it.nameOfCity }, it.key) }
    }

Result(json):

[
  {
    language = "English",
    nameOfCities= [ 
      "New York"
    ]
  },
  {
    language = "German",
    nameOfCities= [
      "Berlin"
    ]
  }
]

Result(Classes):

[
  newClass(nameOfCities=[New York], language=English), 
  newClass(nameOfCities=[Berlin], language=German)
]

Answer 2

Maybe this one will be helpfull:

        list2.filter { list2Element -> !list1.any { it.city == list2Element.nameOfCity} }
        .groupBy({it.language}, {it.nameOfCity}).map { newClass(it.value, it.key) }

Instead of iterating and filtering you can simply use .any()