```data class City ( val name : String = "", val country : String = "", val population :...

# getting-started

[Daniele B]

data class City (
    val name : String = "",
    val country : String = "",
    val population : Int = 0,
)

val cities = { listOf(
    City("a", "1", 1305770),
    City("b", "1", 556934),
    City("c", "2", 1924701),
    City("d", "3", 5785861),
    City("e", "3", 4467118),
) }

[Quincy]

https://pl.kotl.in/3pwJqe5dY

data class City (
    val name : String = "",
    val country : String = "",
    val population : Int = 0,
)

fun main() {
    val cities = listOf(
        City("a", "1", 1305770),
        City("b", "1", 556934),
        City("c", "2", 1924701),
        City("d", "3", 5785861),
        City("e", "3", 4467118),
    )
    
    val populationByCountry = cities.groupBy { it.country }
        .map { (country, cities) -> 
            country to cities.sumBy { it.population }
        }.toMap()
    
    println("byPop -> $populationByCountry")
}

[ephemient]

in a single step: https://pl.kotl.in/mg7NA4blA

Untitled

[ephemient]

.groupBy() and .groupingBy() are subtly different. you can get .groupBy() from .groupingBy(), but you can also get much more.

[ephemient]

using Grouping means you can skip creating the intermediate data structures that .groupBy() forces, if you were going to map over the results anyway

[nanodeath]

if you do use groupBy, .groupBy.mapValues is slightly better than .groupBy.map in this case, btw.