A modern programming language that makes developers happier.

kotlinlang

```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),
) }```


<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) -&gt; 
            country to cities.sumBy { it.population }
        }.toMap()
    
    println("byPop -&gt; $populationByCountry")
}```

Untitled

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

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

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

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