如何在Kotlin中获得groupBy操作后属于组的值列表?

Question

我有以下数据:

import kotlin.test.*
import java.util.*

data class Sales(val year: Int, val price: Int)
val myList = listOf(
Sales(2017, 10),
Sales(2017, 19),
Sales(2020, 15),
Sales(2021, 100),
Sales(2020, 20),
)

我想看看如何获得每个组的值列表。例如，我希望结果为

{2017=[10, 19], 2020=[15, 20], 2021=[100]}

Answer 1

您可以使用以下命令:

import kotlin.test.*
import java.util.*

data class Sales(val year: Int, val price: Int)
val myList = listOf(
Sales(2017, 10),
Sales(2017, 19),
Sales(2020, 15),
Sales(2021, 100),
Sales(2020, 20),
)
fun main () {
val reduced = myList.groupBy({ it.year } , { it.price })
print(reduced)  //result: {2017=[10, 19], 2020=[15, 20], 2021=[100]}
// further - just for understanding
val reduced2 = myList.groupBy({ it.year } , { it})
print(reduced2)  
//result: 
//{
//2017=[Sales(year=2017, price=10), Sales(year=2017, price=19)], 
//2020=[Sales(year=2020, price=15), Sales(year=2020, price=20)], 
//2021=[Sales(year=2021, price=100)]
//}
}

Answer 2

data class Sales(val year: Int, val price: Int)
val myList = listOf(
Sales(2017, 10),
Sales(2017, 19),
Sales(2020, 15),
Sales(2021, 100),
Sales(2020, 20)
)
val result = myList
.groupBy { it.year }
.mapValues { (_, value) -> value.map { it.price } }
println(result)   // Output: {2017=[10, 19], 2020=[15, 20], 2021=[100]}