Kotlin - 来自两个集合的过滤器/映射

Question

是否可以根据所需的条件匹配来过滤和映射两个集合，如下所示：

fun main(args: Array<String>) {
    val selectedDates = listOf("2018-08-12", "2018-08-13", "2018-08-14")

    val expenses = listOf(Expense("Food", "2018-08-12"),
    Expense("Transportation", "2018-08-15"),
    Expense("Misc.", "2018-08-13"),
     Expense("Uber", "2018-08-12"),
    Expense("Clothing", "2018-08-16"))

    val listOfExpensesInSelectedDate = mutableListOf<Expense>()

    for (date in selectedDates){
        listOfExpensesInSelectedDate.addAll(expenses.filter { it.date==date })

    }
    println(listOfExpensesInSelectedDate)

}

data class Expense(
        val expense:String,
        val date: String
)

提供上面给定的代码，我试图返回与另一个字符串列表中的日期匹配的费用列表。在上面的示例中，我同时使用了 for Loop 和 filter 函数来获得我想要的结果。但是是否可以避免 for 循环和过滤器并在单行代码中映射两个集合？

Answer 1

您可以只使用in 进行过滤：

val listOfExpensesInSelectedDate = expenses.filter { it.date in selectedDates }

编辑：自从hotkey发布了关于最佳解决方案的评论，我已经在我的电脑上尝试过，任何有兴趣的人也可以尝试一下：

(1st) 我发布的答案：

val start = Date().time
for (i in 1..10000) {
    val listOfExpensesInSelectedDate = expenses.filter { it.date in selectedDates }
}
val end= Date().time

println(end - start)

平均时间结果：26ms (23ms-35ms)

(2nd) 我对热键建议使用集合的回答：

val start = Date().time
for (i in 1..10000) {
    val expSet = selectedDates.toSet()
    val listOfExpensesInSelectedDate = expenses.filter { it.date in expSet }
}
val end= Date().time

println(end - start)

平均时间结果：70ms (50ms-86ms)

(3d) 热键答案：

val start = Date().time
for (i in 1..10000) {
    val groups = expenses.groupBy { it.date }
    val listOfExpensesInSelectedDate = selectedDates.flatMap { groups[it].orEmpty() }
}
val end= Date().time

println(end - start)

平均时间结果：100ms (74ms-150ms)

Answer 2

您可以通过首先将expenses 按date 分组，然后选择并合并在selectedDates 中具有键的组来简化和优化您的代码，如下所示：

val selectedDates = listOf("2018-08-12", "2018-08-13", "2018-08-14")
val expenses: List<Expense> = TODO("content omitted")

val groups = expenses.groupBy { it.date }
val listOfExpensesInSelectedDate = selectedDates.flatMap { groups[it].orEmpty() }

请参阅：groupBy、flatMap