java - 来自数组的 Scala HashMap

Question

我有一个像这样的二元组的 Scala 数组:

(("A", "2015-11-01"), ("B", "2016-11-11"), ("A", "2017-11-01"), ("B", "2013-11-11"))

我想创建一个映射，其中键映射到最新日期。所以，在上面的例子中，结果应该是:

Map ("A" -> "2017-11-01", "B" -> "2016-11-11")

我知道如何迭代地完成它 - 但什么是 Scala 方式(功能方式)来做到这一点？

Answer 1

首先 groupBy 键，然后选择最新的日期。

arr
  .groupBy(_._1)
  .map { case (k, v) => k -> v.maxBy(_._2)._2 }

使用 mapValues 使其更短

arr.groupBy(_._1).mapValues(_.maxBy(_._2)._2)

由于日期(字符串)格式正确，最大日期是最新日期。您无需将日期转换为以毫秒为单位的时间来确定最大日期。

Scala REPL

scala> val arr = Array(("A", "2015-11-01"), ("B", "2016-11-11"), ("A", "2017-11-01"), ("B", "2013-11-11"))
arr: Array[(String, String)] = Array((A,2015-11-01), (B,2016-11-11), (A,2017-11-01), (B,2013-11-11))

scala> :paste
// Entering paste mode (ctrl-D to finish)

arr
  .groupBy(_._1)
  .map { case (k, v) => k -> v.maxBy(_._2)._2 }


// Exiting paste mode, now interpreting.

res0: scala.collection.immutable.Map[String,String] = Map(A -> 2017-11-01, B -> 2016-11-11)

不需要日期转换，但如果您想转换它，请继续。

日期转换:

//ensure correct date format is given to this method if not it will throw match error at runtime.
def convertStringDateToMillis(str: String): Long = {
 val regex = "(\\d{4})-(\\d{2})-(\\d{2})".r.unanchored
 val regex(year, month, day) = str
 val calendar = Calendar.getInstance()
 calendar.clear()
 calendar.set(Calendar.MONTH, month.toInt)
 calendar.set(Calendar.YEAR, year.toInt)
 calendar.set(Calendar.DAY_OF_MONTH, month.toInt)
 calendar.getTimeInMillis();
}

解决方法:

val arr = Array(("A", "2015-11-01"), ("B", "2016-11-11"), ("A", "2017-11-01"), ("B", "2013-11-11"))

arr.groupBy(_._1).map { case (k, v) => k -> v.maxBy(convertStringDateToMillis(_._2))._2 }

Scala REPL

scala> def convertStringDateToMillis(str: String): Long = {
     |  val regex = "(\\d{4})-(\\d{2})-(\\d{2})".r.unanchored
     |  val regex(year, month, day) = str
     |  val calendar = Calendar.getInstance()
     |  calendar.clear()
     |  calendar.set(Calendar.MONTH, month.toInt)
     |  calendar.set(Calendar.YEAR, year.toInt)
     |  calendar.set(Calendar.DAY_OF_MONTH, month.toInt)
     |  calendar.getTimeInMillis();
     | }
convertStringDateToMillis: (str: String)Long

scala> val arr = Array(("A", "2015-11-01"), ("B", "2016-11-11"), ("A", "2017-11-01"), ("B", "2013-11-11"))
arr: Array[(String, String)] = Array((A,2015-11-01), (B,2016-11-11), (A,2017-11-01), (B,2013-11-11))


scala> arr.groupBy(_._1).map { case (k, v) => k -> v.maxBy(x => convertStringDateToMillis(x._2))._2 }
res3: scala.collection.immutable.Map[String,String] = Map(A -> 2017-11-01, B -> 2016-11-11)