scala - 为什么我的 Scala 列表在以下代码中消失了？

Question

val files = new File("data").list()
val filtered = files.filter(name => name.contains("txn"))
val masterList = new ListBuffer[String]
for (file <- filtered) {
  val lines = Source.fromFile(new File("data\\" + file), "UTF-16").getLines
  val cleaned = lines.filter(!masterList.contains(_))
  println("*" + cleaned.length)
  cleaned.foreach(println(_))
  println("**" + cleaned.length)
  cleaned.foreach(masterList.append(_))
}

代码的输出如下

*175
**0

我不明白为什么我的名单消失了

Answer 1

cleaned是一个迭代器。

scala> val cleaned = lines.filter(_!="")
cleaned: Iterator[String] = non-empty iterator

分配后它是非空的。 scala 中的迭代器是单次使用的——一旦你遍历它(例如通过应用 length 方法)它就变成空的:

scala> cleaned.length
res0: Int = 13

scala> cleaned.length
res1: Int = 0

您可以通过转换为 List 或 Seq(懒惰)来修复该行为:

scala> val lines=Source.fromFile("bla.txt").getLines
lines: Iterator[String] = non-empty iterator

scala> val cleaned = lines.filter(_!="").toSeq
cleaned: Seq[String] = Stream(first, ?)

scala> cleaned.length
res4: Int = 13

scala> cleaned.length
res5: Int = 13