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会消失

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中的迭代器是单独使用的 - 一旦你遍历它（例如通过应用length方法）它就变空了：

scala> cleaned.length
res0: Int = 13

scala> cleaned.length
res1: Int = 0

您可以通过转换为List或Seq（lazy）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

Answer 2

Source.fromFile(new File("data\\" + file), "UTF-16").getLines会返回Iterator[String]。过滤仍会返回Iterator。在迭代器上调用length将导致它被完全读取，直到hasNext返回false。第二次，它是空的。类似于：

scala> val cleaned = List(1,2).iterator
cleaned: Iterator[Int] = non-empty iterator

scala> cleaned.length
res10: Int = 2

scala> cleaned.length
res11: Int = 0

为什么我的Scala列表会在以下代码中消失？

2 个答案: