CODEWITHSUNDEEP
scala
deltanovember
deltanovember

Reputation: 44051

Why does my Scala list disappear in the following code?

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(_))
}

The output from the code is as follows

*175
**0

I don't understand why my list disappears

Upvotes: 3

Views: 2769

Answers (2)

huynhjl
huynhjl

Reputation: 41646

Source.fromFile(new File("data\\" + file), "UTF-16").getLines returns an Iterator[String]. Filtering still returns an Iterator. Calling length on an iterator will cause it to be read fully until hasNext returns false. The second time around, it's empty. Similar to:

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

Upvotes: 6

om-nom-nom
om-nom-nom

Reputation: 62835

cleaned is an iterator.

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

Right after assigning it is non-empty. Iterators in scala are single-used - once you traversed it (e.g. by applying length method) it becomes empty:

scala> cleaned.length
res0: Int = 13

scala> cleaned.length
res1: Int = 0

You can fix that behavior by converting to List, or to Seq (lazy):

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

Upvotes: 19

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