如何使用 scala 读取和更改文件中行的 position

Question

I have a file, lets say file1:

A
B
C
D
E

I have to read this file and want to move 1st and 2nd line from the file to 3rd and 4th line in a file, like:

C
D
A
B
E

Getlines function can get the lines and probably print it. But how to change position of lines in a file using Scala?

Answer 1

Let's say you can't, or just don't want to, read the entire file into memory. Or, on the other hand, what if the file has fewer than 4 lines? Can the swapping still be done safely?

import java.nio.file.{Files, Paths}

util.Using.Manager { use =>  //Scala 2.13
  val input  = use(io.Source.fromFile("inFile.txt"))
  val output = use(Files.newBufferedWriter(Paths.get("outFile.txt")))

  val itr = input.getLines()
  val linesAB = Seq.fill(2)(util.Try(itr.next()))
  val linesCD = Seq.fill(2)(util.Try(itr.next()))
  linesCD.foreach(_.foreach(s => output.write(s + "\n")))
  linesAB.foreach(_.foreach(s => output.write(s + "\n")))
  while (itr.hasNext) output.write(itr.next() + "\n")

}.fold(println,identity)  //report failure

result:

~> head *File.txt  # a 3-line file
==> inFile.txt <==
A
B
C

==> outFile.txt <==
C
A
B

Answer 2

Here is a pragmatic solution:

  val newList =Source.fromFile("file.txt").getLines().toList match {
    case a::b::c::d::rest => c::d::a::b::rest // reorganize your list
    case other => other // don't do anything if the List has not at least 4 elements
  }
  // persist newList

Use the Pattern Matching of the List type.