How to read from standard input line by line?

问题:

What's the Scala recipe for reading line by line from the standard input ? Something like the equivalent java code :

import java.util.Scanner; 

public class ScannerTest {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        while(sc.hasNext()){
            System.out.println(sc.nextLine());
        }
    }
}

回答1:

The most straight-forward looking approach will just use readLine() which is part of Predef. however that is rather ugly as you need to check for eventual null value:

object ScannerTest {
  def main(args: Array[String]) {
    var ok = true
    while (ok) {
      val ln = readLine()
      ok = ln != null
      if (ok) println(ln)
    }
  }
}

this is so verbose, you'd rather use java.util.Scanner instead.

I think a more pretty approach will use scala.io.Source:

object ScannerTest {
  def main(args: Array[String]) {
    for (ln <- io.Source.stdin.getLines) println(ln)
  }
}

回答2:

For the console you can use Console.readLine. You can write (if you want to stop on an empty line):

Iterator.continually(Console.readLine).takeWhile(_.nonEmpty).foreach(line => println("read " + line))

If you cat a file to generate the input you may need to stop on either null or empty using:

@inline def defined(line: String) = {
  line != null && line.nonEmpty
}
Iterator.continually(Console.readLine).takeWhile(defined(_)).foreach(line => println("read " + line))

回答3:

val input = Source.fromInputStream(System.in);
val lines = input.getLines.collect

回答4:

Can you not use

var userinput = readInt // for integers
var userinput = readLine 
...

As available here : Scaladoc API

回答5:

A recursive version (the compiler detects a tail recursion for improved heap usage),

def read: Unit = {
  val s = scala.io.StdIn.readLine()
  println(s)
  if (s.isEmpty) () else read 
}

Note the use of io.StdIn from Scala 2.11 . Also note with this approach we can accumulate user input in a collection that is eventually returned -- in addition to be printed out. Namely,

import annotation.tailrec

def read: Seq[String]= {

  @tailrec
  def reread(xs: Seq[String]): Seq[String] = {
    val s = StdIn.readLine()
    println(s)
    if (s.isEmpty()) xs else reread(s +: xs) 
  }

  reread(Seq[String]())
}