Command-Line I/O
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To get ready to show for loops, if expressions, and other Scala constructs, let’s take a look at how to handle command-line input and output with Scala.
Writing output
As we’ve already shown, you write output to standard out (STDOUT) using println:
println("Hello, world")
That function adds a newline character after your string, so if you don’t want that, just use print instead:
print("Hello without newline")
When needed, you can also write output to standard error (STDERR) like this:
System.err.println("yikes, an error happened")
Because
printlnis so commonly used, there’s no need to import it. The same is true of other commonly-used data types likeString,Int,Float, etc.
Reading input
There are several ways to read command-line input, but the easiest way is to use the readLine method in the scala.io.StdIn package. To use it, you need to first import it, like this:
import scala.io.StdIn.readLine
To demonstrate how this works, let’s create a little example. Put this source code in a file named HelloInteractive.scala:
import scala.io.StdIn.readLine
object HelloInteractive extends App {
print("Enter your first name: ")
val firstName = readLine()
print("Enter your last name: ")
val lastName = readLine()
println(s"Your name is $firstName $lastName")
}
Then compile it with scalac:
$ scalac HelloInteractive.scala
Then run it with scala:
$ scala HelloInteractive
When you run the program and enter your first and last names at the prompts, the interaction looks like this:
$ scala HelloInteractive
Enter your first name: Alvin
Enter your last name: Alexander
Your name is Alvin Alexander
A note about imports
As you saw in this application, you bring classes and methods into scope in Scala just like you do with Java and other languages, with import statements:
import scala.io.StdIn.readLine
That import statement brings the readLine method into the current scope so you can use it in the application.