Command line arguments

Consider the following little script (triangle1.kts):

fun triangle(n: Int) {
  for (i in 1 .. n) {
    for (j in 1 .. i)  
      print("*")
    println()
  }
}

val size = 5
triangle(size)

As you can guess, it produces this output:

$ kts triangle1.kts 
*
**
***
****
*****
$

I can change the size of the triangle by editing the script and changing the value of size. Wouldn't it be much nicer if I could provide the size of the triangle when I call the script?

Command line arguments make this possible. Every time Kotlin executes a script, it takes all the arguments on the command line and packs them inside a list named args (for "arguments"). So we can change our script as follows (triangle2.kts):

val size = args[0].toInt()
triangle(size)

And now we can run it as we like:

$ kts triangle2.kts 3
*
**
***
$ kts triangle2.kts 5
*
**
***
****
*****
$ kts triangle2.kts 10
*
**
***
****
*****
******
*******
********
*********
**********

You can use as many command line arguments as you want (of course they have to fit on your command line when you enter them...). A simple test script is (arguments.kts) as follows:

for (s in args)
  println(s)

And you could use it like this:

$ kts arguments.kts I love CS109!
I
love
CS109!
$ kts arguments.kts CS109    is    the  best  course   at   KAIST...
CS109
is
the
best
course
at
KAIST...
$ kts arguments.kts 
$ kts arguments.kts 13 + 21
13
+
21

Note that the number of spaces between arguments doesn't matter—only the arguments themselves are passed inside args. Also notice that when I call the script without extra arguments, nothing is printed. That's because in this case args is a list of length zero, and so the for-loop does nothing.

Going back to our triangle2.kts script, it's not perfect:

$ kts triangle2.kts
java.lang.ArrayIndexOutOfBoundsException: 0
	at Triangle2.<init>(triangle2.kts:9)

What happened? I forgot to provide the size of the triangle, and so accessing args[0] failed.

So we should add some error checking to our script. Here is a better version (triangle3.kts):

if (args.size == 1) {
  val size = args[0].toInt()
  triangle(size)
} else
  println("Usage: kts triangle.kts <size>")

Now it behaves in a more civilized way:

$ kts triangle3.kts
Usage: kts triangle3.kts <size>

The script is still not perfect, but we'll cover that later when we know more about exceptions.