Arabic-Roman numerals converter written in Scala

A simple arabic to roman numeral converter written in Scala

import scala.collection.SortedMap

def roman(x:Int): String = {

  val romanLiteral = SortedMap(1000 -> "M", 900 -> "CM", 500 -> "D", 400 -> "CD",
    100 -> "C", 90 -> "XC", 50 -> "L", 40 -> "XL", 10 -> "X", 9 -> "IX", 5 ->
    "V", 4 -> "IV", 1 -> "I")

  val arab = romanLiteral.keySet.toList.reverse
  
  def r(n:Int,idx:Int,romanNumber:StringBuilder): String = {

     val a = arab(idx)
  
     if (n==0)
       romanNumber.toString
     else if ( n >= a )
        r( n-a, idx, romanNumber append romanLiteral(a) )
     else
       r( n, idx+1, romanNumber )
  } 

  r(x,0,new StringBuilder())

}

(1 to 100).foreach( x => printf( "%s=%s\n", x, roman(x) ))