Using maps in Scala - scala

Question

val nums = Map('M' -> 1000, 'D' -> 500, 'C' -> 100, 'L' -> 50, 'X' -> 10, 'V' -> 5, 'I' -> 1)
def convert(data : String):Int = {
var count = 0;
var index = 0;
var list: List[Char] = List();
for((value,index) <- data.reverse.zipWithIndex){
list = value :: list;
}
def calcRM(rm : List[Char]): List[Char] = rm match{
case x :: Nil => x
case x :: tail => ???
case Nil => Nil
}
return count
}
}
I am wondering how I can convert the value I get from x(head) and tail.head by using the Map provided so I can convert the string to the value in Roman Numerals.

Answer 1

To access a map, you simply use its get method:
nums.get(x)
which returns an Option that you would have to unwrap, so you can assume all good values and use the apply:
nums(x) //Throws if key not found
or you could use a default:
nums.getOrElse(x, 0)

Answer 2

You are looking for the apply method on Map. Recall that you can call apply simply by adding parentheses:
nums('M') //returns 1000
nums.apply('M') //equivalent in Scala
You can also use get to return an Option[Int] if you aren't sure the key is available in the Map, and you want to avoid the exception that would otherwise be thrown by apply:
nums.get('M') //returns Some(1000)
nums.get('K') //returns None
nums('K') //throws an exception
I would also recommend looking into some of the more idiomatic paradigms in Scala (e.g. using val). For your example, you might consider something like this, which uses inner methods and values, slightly more complex pattern matching, the ever important map method, the implicit apply used in the map, and recursion:
def convert(rm: String) = {
val nums = Map('M' -> 1000, 'D' -> 500, 'C' -> 100, 'L' -> 50, 'X' -> 10, 'V' -> 5, 'I' -> 1)
def calcRM(rm: Seq[Int]): Int = rm match {
case x :: y :: tail if x < y => y - x + calcRM(tail)
case x :: tail => x + calcRM(tail)
case Nil => 0
}
calcRM(rm.map(nums).toList)
}