Cross product of two Strings - scala

Question

I am just starting out in Scala and for my first project, I am writing a Sudoku solver. I came across a great site explaining Sudoku and how to go about writing a solver: http://norvig.com/sudoku.html and from this site I am trying to create the corresponding Scala code.
The squares of a Sudoku grid are basically the cross product of the row name and the column name, this can be generated really easily in Python using a list comprehension:
# cross("AB", "12") = ["A1", "A2", "B1", "B2"]
def cross(A, B):
"Cross product of elements in A and elements in B."
return [a+b for a in A for b in B]
It took me awhile to think about how to do this elegantly in Scala, and this is what I came up with:
// cross("AB", "12") => List[String]("A1", "A2", "B1", "B2")
def cross(r: String, c: String) = {
for(i <- r; j <- c) yield i + "" + j
}.toList
I was just curious if there is a better way to doing this in Scala? It would seem much cleaner if I could do yield i + j but that results in an Int for some reason. Any comments or suggestions would be appreciated.

Answer 1

Yes, addition for Char is defined by adding their integer equivalents. I think your code is fine. You could also use string interpolation, and spare the toList (you will get an immutable indexed sequence instead which is just fine):
def cross(r: String, c: String) = for(i <- r; j <- c) yield s"$i$j"
EDIT
An IndexedSeq is at least as powerful as List. Just check your successive usage of the result. Does it require a List? E.g. do you want to use head and tail and pattern match with ::. If not, there is no reason why you should need to enforce List. If you use map and flatMap on the input arguments instead of the syntactic sugar with for, you can use the collection.breakOut argument to directly map to a List:
def cross(r: String, c: String): List[String] =
r.flatMap(i => c.map(j => s"$i$j"))(collection.breakOut)
Not as pretty, but faster than an extra toList.