How to make Reverse polish notation in Scala with Recursion? - algorithm

Question

I tried to make RPN in scala but i got Exception in console
Exception in thread "main" scala.MatchError: 1 (of class
java.lang.Character)
and i stop here because Intelij don't show where is problem . I think that i 'am wrong making
if (!expr.isEmpty) {
expr.head match
but i spend some hours thinking how to exchange this and i din't invent anything
i make this program in Java but in functional programming i don't know how to correctly use this match with tail Recursive
object RPN extends App {
print(evaluate("123++"))
def evaluate( expr : String) {
val stack = Stack[Double]()
var a,b :Int=0
#tailrec
def helper ( asset : String){
if (!expr.isEmpty) {
expr.head match {
case it if 0 until 9 contains it => stack.push(expr.head); helper(asset.tail)
case '+' => b = stack.pop.toString().toInt; a = stack.pop.toString().toInt; stack.push(a + b)
case '-' => b = stack.pop.toString().toInt; a = stack.pop.toString().toInt; stack.push(a - b)
case '*' => b = stack.pop.toString().toInt; a = stack.pop.toString().toInt; stack.push(a * b)
case '/' => b = stack.pop.toString().toInt; a = stack.pop.toString().toInt; stack.push(a / b)
}
}
else return stack.pop
}
helper(expr)
}
}

Answer 1

Your code is referencing expr inside of helper(), where you probably want to reference asset instead.
Also, you don't need the return or the vars.
def evaluate(expr :String) :Double = {
val stack = collection.mutable.Stack[Double]()
#annotation.tailrec
def helper(asset :String) :Double =
if (asset.isEmpty) stack.pop
else {
asset.head match {
case c if c.isDigit =>
stack.push(c.asDigit)
case '+' => stack.push(stack.pop + stack.pop)
case '-' => stack.push(-stack.pop + stack.pop)
case '*' => stack.push(stack.pop * stack.pop)
case '/' => stack.push(1/stack.pop * stack.pop)
case c => throw new Error(s"Bad Char: $c")
}
helper(asset.tail)
}
helper(expr)
}
testing:
evaluate("123++") //res0: Double = 6.0
evaluate("954--") //res1: Double = 8.0
evaluate("423*+") //res2: Double = 10.0
evaluate("28/") //res3: Double = 0.25
evaluate("73/") //res4: Double = 2.333333333333333