The function parameters are val
s in Scala, so you cannot modify them.
Write your function recursively. Something like:
def functionA(c: Double, x0: Double, n: Int): Double = {
require(x0 > 0) //This is a must
if (n < 0) {
x0
} else {
functionA(c, functionB(c, x0), n - 1)
}
}