Recursive function - reverse iterate through List - list

Question

I have recently started learning functional programming on a SCALA. I used the recursive function to print elements of the list, now I would like to print elements starting from the end ("Ann" ...) but also using recursive function, can anyone please help me to transform the code?
object Scala {
val names: List[String] = List("Adam", "Mick", "Ann");
def main(args: Array[String]) {
println(printNames(names))
def (printNames (name: List[String] ) {
if(names.isEmpty) ""
else names.head + (printNames(names.tail);
}
}
}

Answer 1

You can use the following definition:
def printNames(names: List[String]): String = {
if(names.isEmpty) ""
else printNames(names.tail) + " " + names.head
}
Note that this implementation is not tail recursive and cannot be optimized by scala compiler. I strongly advise using an accumulator for the resulting String.
Luis Miguel Mejía Suárez suggests using StringBuilder to avoid expensive String concatenation:
def printNames(names: List[String]): StringBuilder = {
if(names.isEmpty) new StringBuilder("")
else printNames(names.tail).append(" ").append(names.head)
}

Answer 2

def printNames(names: List[String]): String = names.reverse.mkString(" ")
You can reverse the name list and then use mkString with space as a separator. The above one can be used to achieve the same without recursion, as mkString by default uses StringBuilder.
As you were looking for a tail-recursive solution
import scala.annotation.tailrec
def printNames(names: List[String]): String = {
#tailrec
def printNamesHelper(
names: List[String],
acc: StringBuilder = new StringBuilder("")
): String = {
names match {
case Nil => acc.toString.trim
case head :: tail => printNamesHelper(tail, acc.append(head).append(" "))
}
}
printNamesHelper(names)
}
def printNamesReverse(names: List[String]): String = {
#tailrec
def printNamesReverseHelper(
names: List[String],
accum: List[String] = List.empty
): String = {
names match {
case Nil => printNames(accum)
case head :: tail => printNamesReverseHelper(tail, head :: accum)
}
}
printNamesReverseHelper(names)
}
printNames(List("Adam", "Framk"))
printNamesReverse(List("Adam", "Framk"))
Added scastie snippet below:
<script src="https://scastie.scala-lang.org/shankarshastri/0dxWt7K3S8CkkAffliLICQ/7.js"></script>