List in scala not getting updated

Question

I am new to Scala Collections and currently I want to separate a given list of strings into a tuple of two lists (List[String], List[String]), which contains list of palindrome strings and rest of the of the input strings.

For example, if input is List("racecar", "abcd", "lilil", "effg") output should be (List("racecar", "lilil"), List("abcd", "effg"))

I have got a solution using filter. But, currently, trying to refine my solution using foldLeft. My new approach is as follows:

def stringTuples2(strings: List[String]): (List[String], List[String]) = {
strings.foldLeft((List[String](), List[String]()))((b, a) => {
  if (a.equals(a.reverse)) { b._1 :+ a; b }
  else { b._2 :+ a; b }
})}

I am not sure, what I am doing wrong, but the output for this solution is Tuple of two empty lists, i.e. (List(), List()).

Help is appreciated. Thanks!

Answer 1

Your attempt to modify b creates a new List, you then throw the new List away and return b, which is unchanged. Take out the ;b part and return the updated tuple: (b._1 :+ a, b._2) or (b._1, b._2 :+ a)

BTW, here's a different approach to the solution.

List("racecar", "abcd", "lilil", "effg").partition(s => s == s.reverse)