Using Generics To Group A List Of Elements-CodePudding

I'm trying to group elements in a list, when I tried doing this with only one type, the first function works. But when I tried to use generics so I could group elements no matter the type, the function below doesn't do the work. Can you explain how can I do that?

def groupString (list: List[String]): List[List[String]] = {
    def groupStringHelper(list: List[String], n: String, outputList:List[List[String]], outputListCurrent: List[String]):List[List[String]] =
      if (list.isEmpty) outputList:  outputListCurrent
      else if (list.head == n) groupStringHelper(list.tail, n, outputList, outputListCurrent :  list.head)
      else groupStringHelper(list.tail, list.head, outputList : outputListCurrent, List(list.head))

    groupStringHelper(list, null, List(), null).tail
  }
  def group [A](list: List[A]): List[List[A]] = {
    def groupHelper(list: List[A], n: A, outputList:List[List[A]], currentList: List[A]):List[List[A]] =
      if (list.isEmpty) outputList:  outputListCurrent
      else if (list.head == n) groupHelper(list.tail, n, outputList, currentList :  list.head)
      else groupHelper(list.tail, list.head, outputList : currentList, List(list.head))
    groupHelper(list, null, List(), null).tail
  }

CodePudding user response：

You would be wise to follow the advice (comment) from @Luis Miguel.

Here's a different approach you could take. (Scala 2.13.x)

def group[A](list: List[A]): List[List[A]] =
  List.unfold(list){ lst =>
    Option.when(lst.nonEmpty)(lst.span(_ == lst.head))
  }

CodePudding user response：

The reason your code not compiled is not all type A is nullable, e.g. Int. So you can make your code work as:

  def group [A >: Null](list: List[A]): List[List[A]] = {
    def groupHelper(list: List[A], n: A, outputList:List[List[A]], currentList: List[A]):List[List[A]] =
      if (list.isEmpty) outputList:  currentList
      else if (list.head == n) groupHelper(list.tail, n, outputList, currentList :  list.head)
      else groupHelper(list.tail, list.head, outputList : currentList, List(list.head))
    groupHelper(list, null, List(), null).tail
  }

However, the above code would not work for group[Int](List(1, 2, 3)).

Here is a solution based on your code with removing null:

def group[A](list: List[A]): List[List[A]] = {
  def groupHelper(list: List[A], n: A, outputList:List[List[A]], currentList: List[A]):List[List[A]] =
    if (list.isEmpty) outputList:  currentList
    else if (list.head == n) groupHelper(list.tail, n, outputList, currentList :  list.head)
    else groupHelper(list.tail, list.head, outputList : currentList, List(list.head))
  if(list.isEmpty) Nil else groupHelper(list.tail, list.head, Nil, list.take(1))
}