appending elements to list of list in scala

Question

i have created a empty scala mutable list

import scala.collection.mutable.ListBuffer
val list_of_list : List[List[String]] = List.empty

i want to append elements to it as below

filtered_df.collect.map(
          r => {

            val val_list = List(r(0).toString,r(4).toString,r(5).toString)
            list_of_list += val_list
          }
        )

error that i am getting is

Error:(113, 26) value += is not a member of List[List[String]]
  Expression does not convert to assignment because receiver is not assignable.
            list_of_list += val_list

Can someone help

You have to change that `val list_of_list` to `var list_of_list` and also change the type of `list_of_list` into a mutable alternative — Sparker0i, May 16 '20 at 12:13

Duelist · Answer 1 · 2020-05-16T13:17:21.087

Your declaration seems wrong:

val list_of_list : List[List[String]] = List.empty

means that you've declared scala.collection.immutable.List whose operations return a new list without changing the current.

To fix the error you need to change the outer List type to ListBuffer that you imported above the declaration as follows:

val list_of_list : ListBuffer[List[String]] = ListBuffer.empty

Also it looks like you don't to use map here unless you want to modify your data collected from DataFrame, so you can change it to foreach:

filtered_df.collect.foreach {
  r => {
    val val_list = List(r(0).toString,r(4).toString,r(5).toString)
    list_of_list += val_list
  }
}

Furthermore you can make it in a functional way without resorting to ListBuffer, by using immutable List and foldRight as follows:

val list_of_list: List[List[String]] = 
  filtered_df.collect.toList
    .foldRight(List.empty[List[String]])((r, acc) => List(r(0).toString,r(4).toString,r(5).toString) :: acc)

toList is used to achieve a stack safety when calling foldRight, because it's not stack safe for Arrays

More info about foldLeft and foldRight

Unfortunately `foldRight` is not stack-safe I would prefer `foldLeft` instead `foldRight`, `foldLeft` is stack safe — Boris Azanov, May 16 '20 at 13:10
@BorisAzanov you're right. It's stack-safe only for immutable `List`. Edited the answer — Duelist, May 16 '20 at 13:18

score 0 · Answer 2 · answered May 16 '20 at 12:20

0

You have to change that val list_of_list to var list_of_list. That alone would not be enough as you also have to change the type of list_of_list into a mutable alternative.

answered May 16 '20 at 12:20

Sparker0i

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appending elements to list of list in scala

2 Answers2