Split list into multiple lists with fixed number of elements

Question

I think you're looking for grouped. It returns an iterator, but you can convert the result to a list,

scala> List(1,2,3,4,5,6,"seven").grouped(4).toList
res0: List[List[Any]] = List(List(1, 2, 3, 4), List(5, 6, seven))

There is much easier way to do the task using sliding method. It works this way:

val numbers = List(1, 2, 3, 4, 5, 6 ,7)

Lets say you want to break the list into smaller lists of size 3.

numbers.sliding(3, 3).toList

will give you

List(List(1, 2, 3), List(4, 5, 6), List(7))

Or if you want to make your own:

def split[A](xs: List[A], n: Int): List[List[A]] = {
  if (xs.size <= n) xs :: Nil
  else (xs take n) :: split(xs drop n, n)
}

Use:

scala> split(List(1,2,3,4,5,6,"seven"), 4)
res15: List[List[Any]] = List(List(1, 2, 3, 4), List(5, 6, seven))

edit: upon reviewing this 2 years later, I wouldn't recommend this implementation since size is O(n), and hence this method is O(n^2), which would explain why the built-in method becomes faster for large lists, as noted in comments below. You could implement efficiently as follows:

def split[A](xs: List[A], n: Int): List[List[A]] =
  if (xs.isEmpty) Nil 
  else (xs take n) :: split(xs drop n, n)

or even (slightly) more efficiently using splitAt:

def split[A](xs: List[A], n: Int): List[List[A]] =
  if (xs.isEmpty) Nil 
  else {
    val (ys, zs) = xs.splitAt(n)   
    ys :: split(zs, n)
  }

I am adding a tail recursive version of the split method since there was some discussion of tail-recursion versus recursion. I have used the tailrec annotation to force the compiler to complain in case the implementation is not indeed tail-recusive. Tail-recursion I believe turns into a loop under the hood and thus will not cause problems even for a large list as the stack will not grow indefinitely.

import scala.annotation.tailrec


object ListSplitter {

  def split[A](xs: List[A], n: Int): List[List[A]] = {
    @tailrec
    def splitInner[A](res: List[List[A]], lst: List[A], n: Int) : List[List[A]] = {
      if(lst.isEmpty) res
      else {
        val headList: List[A] = lst.take(n)
        val tailList : List[A]= lst.drop(n)
        splitInner(headList :: res, tailList, n)
      }
    }

    splitInner(Nil, xs, n).reverse
  }

}

object ListSplitterTest extends App {
  val res = ListSplitter.split(List(1,2,3,4,5,6,7), 2)
  println(res)
}

I think this is the implementation using splitAt instead of take/drop

def split [X] (n:Int, xs:List[X]) : List[List[X]] =
    if (xs.size <= n) xs :: Nil
    else   (xs.splitAt(n)._1) :: split(n,xs.splitAt(n)._2)