Algorithm for splitting a list into N sublists

You might use Enum.chunk_every/2 on a collection.

iex> 1..10 |> Enum.chunk_every(3) |> IO.inspect(charlists: :as_list)
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]

You could also use remainder like this

iex> for x <- 1..10, do: rem x, 3
[1, 2, 0, 1, 2, 0, 1, 2, 0, 1]

def split(list), do: split(list, [], [], [])

defp split([x1, x2, x3 | rest], l1, l2, l3), 
  do: split(rest, [x1 | l1], [x2 | l2], [x3 | l3])
defp split([], l1, l2, l3), 
  do: {Enum.reverse(l1), Enum.reverse(l2), Enum.reverse(l3)}
defp split([x1, x2], l1, l2, l3), 
  do: {Enum.reverse(l1, [x1]), Enum.reverse(l2, [x2]), Enum.reverse(l3)}
defp split([x1], l1, l2, l3), 
  do: {Enum.reverse(l1, [x1]), Enum.reverse(l2), Enum.reverse(l3)}

iex> Test.split(Enum.to_list(1..10))
{[1, 4, 7, 10], [2, 5, 8], [3, 6, 9]}

If your list has always length divisible by 3, you don’t need the last 2 clauses which account for uneven split.

N sub-lists. N is a variable

That won’t work for me because:

1. it’ll create one additional sub-list when there’s a reminder: rem(7, 3) = 1

2. I don’t know the size of each sub-list. N = the amount of sub-lists

If I have 7 elements and I want to split them into N = 3 sub-lists, there should be created 3 sublists:

[[3 elements], [2 elements], [2 elements]] 

Namely, I want to preserve all the elements

Try this, I think its what you’re after but I suspect there is a better optimisation for the end of a non-uniform list.

defmodule Split do
  def chunk([] = l, n) do
    l
  end
  
  def chunk(list, n) when is_list(list) do
    chunk(Enum.split(list, n), n)
  end
  
  def chunk({l, t}, n) when length(t) <  n * 2 do
    [l | [t]]
  end
    
  def chunk({l, t}, n) do
    [l | chunk(t, n)]
  end
end

If you don’t care about the order in which they’re put in the bins, this should work. Definitely suboptimal because of the list length and integer division at each step, though.

def chunker(list, parts), do: do_chunk(list, parts, [])

defp do_chunk(_, 0, chunks), do: chunks

defp do_chunk(to_chunk, parts, chunks) do
  chunk_length = to_chunk |> length() |> div(parts)
  {chunk, rest} = Enum.split(to_chunk, chunk_length)
  do_chunk(rest, parts - 1, [chunk | chunks])
end

iex> chunker([1, 2, 3, 4, 5, 6, 7], 4)
[[6, 7], [4, 5], [2, 3], [1]]
iex> chunker([1, 2, 3, 4, 5, 6, 7, 3)
[[5, 6, 7], [3, 4], [1, 2]]
iex> chunker([1, 2, 3, 4, 5, 6, 7, 10)
['\a', [6], [5], [4], [3], [2], [1], [], [], []]

I‘d do it like this:

columns = 
  list
  |> Enum.with_index()
  |> Enum.map(fn {_, i} -> rem(i, 3) end)

Enum.zip(list, columns)

Did you end up with a solution to your problem? I’m in the same situation with creating N = 4 sub-lists.

I ended up using this example. It solves my need for now. Thank you.