Macro for generating combos of variable length from a given collection

Questions / Help
1

In an earlier AOC thread @LostKobrakai posted the following code for generating pairs from a collection:

combinations =
        for a <- options, b <- options, uniq: true do
          [a, b] |> Enum.sort() |> List.to_tuple()
        end

I am not familiar at all with writing macros in elixir, but I think it would be possible to have a macro that took an argument k such that you could have

combinations =
        for a <- options, b <- options, ... k <- options, uniq: true do
          *stuff*
        end

Is this possible? If so, any tips or suggested resources to look into?

2

You do not need macros for that, just function would do. What you need to notice is that combinations with size N are combinations of size N-1 and other elements. So you can do similarly to my solution

  def run([], _, _), do: nil
  def run(_, _, sum) when sum < 0, do: nil
  def run([a | _], 1, a), do: [a]

  def run([a | rest], n, sum) do
    case run(rest, n - 1, sum - a) do
      nil -> run(rest, n, sum)
      nums when is_list(nums) -> [a | nums]
    end
  end

You just need to replace addition with generating lists and this will result in what you want.

I can provide more complete example later as right now I am on my phone, so it is hard to code something like that.

1 Like
3

That‘s only possible if you know ˋkˋ at compile time. If it‘s dynamic at runtime macros won‘t help.

2 Likes
4

Thanks for clarifying.