I know how to use comprehensions to generate combinations/permutations: (for a <- 0..9, b <- 0..9 |> Enum.reject(&Kernel.==(&1, a)), c <- 0..9 |> Enum.reject(fn i -> i == a or i == b end), do: [a,b,c]) |> apply_criteria() How would I do this lazily so that I stop generating permutations once I fin…

I think you need to go in order, otherwise you won’t know when you have exhausted all possibilities. I would use the recursive approach like such: def gen_perms(criteria), do: gen_perms({0, 0, 0}, criteria) def gen_perms({_, _, 10}, criteria), do: {:no_match, criteria} def gen_perms({a, 10, c}, c…

[image] stevensonmt: (for a <- 0..9, b <- 0..9 |> Enum.reject(&Kernel.==(&1, a)), c <- 0..9 |> Enum.reject(fn i -> i == a or i == b end), do: [a,b,c]) As an aside: filtering is supported out of the box by Elixir’s for comprehension: (for a <- 0..9, b <- 0..9, c <- 0..9, a != b and b != c, do…

[image] stevensonmt: Stream.repeatedly(Enum.random(0..9)) |> Stream.chunk_every(3) |> Stream.filter(fn combo -> Enum.uniq(combo) == combo end) |> apply_criteria() But this only works in the cases for which there is a permutation that satisfies the criteria. If there is no match it will just go …

Even though @APB9785 ’s code is sound and does what it says it does, I am still not sure I understand your requirements. Can you share some more of them? Or ideally a small GitHub projects where you show where are you at currently?

@APB9785 ’s solution is a great illustration of what I’m trying to do. I’m trying to check each combination as they’re created and short circuit at the first correct combination. I had it in my head that it should be possible to do with a Stream of some sort but that recursive approach is fine. The i…

defp gen_combos(len, elements, criteria) do Stream.cycle([0]) |> Enum.take(len) |> Enum.zip(elements) |> next_combo(len, elements, criteria) end defp valid_combo(combo, criteria) do Enum.all?(criteria, fn check -> apply(check, combo) end) end defp next_combo([{hd, c} | …

Here’s a fully-lazy approach: defmodule RecursiveStream do def all_exactly(_, 0), do: [] def all_exactly([], _), do: [] def all_exactly(alphabet, 1) do Stream.map(alphabet, &[&1]) end def all_exactly(alphabet, n) do streams = Enum.map(alphabet, &prepend(&1, all_exactly(alphabet -…

Thanks for that tip. Because I’m generating permutations of N length from a set of M items I had to change the terminating case from a map size of zero to map size of M - N. It’s about 30% faster than my original version. I also threw in a Enum.shuffle(M) when generating the permutations. This rando…

I can’t seem to get this to work. The problem is that I don’t want any permutations of the wrong length, so I’d have to introduce a filtering step after generating them all, which seems terribly inefficient. For some reason I’m finding it very difficult to follow how your code works so I don’t know …

Elixir Programming Language Forum

Lazily generate permutations

Questions & Help Questions

thepeoplesbourgeois May 31, 2021, 8:33am 9

You’ll generate the permutations faster with a map