I’m new to Elixir, and relatively new to functional programming. I’ve just written this little algorithm to take a weighted sample from a list of tuples; # items = [apples: 10, oranges: 30, bananas: 60] defp weight_based_choice(items) do max = sum_weights(items) state = %{ current: 0…

Welcome to the forum! Not necessarily idiomatic, just a different way of doing things: defmodule Demo do def weight_based_choice(items) do value = items |> sum_weights(0) |> :rand.uniform() select_item(items, value, 0) end defp select_item([{name, _}], _, _) do …

Can’t we write select_item/3 roughly like this, making it effectively a select_item/2? defp select_item([{name, weight} | _], idx) when idx < weight, do: name defp select_item([{_, weight} | tail], idx), do: select_item(tail, idx - weight) While also making use of Enum.reduce/3 in sum_weights/2 we…

So defmodule Demo do def weight_based_choice(items) do value = items |> Enum.reduce(0, &acc_weight/2) |> :rand.uniform() select_item(items, value) end # base cases defp select_item([{name, _}], _), do: name defp select_item([{name, weight} | _], index) whe…

That’s a fun little problem. Here’s my take, from playing around in iex iex(109)> items = [apples: 10, oranges: 30, bananas: 60] [apples: 10, oranges: 30, bananas: 60] iex(110)> accumulated_weights = Enum.scan(items, fn {k, w}, {_, w_sum} -> {k, w + w_sum} end) [apples: 10, oranges: 40, bananas: 10…

If I were to really use this in production, I’d likely wrap it in its own struct module like this: defmodule WeightedList do defstruct [:acc_list, :max] def new(kw_list) do # Might want to verify it is sorted lowest to highest weight acc_list = Enum.scan(kw_list, fn {k, w}, {_, w_sum} -> {…

[image] peerreynders: To a certain degree choosing reduce when a simple recursive function will do feels a bit like “premature pessimization” especially as recursion is how iteration is accomplished in Elixir/Erlang - but I know I’m in the minority there. I do see it quite the opposite. reduc…

FYI … :smile: Of course, one should be able to implement some functions in Enum on their own (for lists at least). map , reduce , reverse , concat , those are my personal favorites … When learning to a certain degree it can be helpful to make Enum off-limits until recursion (body and tail)…

Yes. I sign this. During learning you are totally correct. But the question was about idiomatic code, not what would I write when I learn about recursion. And using Enum and Stream is by far the most idiomatic way dealing with lists and other “iterables”.

I side with you. I see @peerreynders ’s point but I’ve done my exercises and re-implemented most of Enum. For such problems, start with idiomatic Elixir code and only go for hand-rolled algorithms if performance is of utmost importance.

Thank you so much everyone! This is way more than I expected. It’s a lot to digest, I’ve learned so much. I like @gregvaughn ’s suggestion the most, not because of the struct idea, but because it seems to me to realise the thought process in my original approach, except without all the noobish bloat.…

Weight-based random sampling

Questions & Help Questions

tombh June 28, 2019, 5:52am 17

That really helped clarify it for me, thank you