Iterating through a nested map without changing the structure

Enum.max_by returns at max 1 element =)
If you want to do it in a single cycle you probably want to do some custom reduce like

Enum.reduce(map, [], fn 
  item, [] -> [item]
  {_, val} = item, [{_, acc_val} | _] when length(val) > length(acc_val) -> [item]
  {_, val} = item, [{_, acc_val} | _] = acc when length(val) == length(acc_val) -> [item | acc]
  _, acc -> acc
end)

P. S. naming is hard

Seems that max_by won’t work for you here, because:

If multiple elements are considered maximal, the first one that was found is returned.

So you need something like:

{count, values} =
  map
  |> Enum.group_by(fn {_key, values} -> length(values) end)
  |> Enum.max_by(fn {count, _values} -> count end)

Yeah it makes sense now. max_by will output the only instance maximum output. Enum.reduce seems like a better option.

Thats right … as @RudManusachi mentioned as well.

To be honest, if Enum.reduce is a better option or not - I’m not sure

I like the example of @stefanchrobot more, because it looks simpler and reads better =)