Enum.sort does not work as expected

Questions / Help
,
1

I have a situation that I can not figure out yet for some time and it drives me nuts. Either Enum.sort is not working as expected either I am doing it wrong.
I have some rules have a name and depend on each other. Source is the name of the dependency. I am trying to sort them ascending by dependency to be able to use them later to compute some amounts:

rules = [
  %{
    name:  "ambassador1",
    source: "ambassadors"
  },
  %{
    name: "zuppler fee",
    source: "total"
  },
  %{
    name:  "ambassadors",
    source: "zuppler fee"
  }
]

sorter = fn
  sorter = fn
  %{name: name1}, %{source: source2} when name1 == source2 -> true
  %{source: source1}, %{name: name2} when source1 == name2 -> false
  %{name: name1, source: source1}, %{name: name2, source: source2} when name1 == name2 and source1 == source2 -> true
  %{name: name1, source: source1}, %{name: name2, source: source2} -> name1 < name2 and source1 < source2
end
  _, _ -> false
end
rules |> Enum.sort(sorter) |> IO.inspect()

Result is:

[
  %{name: "ambassadors", source: "zuppler fee"},
  %{name: "zuppler fee", source: "total"},
  %{name: "ambassador1", source: "ambassadors"}
]

when the expected result would be something like:

[
  %{name: "zuppler fee", source: "total"},
  %{name: "ambassadors", source: "zuppler fee"},
  %{name: "ambassador1", source: "ambassadors"}
]

Can somebody give me a hint?

1 Like
2

Function passed to Enum.sort/2 need to provide strict ordering, which mean that func.(a, b) == not func.(b, a) or func.(a, a) == true. This is not a case in your example.

3

Also souce in your map keys vs. source in your pattern matches.

2 Likes
5

This is a typo error. But it real code it’s typed ok and still does not work.

6

I updated my function to handle all comparison cases. I tried to cover all comparison cases that can appear between two elements. It may not be exactly strict ordering but taking whatever two elements it orders them correctly.
Also the docs does not specify too many requirements for this function: https://hexdocs.pm/elixir/Enum.html#sort/2

This function uses the merge sort algorithm. The given function should compare two arguments, and return true if the first argument precedes or is in the same place as the second one.

I think it complies with this requirement.

7

I’m going to suggest it is the order in which you have defined the conditions or the order of items in the list to begin with. The first two conditions/guards (when clauses) lead to a situation where item 2 > item 3 and item 3 > item 1 but the comparison between item 1 and item 2 gives item 1 > item 2 based on the last condition/guard clause. This leads to ambiguity that seems to be resolved based on the order of items in the list.

You can see this if you alter the order of the original list and apply the same sort. You get a different sorted order.

8

Calling sorter with %{name: "zuppler fee", source: "total"} and %{name: "ambassador1", source: "ambassadors"} is going to return false, so I don’t think Enum.sort would find the configuration you’re looking for. The position of %{name: "ambassador1", source: "ambassadors"} in the result depends on the existence of %{name: "ambassadors", source: "zuppler fee"}…

Consider topological sort + tree traversal algorithms for this one.

2 Likes
9

Thanks for the tip.

10

@silviurosu Is this what you want?

Enum.sort_by(rules, &(&1.name), :desc)