Elixir获取地图，其中包含特定字段的最大值

Question

I'm a beginner in Elixir programming language.

I have an object something like this:

{
  id: uuid,
  created_at: DateTime.t,
  updated_at: DateTime.t,
  type: my_type
}

let's say my_type is one of ~w[ABCD]

I want to write a function which takes a list of these objects and returns a following map:

%{
  A: 120,
  B: 220,
  C: 560,
  D: 0,
  any: 560
} 

The values here has to be the MAXIMUM difference between updated_at and created_at columns( Timex.diff(updated_at, created_at, :seconds) ) per my_type + any in addition. In case of any the my_type is not considered and takes the maximum among all objects in the list.

What is the best way to do it in Elixir? Thanks.

Answer 1

The following will group the list by its type, then calculate the max difference for each group, and finally result in a map containing each type as the key and max difference as the value.

map = list
|> Enum.group_by(& &1.type)
|> Enum.map(fn {type, values} ->
  max =
    values
    |> Enum.map(fn %{created_at: created_at, updated_at: updated_at} ->
      # return the difference here
    end)
    |> Enum.max
  {type, max}
end)
|> Map.new

This should give you something like:

%{
  A: 120,
  B: 220,
  C: 560,
  D: 0
}

You can calculate the value for any now by doing map |> Map.values |> Enum.max .

Answer 2

I am posting this here for the sake of formatting and diversity. The answer by @Dogbert is likely better fit, although this approach could be more straightforward and arguably flexible.

list = [%{type: :a, v1: 10, v2: 20},
        %{type: :a, v1: 10, v2: 30},
        %{type: :b, v1: 10, v2: 20},
        %{type: :c, v1: 10, v2: 20}]

kw = for %{type: t, v1: v1, v2: v2} <- list,
       do: {t, v2 - v1}, into: []
#⇒ [a: 10, a: 20, b: 10, c: 10]

kw
|> Enum.sort_by(fn {_, v} -> v end, &>=/2)
|> Enum.uniq_by(fn {k, _} -> k end)
#⇒ [a: 20, b: 10, c: 10]