从列表中过滤单个项目

Question

Given:

iex(9)> list_of_maps = [%{"a" => 1, "b"  => 2, "c" => 3},%{"a" => 66, "b"  => 1, "c" => 9},%{"a" => 66, "b"  => 20, "c" => 8}]

I can do:

iex(10)> Enum.filter(list_of_maps, &(&1["a"] == 1))                                                                                                                                                                
Enum.filter(list_of_maps, &(&1["a"] == 1))                                                                                                                                                                         
[%{"a" => 1, "b" => 2, "c" => 3}]

However now comes the part I dread writing in every language - getting the first value of this list to extract the single item.

Is there some standard function in elixir that filters a list, returning single item if there is only one item after a filter is applied, or a list of items if there are numerous items returned after the filter is applied? Like:

iex(11)> Enum.filterr(list_of_maps, &(&1["a"] == 1))                                                                                                                                                                
Enum.filter(list_of_maps, &(&1["a"] == 1))                                                                                                                                                                         
%{"a" => 1, "b" => 2, "c" => 3}

iex(12)> Enum.filterr(list_of_maps, &(&1["a"] == 66))                                                                                                                                                                
Enum.filter(list_of_maps, &(&1["a"] == 66))                                                                                                                                                                         
[%{"a" => 66, "b"  => 1, "c" => 9},%{"a" => 66, "b"  => 20, "c" => 8}]]

Answer 1

Find one item from a List

If you want to filter a list to get only one item , use Enum.find/2 :

Enum.find(list_of_maps, fn m -> m["a"] == 1 end)

---

Get one or a list of matches

To handle both cases, pattern matching is the way to go:

defmodule MyEnum do
  def filter(list, fun) do
    list
    |> Enum.filter(fun)
    |> normalize
  end

  defp normalize([item]), do: item
  defp normalize(list),   do: list
end

You can then use it like this:

MyEnum.filter(list_of_maps, &(&1["a"] == 1))

It will return a list if there are multiple matches or the map itself if there's only one match.

Answer 2

Just pipe your filtered list into Enum.at/2

first = list_of_maps |> Enum.filter(&(&1["a"] == 66)) |> Enum.at(0)

That will get the first element of the filtered list regardless of size.

Also, as Dogbert mentioned in the comments you can just use Enum.find/2 instead of filter to just find the first match.

Enum.find(list_of_maps, &(&1["a"] == 66))

Answer 3

Is there some standard function in elixir that filters a list, returning single item if there is only one item … or a list of items if there are numerous items

No, but pattern matching makes it trivial:

def single_or_many([single]), do: single
def single_or_many(many), do: many

And if you wanted to treat the empty list specially (before the 'many' case):

def single_or_many([]), do: :nil

Then, the function call chain would be:

list_of_maps
|> Enum.filter(&(&1["a"] == 1))
|> single_or_many