Enum find by id?

Questions / Help
1

How to find index of element inside enum by property?

I have this enum and I need to find the index of an item by its id(or another property).
[%{“code” => “ABEV3”, “comparison” => “>”, “field” => “close”, “id” => “4a9d96ce-e572-11eb-a557-0242ac170003”, “message” => “ABEV3 Close price is higher than 11.9!”, “value” => 11}]

How to do this?

2

see: Enum.find_index/2

EDIT: OP did not post a map.

Note, that your map-items will be passed as {key, value} to the function.

See the example at the beginning of the Enum documentation:

map = %{"a" => 1, "b" => 2}
Enum.map(map, fn {k, v} -> {k, v * 2} end)
[{"a", 2}, {"b", 4}]
3

Didn’t get it… I understand that I can use find when I already have the element, like this /

Enum.find(alerts, fn x -> equal?(x, alert) end)
    |> case do
      nil -> alerts
      idx -> List.delete_at(alerts, idx)
    end

But when I want to find it by the property, I’m not being able to, tryed

 Enum.find(alerts, fn x -> equal?({:id}, alert_id) end)
    |> case do
      nil -> alerts
      idx -> List.delete_at(alerts, idx)
    end

and

Enum.find(alerts, fn {:id => id} -> equal?(id, alert_id) end)
    |> case do
      nil -> alerts
      idx -> List.delete_at(alerts, idx)
    end

But didn’t work

4

Sorry I overlooked that you want the index. I updated my answer.

5

But the question is how to find by property… The examples show how to find by equality, I currently don’t have the entire object, only the property(id) value

6

You want to (eg) find the index of the item with id==2 right?

[%{id: 1, foo: :bar}, %{id: 2, foo: :baz}, %{id: 3, foo: :foobar}]

So you want the index of {id: 2, foo: baz} which is 1…?

Edit: too much python today.

7

Exactly

8

find_index/2 will return the index of the first element, where the function you provide returns true.
So write that function.

9

Don`t like python, but C, C#, Javascript… I’m just new to Elixir, and didn’t spend enough time to learn it yet, but thank you for your help

10

Sorry I most likely confused you with the hint that map-items are given as {k, v} to the function.
You have a list of maps, So you just do:

e = [%{foo: :bar, id: 1}, %{foo: :baz, id: 2}, %{foo: :foobar, id: 3}]
iex(3)> Enum.find_index(e, fn v -> v.id == 1 end)                           
0

So find_index goes through the list element by element, calls the function you provide and if this function return true, the index is returned.

1 Like
11

take a look at the great visual Elixir reference:

https://superruzafa.github.io/visual-elixir-reference/Enum/find_index/2/

3 Likes
12

you can also use pattern matching

Enum.find_index(list, fn %{id: ^id} -> true; _ -> false end)

you could also use filter to just remove the item

Enum.filter(list, fn %{id: ^id} -> false; _ -> true end)
1 Like
13

I didn’t get it…

In this case, what does ‘^’ mean on the id?

14

Pinning, so it will match the already bound value instead of rebinding. However in this case I think that it would be more readable to do:

Enum.find(list, fn elem -> elem.id == id end)
2 Likes
15

As @hauleth said, you can pin the variable. So if this is wrapped into a function

def remove_alert(alerts, id) do
    Enum.filter(alerts, fn %{id: ^id} -> true; _ false end)
end

You can match on the id variable in the parent scope, without the ^, id just gets assigned to whatever the id of the alert is in the fn scope.