CODEWITHSUNDEEP
elixir
Junaid Farooq
Junaid Farooq

Reputation: 2598

Find last index value for duplicate key item in list Elixir

I am trying to find index_value for an element in the list which is duplicate

[%{day: "mon", name: "Doc"}, %{day: "mon", name: "Doctor"}, %{day: "mon", name: "Doctor"}, %{day: "wed", name: "Doc"}, %{day: "wed", name: "Doc"}]

I am only concerned about day.

When I try

list |> Enum.find_index(fn hour -> hour.day == "mon" end)

this gives me 0, where "mon" has 3 items.

I want to get the very last index value, anyhow my main purpose is to get use that index to get the very next value to it.

In this case , I am trying to get

%{day: "wed", name: "Doc"}

any help would be grateful, thank you.

Upvotes: 2

Views: 317

Answers (1)

Aleksei Matiushkin
Aleksei Matiushkin

Reputation: 121000

There is no such thing as index for linked lists. Linked lists are not arrays.

If you are trying to get to the next value after a bunch of %{day: "mon"}, you might want to reduce

Enum.reduce_while(input, :before, fn
  %{day: "mon"}, _ -> {:cont, :in}
  result, :in -> {:halt, result}
end)
#⇒ %{day: "wed", name: "Doc"}

or, which is arguably more readable, but less performant, Enum.chunk_every/4 in combination with Enum.find/2

input
|> Enum.chunk_every(2, 1, :discard)
|> Enum.find(&match?([%{day: "mon"}, %{day: day}] when day != "mon", &1))
#⇒ [%{day: "mon", name: "Doctor"}, %{day: "wed", name: "Doc"}]

The second/last element of the array returned is the result.

Upvotes: 2

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