Delete the 1st pair in a map

AFAIK maps are unordered, so there is no “first”. You could enumerate over it, collecting all but the first in the enumeration, but I think it would be an arbitrary selection so I’m not sure there’s much difference between that and Map.delete(map, random_key)

In this situation, my keys are atoms or lists, or at least I need to iterate through a map except 1st one, maybe something like Enum.each(map, fn key, value -> if key == Enum.at(map,0) do end end ?

map
|> Enum.to_list()
|> tl()
|> Enum.into(%{})

A simple way but maybe not most performant and doesn’t solve the problem I stated above

Please post your code showing how your map is constructed. As @msimonborg said, there is no such concept of the “first” item in a map.

Actually, I can do this trick [ first_key_to_delete | _] = Enum.to_list(my_map) thanks to @msimonborg

Just for the sake of experiment, try the following in iex

m1 = Enum.map(1..100, &{&1, &1}) |> Map.new()
m2 = Enum.map(1..1000, &{&1, &1}) |> Map.new()
Enum.at(m1, 0) == Enum.at(m2, 0)

There is no consistent concept of the “first” in a map, otherwise this should return true no matter how large or small you make the range. If you are trying to skip or remove the first pair of a map, you will most likely get inconsistent results as the size and composition of the map changes.

One option for deterministic behaviour is to sort the keys before picking the first one to use for the delete.

map
|> Enum.sort() # use deafult erlang ordering
|> tl() # get the tail, the rest but the first
|> Map.new()

Check also Enum.sort_by/3 for custom ordering

Which key do you call the first key of a map and how you know it is still the first key ? I think that’s what the other replies are trying to tell you.

iex(13)> %{a: 1, b: 2} == %{b: 2, a: 1}
true

You can see that for Elixir even if I invert the order of the above maps there are considered to be the same map.

iex(14)> %{b: 2, a: 1}
%{a: 1, b: 2}

And above you can notice that when Elixir is evaluating %{b: 2, a: 1} it sorts the keys differently. So if one thinks that :b will always be the first key, he could be totally wrong.

I think it’s better to know the name of the key you want actually to be deleted when manipulating maps.

Ah yep, in this case, all the keys’ properties are quite similar, and I just want to delete first one, that’s all