Dealing with an array in phoenix

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1

Hello again with my “questions” :slight_smile:
Having this in my live phx1.6

def handle_event("do_something", %{"array" => attrs}, socket) do
    (...)
end

where attrs would be:

%{user1_id: 1, user2_id: 2}

I can do this for example:

attrs["user1_id"]

which will obviously return 1. But how do I update this user1_id, let’s say I want to change the value of user1_id to 5.

attrs["user1_id"] = 5

won’t do it. Tried this: Map.update!(%{user1_id: 1}, :user1_id, &(&5)), also array |> Map.replace!(:user1_id, "8") Stupit question again, but can’t figure it out :slight_smile:

Best Regards

2

First, attrs is not an array, it’s a map. And you can use:

attrs = Map.replace(attrs, :user1_id, 5).

Also, about Map.update!/3, that’s not how the capture operator (&) works. The anonymous function in that function only provides a single parameter (the current value). If you use &(&5), you’re trying to access the fifth parameter being provided, which does not exist in this case. Doing it like this Map.update!(attrs, :user1_id, &(&1 + 1)) would means that i’m taking the current value, and adding one to it.

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3

One way to update a value in a map is to replace it using Map.put/3. It will replace or add any key-value pair. There is also Map.put_new if you don’t want to create a new key-value- pair.

attrs = %{ "user_id" => 1 }

 Map.put(attrs, "user_id", 5)
# %{ "user_id" => 5 }
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4

attrs["user1_id"] will not work with %{user1_id: 1, user2_id: 2} as the first expression uses string keys while the map is defined with atom keys.

5

While this is very true, I want to point out the likelihood that OP has the opposite issue: attrs likely uses string keys, coming from a Phoenix form, and can’t be updated or accessed using atoms.

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