Get specific key and the rest of map with Pattern Matching in Elixir

Questions / Help
,
1

Im trying to get one element and the rest of the map with pattern matching but I’m only get compile errors.

I came up with this:

%{“One” => one | tail} = %{“One" => 1, "Three" => 3, "Two" => 2}

But I got compile errors saying that it was expected key-value pairs.

The behavior that I’m trying to achieve is:

%{“One” => one | tail} = %{“One" => 1, "Three" => 3, "Two" => 2}
one = 1
tail = %{"Three" => 3, "Two" => 2}

In elixir there is a way to acomplished that?

2

It sounds like you want to use Map.pop/3

iex(1)> {value, map} = Map.pop(%{"One" => 1, "Three" => 3, "Two" => 2}, "One")
{1, %{"Three" => 3, "Two" => 2}}
iex(2)> value
1
iex(3)> map
%{"Three" => 3, "Two" => 2}
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3

Note that you get that error because on %{"One" => one | tail} the | is used to update the key, in your case is like that you are trying to update the tail key but missing the value.

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4

A bit late to the party, but if you want to pattern match in a function definition, you can also capture the whole map:

defmodule Foo do
  def foo(map = %{"One" => one}) do
    rest = Map.delete(map, "One")
    IO.inspect(%{one: one, rest: rest})
  end
end

 Foo.foo(%{"One" => 1, "Three" => 3, "Two" => 2})
# > %{one: 1, rest: %{"Three" => 3, "Two" => 2}}
3 Likes