Question: Being new to Elixir I'm having some problems understanding pattern matching.

If I have an Elixir data structure like this:

```
list_with_maps = [%{"id" => 1, "name" => "a"}, %{"id" => 2, "name" => "b"}]

```

What would be the best way to get values of all id fields from it?

Answer: 
A more generic (and simpler) way of getting a subset of the keys of a `Map` is to use `Map.take/2`, which you can use like this:

```
map = %{"id" => 1, "name" => "a"}
Map.take(map, ["id"])
> %{"id" => 1}

```

As you can see, it takes an array of keys and returns a new map with only the keys you want.

Now, applying this to a list is as simple as using a map, and then using the `Map.take/2` in mapper function. As has been pointed out, you can do this using either a lambda:

```
Enum.map(list_with_maps, fn (map) -> Map.take(map, ["id"]) end)

```

Or you can use a capture:

```
Enum.map(list_with_maps, &(Map.take(&1, ["id"])))

```

This will create more intermediate maps, but for most situations that won't be a problem, as Elixir is pretty smart about memory re-usage and won't actually create these objects a lot of the times, unles
For sake of completeness for the answers for this question you could also do something like this:

```
defmodule Test do
  def get_all_ids([head | tail ]) do
    IO.puts head["id"]
    get_all_ids(tail)
  end

  def get_all_ids([]) do
    IO.puts "end"
  end
end

```

Which would be used like so:

```
iex(7)> Test.get_all_ids(list_with_maps)
1
2
end
:ok

```

Although I think @Gazler's answer is the better answer in this case.

Oh and since you specifically mentioned pattern matching, this would also work:

```
defmodule Test do
  def get_all_ids([%{"id" => id} = m  | tail ]) do
    IO.puts id
    get_all_ids(tail)
  end

  def get_all_ids([]) do
    IO.puts "end"
  end
end

```

The call would be exactly the same; the difference in the second approach is that it's using a pattern match to parse the map in the argument list. 

You might also change the argument list in this line: `def get_all_ids([%{"id" => id} = m | tail ]) do` to this: `def get_all_ids([%{"id" => id} = _m | tail ]) do` just to avoid the warning about `m` being unused.