List with nested maps

Questions / Help
, , ,
1

How would I map over something like this?

results = [
  {"John Smith", %{"twitter" => "jsmith"}},
  {"Lisa Smith", %{"twitter" => "lsmith", "website" => "lsmith.com"}}
]

Into something like this

new_results = [
  %{name: "John Smith", twitter: "jsmith", website: ""},
  %{name: "Lisa Smith", twitter: "lsmith", website: "lsmith.com"}
]

Each entry might not have a twitter_handle or website, but I still need to have those entries. Even if the values of them are blank.

I’m think I need to Pattern match to get the values out of the results?

# Pattern Match A
{username, %{"twitter"=> twitter_handle}} = {"John Smith", %{"twitter" => "jsmith"}}

username = "John Smith"
twitter_handle = "jsmith"

# Pattern Match B
{username, %{"twitter"=> twitter_handle, "website"=> website}} = {"Lisa Smith", %{"twitter" => "lsmith", "website" => "lsmith.com"}}

username = "Lisa Smith"
twitter_handle = "lsmith"
website = "lsmith.com"

And then Enum.map into a new list?

Looking for some feedback on how I’m approaching this problem.

2 
iex> results |> Enum.map(fn {username, map} = _tuple -> %{name: username, twitter: Map.get(map, "twitter"), website: Map.get(map, "website") } end) 
[%{name: "John Smith", twitter: "jsmith", website: nil},
 %{name: "Lisa Smith", twitter: "lsmith", website: "lsmith.com"}]

if You want to avoid nil value, you can use map get with default value.

1 Like
3

Or perhaps more generically if the keys are variable:

defmodule Mapper do
  @list [
    {"John Smith", %{"twitter" => "jsmith"}},
    {"Lisa Smith", %{"twitter" => "lsmith", "website" => "lsmith.com"}}
  ]

  def merge do
    Enum.map @list, fn {name, map} ->
      map
      |> Map.put("name", name)
      |> atomize_keys
    end
  end

  def atomize_keys(map) do
    map
    |> Enum.map(fn {k, v} -> {String.to_atom(k), v} end)
    |> Enum.into(%{})
  end
end
4

Your solution does not create needed blank keys :slight_smile:

5

This solution works. Some follow up questions:

What does _tuple mean in this context?

6

it means nothing special, it says {username, map} comes from a tuple which will not be used later. It could be useful if you also want to use this tuple.

{u, m} = t is the destructuring notation

i could have used this short one without the need of it.

iex> results |> Enum.map(fn {u, m} -> %{name: u, twitter: Map.get(m, “twitter”), website: Map.get(m, “website”) } end)

7

Ah, good call. Here’s the update:

defmodule Mapper do
  @list [
    {"John Smith", %{"twitter" => "jsmith"}},
    {"Lisa Smith", %{"twitter" => "lsmith", "website" => "lsmith.com"}}
  ]
  
  @default_keys %{"twitter" => "", "website" => ""}

  def merge do
    Enum.map @list, fn {name, map} ->
      map
      |> Map.put("name", name)
      |> Map.merge(@default_keys, fn _k, v1, v2 -> v1 end)
      |> atomize_keys
    end
  end

  def atomize_keys(map) do
    map
    |> Enum.map(fn {k, v} -> {String.to_atom(k), v} end)
    |> Enum.into(%{})
  end
end
8

def atomize_keys(map) do
map
|> Enum.map(fn {k, v} → {String.to_atom(k), v} end)
|> Enum.into(%{})
end

When I see a map into, I am tempted to prefer reduce like this

iex> map |> Enum.reduce(%{}, fn ({k, v}, acc) -> Map.put(acc, String.to_atom(k), v) end)
1 Like