ARRG! Still stuck in the imperative world! Help! :)

czrpb

Have a Map whose values are MapSets: Best way to update?

Currently have:

keys
|> Enum.reduce(
  %{},
  fn key, acc ->
    acc
    |> Map.put(
      key,
      Map.get(acc, key, MapSet.new())
      |> MapSet.put(get_value_to_add(key))
    )
  end
)

14 comments

#maps #mapset

0 1164 14

2020-07-18 08:32:10 UTC

Showing Posts 1 to 10

Show Best Posts
Show All Posts (oldest first)
Show All Posts (newest first)

kokolegorille

It is not very clear what You want to achieve… better put some data and expected result.

Anyway, this looks wrong (bad parens placement…)

Map.get(acc, key, MapSet.new())
      |> MapSet.put(get_value_to_add(key))

and piping once is something I try not to do. At least two pipes, otherwise I pass the parameter normally.

There is a Map.update function

Post #1

al2o3cr

Could alternatively be spelled:

update_in acc, [Access.key(key, MapSet.new())], &MapSet.put(&1, get_value_to_add(key))

Post #2

czrpb

better way for updating the MapSet within the Map?

["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]
|> Enum.reduce(
  %{},
  fn word, acc ->
    first_letter = String.at(word, 0)
    acc |> Map.put(
      first_letter,
      Map.get(acc, first_letter, MapSet.new()) |> MapSet.put(word)
    )
  end
)
|> inspect
|> IO.puts

$ elixir ~/Downloads/elixir-tmp-1.exs
%{"a" => #MapSet<["apple", "apricot"]>, "b" => #MapSet<["banana", "blackberry", "blueberry"]>, "k" => #MapSet<["kiwi"]>}

Post #3

kokolegorille

Except the MapSet…

iex> list = ["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]
["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]
iex> Enum.group_by(list, &String.first/1)
%{
  "a" => ["apple", "apricot"],
  "b" => ["banana", "blueberry", "blackberry"],
  "k" => ["kiwi"]
}

With MapSet…

list 
|> Enum.group_by(&String.first/1) 
|> Enum.reduce(%{}, fn {k, v}, acc -> Map.put(acc, k, MapSet.new(v)) end)
%{
  "a" => #MapSet<["apple", "apricot"]>,
  "b" => #MapSet<["banana", "blackberry", "blueberry"]>,
  "k" => #MapSet<["kiwi"]>
}

With MapSet, short version

iex> list 
|> Enum.group_by(&String.first/1) 
|> Enum.reduce(%{}, &Map.put(&2, elem(&1, 0), MapSet.new(elem(&1, 1))))  
%{
  "a" => #MapSet<["apple", "apricot"]>,
  "b" => #MapSet<["banana", "blackberry", "blueberry"]>,
  "k" => #MapSet<["kiwi"]>
}

Post #4

ityonemo

A matter of personal preference, but I don’t like that reduce, because I tend to avoid using reduces unless it’s necessary, Enum.into is you much much more readable friend.

list = ["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]

list
|> Enum.group_by(&String.first/1)
|> Enum.map(fn 
  {key, value} -> {key, Enum.into(value, MapSet.new()}
end)
|> Enum.into(%{})

Post #5

kokolegorille

I always think reduce is better than map into… but last time I checked with benchee, I was proved wrong

Post #6

ityonemo

I tend to do things where network is very much the bottleneck, and correctness and easy debug/read legacy code is more important, so I’ve slowly let go of caring about performance =D

My only readability concern with my code is that it’s not immediately obvious that “group_by” returns a map. so I might put a comment to that effect. Also, I’m really surprised there wasn’t a List.to_mapset…

Post #7