Elixir - Looping through and adding to map

Question

I\'m rebuilding something in Elixir from some code I built in C#.

It was pretty hacked together, but works perfectly (although not on Linux, hence rebuild).

Answer 1

This might help too:

count_animals_in_area = fn (area, acc) ->
  acc = case Map.has_key?(area, "duck") do
          true ->
            Map.put(acc, "ducks", (acc["ducks"] + area["duck"]))
          false ->
            acc
        end

  acc = case Map.has_key?(area, "goose") do
          true ->
            Map.put(acc, "geese", (acc["geese"] + area["goose"]))
          false ->
            acc
        end

  acc = case Map.has_key?(area, "cat") do
          true -> 
            Map.put(acc, "cats", (acc["cats"] + area["cat"]))
          false ->
            acc
        end

  acc
end

count_animals_in_areas = fn(areas) ->
  acc = %{ "ducks" => 0,
           "geese" => 0,
           "cats" => 0 }
  IO.inspect Enum.reduce areas, acc, count_animals_in_area
end

t1 = [ %{"duck" => 3, "goose" => 4, "cat" => 1},
       %{"duck" => 7, "goose" => 2},
       %{"goose" => 12}]

IO.puts "JEA: begin"
count_animals_in_areas.(t1)
IO.puts "JEA: end"

Output:

iex(31)> c "count_animals.exs"
JEA: begin
%{"cats" => 1, "ducks" => 10, "geese" => 18}
JEA: end
[]

I am just learning Elixir so the above is undoubtedly suboptimal, but, hopefully slightly informative.

Answer 2

I am also new to Elixir but here is a cute and simple solution that uses pattern matching and recursion.

defmodule YourModule do
   def reduce_list([], reduced) do reduced end

   def reduce_list([first | rest ], reduced) do
      # Do what you need to do here and call the function again 
      # with remaining list items and updated map.
      reduce_list(rest, Map.put(reduced, first, "Done"))
   end
end 

And call the function with just the list you want to map and an empty map

> YourModule.reduce_list(["one", "two", "three"], %{})
  %{"one" => "Done", "three" => "Done", "two" => "Done"}

Answer 3

This is a nice challenge to have and solving it will definitely give you some insight into functional programming.

The solution for such problems in functional languages is usually reduce (often called fold). I will start with a short answer (and not a direct translation) but feel free to ask for a follow up.

The approach below will typically not work in functional programming languages:

map = %{}
Enum.each [1, 2, 3], fn x ->
  Map.put(map, x, x)
end
map

The map at the end will still be empty because we can't mutate data structures. Every time you call Map.put(map, x, x), it will return a new map. So we need to explicitly retrieve the new map after each enumeration.

We can achieve this in Elixir using reduce:

map = Enum.reduce [1, 2, 3], %{}, fn x, acc ->
  Map.put(acc, x, x)
end

Reduce will emit the result of the previous function as accumulator for the next item. After running the code above, the variable map will be %{1 => 1, 2 => 2, 3 => 3}.

For those reasons, we rarely use each on enumeration. Instead, we use the functions in the Enum module, that support a wide range of operations, eventually falling back to reduce when there is no other option.

EDIT: to answer the questions and go through a more direct translation of the code, this what you can do to check and update the map as you go:

Enum.reduce blogs, %{}, fn blog, history ->
  posts = get_posts(blog)
  Enum.reduce posts, history, fn post, history ->
    if Map.has_key?(history, post.url) do
      # Return the history unchanged
      history
    else
      do_thing(post)
      Map.put(history, post.url, true)
    end
  end
end

In fact, a set would be better here, so let's refactor this and use a set in the process:

def traverse_blogs(blogs) do
  Enum.reduce blogs, HashSet.new, &traverse_blog/2
end

def traverse_blog(blog, history) do
  Enum.reduce get_posts(blog), history, &traverse_post/2
end

def traverse_post(post, history) do
  if post.url in history do
    # Return the history unchanged
    history
  else
    do_thing(post)
    HashSet.put(history, post.url)
  end
end