Custom Enum With Reduce

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Custom Enum With Reduce

You’re going to use Enum.reduce/2 and/or Enum.reduce/3 to re-implement several of the Enum module’s functions. Only implement these functions for lists.

Example Solution

defmodule CustomEnum do
  def reverse(list) do
    Enum.reduce(list, [], fn each, acc -> [each | acc] end)
  end

  def map(list, callback_function) do
    Enum.reduce(list, [], fn each, acc ->
      [callback_function.(each) | acc]
    end)
    |> Enum.reverse()
  end

  def filter(list, callback_function) do
    Enum.reduce(list, [], fn each, acc ->
      if callback_function.(each) do
        [each | acc]
      else
        acc
      end
    end)
    |> Enum.reverse()
  end

  def sum(list) do
    Enum.reduce(list, 0, fn each, acc -> acc + each end)
  end

  def join(list_of_strings) do
    Enum.reduce(list_of_strings, "", fn str, acc -> acc <> str end)
  end
end

Implement the CustomEnum module according to the documentation below.

defmodule CustomEnum do
  @moduledoc """
  Documentation for `CustomEnum`.
  Re-implement common [Enum](https://hexdocs.pm/elixir/Enum.html) functions using [Enum.reduce/2](https://hexdocs.pm/elixir/Enum.html#reduce/2) or [Enum.reduce/3](https://hexdocs.pm/elixir/Enum.html#reduce/3).
  """

  @doc """
  Reverse a list

  ## Examples

    iex> CustomEnum.reverse([1, 2, 3])
    [3, 2, 1]

    iex> CustomEnum.reverse([4, 5, 6, 7])
    [7, 6, 5, 4]
  """
  def reverse(list) do
  end

  @doc """
  Map over a list

  ## Examples

    iex> CustomEnum.map([1, 2, 3], fn integer -> integer * 2 end)
    [2, 4, 6]

    iex> CustomEnum.map([5, 4, 3], fn integer -> is_integer(integer) end)
    [true, true, true]
  """
  def map(list, callback_function) do
  end

  @doc """
  Filter elements in a list. Keep elements that return `true` when called with the
  provided callback function.

  ## Examples

    iex> CustomEnum.filter([1, 2, "3"], fn int -> is_integer(int) end)
    [1, 2]

    iex> CustomEnum.filter([1, "2", "3"], fn char -> is_bitstring(char) end)
    ["2", "3"]
  """
  def filter(list, callback_function) do
  end

  @doc """
  Sum a list of integers.

  ## Examples

    iex> CustomEnum.sum([1, 2, 3])
    6

    iex> CustomEnum.sum([1, 1, 1])
    3
  """
  def sum(list_of_integers) do
  end

  @doc """
  Join a list of strings together.

  ## Examples

    iex> CustomEnum.join(["A", "B", "C"])
    "ABC"

    iex> CustomEnum.join(["Hello", ",", " ", "World", "!"])
    "Hello, World!"
  """
  def join(list_of_strings) do
  end
end

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "custom_enum_with_reduce_reading"
    "exercises" -> "custom_enum_with_reduce_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b custom-enum-with-reduce-exercise
$ git add .
$ git commit -m "finish custom enum with reduce exercise"
$ git push origin custom-enum-with-reduce-exercise

Create a pull request from your custom-enum-with-reduce-exercise branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

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