Factorial

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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Factorial

The factorial of a $n$ is the sum of the sequence of numbers leading up to $n$.

flowchart LR
n --*--> n1[n - 1] --*--> n2[n - 2] --*--> n3[n - 3] --*--> 2 --*--> 1

Meaning the factorial of 5 is 5 * 4 * 3 * 2 * 1 which equals 120.

So $factorial(n) = n * factorial(n - 1)$

Example Solution

defmodule Factorial do
  def of(1), do: 1
  def of(n) do
    n * of(n - 1)
  end
end

Our factorial function only needs to handle inputs above 1. Implement the Factorial module as documented below.

defmodule Factorial do
  @doc """
  Find the factorial of an integer.

  ## Examples

    iex> Factorial.of(1)
    1

    iex> Factorial.of(2)
    2

    iex> Factorial.of(3)
    6

    iex> Factorial.of(4)
    24
    
    iex> Factorial.of(5)
    120

    iex> Factorial.of(10)
    3628800
  """
  def of(n) do
  end
end

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "factorial_reading"
    "exercises" -> "factorial_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 factorial-exercise
$ git add .
$ git commit -m "finish factorial exercise"
$ git push origin factorial-exercise

Create a pull request from your factorial-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.

If you are interested in joining the next academy cohort, sign up here to receive more news when it is available.

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