Replacing Nils

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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Purpose

This is a guest exercise written by Quentin Crain.

This is a drill exercise:

> Drill exercises are meant to provide practise of Elixir’s > syntax and important language modules so developers can > type them out as fast as possible. > The problem should not be conceptually difficult to > facilitate this goal. > > Developers need to commit both Elixir’s syntax and core > modules/functions to (working) memory so that when writing > their code the developer is not slowed down having to > remember the syntax or which module function to use. This > is analogous to knowing how to spell and the grammar of > your (human) language.

In this particular iteration of this problem, the goal is to practise writing/using functions with pattern matching in the functions’ heads.

To develop familiarity with functions and pattern matching, you will replace nil values in a list with values from a second list at the same index.

For example:

list1 = [0, nil, 2, 3, nil]
list2 = [:a, :b, :c, :d, :e]

ReplaceNils.replace(list1, list2)
[0, :b, 2, 3, :e]

You can assume the lists are of the same length.

Implement the ReplaceNils module as below:

defmodule ReplaceNils do
  @moduledoc """
  Replace Nils
  """

  @doc """
  replace nil values in the first list with values from the second list in the same position.
  """
  def replace(input1, input2) do
    nil
  end
end

Here are some additional test data to think about:

Testcase 1:
  Input1:   [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  Input2:   [nil, nil, nil, nil, nil, nil, nil, nil, nil, nil]
  Expected: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Testcase 2:
  Input1:   [nil, nil, nil, nil, nil, nil, nil, nil, nil, nil]
  Input2:   [:a, :b, :c, :d, :e, :f, :g, :h, :i, :j]
  Expected: [:a, :b, :c, :d, :e, :f, :g, :h, :i, :j]

Testcase 3:
  Input1: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  Input2: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Testcase 4:
  Input1: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  Input2: [:a, :b, :c, :d, :e, :f, :g, :h, :i, :j]

Testcase 5:
  [1, 2, 3, nil, nil, 6, 7, nil, 9, 10]
  [:a, :b, :c, :d, :e, :f, :g, :h, :i, :j]

Here are some example solutions:

Example Solution 1

defmodule ReplaceNils do
  def replace([i1|input1], [i2|input2]) do
    if input1 == [] do
      if i1 == nil, do: [i2], else: [i1]
    else
      [if i1 == nil, do: [i2], else: [i1]] ++ replace(input1, input2)
    end
  end
end

Example Solution 2

defmodule ReplaceNils do
  def replace([a|as], [b|bs]) do
    case as do
      [] -> [a || b]
      _  -> [a || b] ++ replace(as, bs)
    end
  end
end

Example Solution 3

defmodule ReplaceNils do
  def replace(as, bs, acc \\ [])

  def replace([], [], acc), do: Enum.reverse(acc)
  def replace([nil|as], [b|bs], acc), do: replace(as, bs, [b|acc])
  def replace([a|as], [_b|bs], acc), do: replace(as, bs, [a|acc])
end

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "drill-patternmatching-replace-nils_reading"
    "exercises" -> "drill-patternmatching-replace-nils_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 drill-patternmatching-replace-nils-exercise
$ git add .
$ git commit -m "finish drill-patternmatching-replace-nils exercise"
$ git push origin drill-patternmatching-replace-nils-exercise

Create a pull request from your drill-patternmatching-replace-nils-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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