Drill: Replace nils In A List With Elements From A 2nd List

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Purpose

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.

To develop familiarity with comprehensions, 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
  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(input1, input2) do
    for index <- 0..length(input1)-1 do
      if Enum.at(input1, index) == nil do
        Enum.at(input2, index)
      else
        Enum.at(input1, index)
      end
    end
  end
end

Example Solution 2

defmodule ReplaceNils do
  def replace(input1, input2) do
    for {element, index} <- Enum.with_index(input1) do
      element || Enum.at(input2, index)
    end
  end
end

Example Solution 3

defmodule ReplaceNils do
  def replace(as, bs) do
    for {a, b} <- Enum.zip(as, bs) do
      a || b
    end
  end
end

Mark As Completed

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

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

default = Map.get(existing_progress, file_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, file_name, completed)))
  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 solutions
$ git checkout -b drill-enum2-replace-nils-exercise
$ git add .
$ git commit -m "finish drill-enum2-replace-nils exercise"
$ git push origin drill-enum2-replace-nils-exercise

Create a pull request from your drill-enum2-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 instructor by including @BrooklinJazz in your PR description to get feedback. You (or your instructor) may merge your PR into your solutions branch after review.

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