Ranges

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

Navigation

Return Home Report An Issue

Setup

Ensure you type the ea keyboard shortcut to evaluate all Elixir cells before starting. Alternatively, you can evaluate the Elixir cells as you read.

Review Questions

Upon completing this lesson, a student should be able to answer the following questions.

• How do we represent a range of numbers in Elixir?
• What is the step of a range, and how do we change it?
• How can we pattern match on a range?

Ranges

Ranges are useful for representing an ascending or descending sequence of numbers, without needing to manually type each number in a list.

For example, you could have a range from 1 to 10.

1..10

flowchart LR
  1 --> 2 --> 3 --> 4 --> 5 --> 6 --> 7 --> 8 --> 9 --> 10

or 5 to -5.

5..-5

flowchart LR
  5 --> 4 --> 3 --> 2 --> 1 --> 0 --> -1 --> -2 --> -3 --> -4 --> -5

You can visualize a range as representing an ascending or descending list of integers.

On its own, it might not seem like much. But ranges will be incredibly useful with the Enum module. You’ll learn more about the Enum module in a future lesson. For now, it’s enough to understand the relationship between ranges and lists.

For example the range 1..5 equates to [1, 2, 3, 4, 5] as a list.

To establish that relationship, you can use Enum.to_list/1 to convert a range from the list.

Enum.to_list(1..5)

Ranges can have a step. By default ranges have a step of 1. That means that by default they include each whole number in the range.

You can override the default step of the range using //:

0..10//2

This creates a range from 0 to 10 with a step of 2.

0 --> 2 --> 4 --> 6 --> 8 --> 10

That means the range 0..10//2 correlates to the list [0, 2, 4, 6, 8, 10]

Here’s the same range converted into a list to help you visualize it. Once again, you will learn more about what Enum.to_list is in a future lesson.

Enum.to_list(0..10//2)

Your Turn

In the Elixir cell below, use Enum.to_list/1 to convert a range from 3 to 9 with a step of 3 into a list.

Example solution

  Enum.to_list(3..9//3)

Pattern Matching With Ranges

We can pattern match on ranges just like any other data type.

For the most strict match, the left and right side must be exactly equal.

1..10 = 1..10

We can also bind the start and end of the range to a variable in our pattern match.

start..finish = 1..10

start

finish

We can also bind the step.

start..finish//step = 0..9//3

start

finish

step

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "ranges_reading"
    "exercises" -> "ranges_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 ranges-reading
$ git add .
$ git commit -m "finish ranges reading"
$ git push origin ranges-reading

Create a pull request from your ranges-reading 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.

Up Next