SafeRange

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  {:utils, path: "#{__DIR__}/../utils"}
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A Safe Range Function

In the lesson on creating a range of numbers using the .. operator:

iex > Enum.to_list(1..5)
[1, 2, 3, 4, 5]

You also learned that it can optionally take a step value:

iex > Enum.to_list(1..5//2)
[1, 3, 5]

In this way, the programmer may create a sequence of integers starting at the first number and returning increments by step until the last number is reached or exceeded.

But, what happens when the step attempts to create a sequence that is not incrementing towards the last number, such as:

iex > Enum.to_list(1..5//-1)

In this case, an empty list ([]) is returned as there are no numbers between 1 and 5 starting at 1 and incrementing by -1.

As was brought up* in a Beta release of this course, why would Elixir allow such code? Does it compile? Does it evaluate?

In this bonus exercise, you will be recreating the range function AND creating a safe version.

In Elixir, there is already a standard on providing safe versions of functions: When you want Elixir to throw an exception instead of returning a reasonable or error value, the name of the function will end with !.

Two examples from the standard library may be referred to:

Enum.fetch & Enum.fetch!

Enum.html

The Enum module provides two functions to retrieve an element from a list ([]) at a given index. The difference is in how they behave when the index is invalid; ex:

iex> list = [1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]
iex> Enum.fetch(list, 10)
:error
iex> Enum.fetch!(list, 10)
** (Enum.OutOfBoundsError) out of bounds error
    (elixir 1.14.0) lib/enum.ex:1072: Enum.fetch!/2
    iex:27: (file)

Map.fetch & Map.fetch!

Map.html

When accessing a Map (%{}) with a key, the Map module provides these two functions. The difference is in how they behave when the key is not present in the map; ex:

iex> map = %{a: 1, b: 2}
%{a: 1, b: 2}
iex> Map.fetch(map, :c)
:error
iex> Map.fetch!(map, :c)
** (KeyError) key :c not found in: %{a: 1, b: 2}
    (stdlib 4.0.1) :maps.get(:c, %{a: 1, b: 2})
    iex:29: (file)

Your assignment is to implement the SafeRange module with 2 functions called range and range! where:

• SafeRange.range behaves as the existing ..//STEP function

• SafeRange.range! verifies that the given step “makes sense” with the given first and last values, and if not a RuntimeError exception is thrown.

In Elixir exceptions may be thrown using the raise function, for example here is how to throw a RuntimeError exception:

iex> raise "Houston, we have a problem."
** (RuntimeError) Houston, we have a problem.

defmodule SafeRange do
  @doc """
  Generate a sequences of integers from 'first' to 'last' with an
  option 'step' value.

  If incrementing by 'step' will not end at or beyond 'last',
  just return an empty list ([]).

  ## Examples

    iex> SafeRange.range(1, 5)
    [1, 2, 3, 4, 5]

    iex> SafeRange.range(1, 5, 2)
    [1, 3, 5]

    iex> SafeRange.range(1, 5, -1)
    []

    iex> SafeRange.range(5, 1, 1)
    []
  """
  def range(first, last, step \\ DEFAULT) do
  end

  @doc """
  Generate a sequences of integers from 'first' to 'last' with an
  option 'step' value.

  If incrementing by 'step' will not end at or beyond 'last',
  a "RuntimeError" will be thrown.

  ## Examples

    iex> SafeRange.range!(1, 5)
    [1, 2, 3, 4, 5]

    iex> SafeRange.range!(1, 5, 2)
    [1, 3, 5]

    iex> SafeRange.range!(1, 5, -1)
    ** (RuntimeError) Invalid step value of: -1
        iex: SafeRange.range!/3

    iex> SafeRange.range!(5, 1, 1)
    ** (RuntimeError) Invalid step value of: 1
        iex: SafeRange.range!/3
  """
  def range!(first, last, step \\ DEFAULT) do
  end
end

Example Solution

defmodule SafeRange do
  def range(first, last, step \\ 1) do
    start..finish//step
  end

  def range!(first, last, step \\ 1) do
    cond do
      step < 0 and first < last ->
        raise "Invalid step value of: #{step}"
      last < first ->
        raise "Invalid step value of: #{step}"
      true -> first..last//step
    end       
  end
end

• Thanks to Jeff Helman for asking the question and pursuing a good answer!

Commit Your Progress

Run the following in your command line from the beta_curriculum folder to track and save your progress in a Git commit.

$ git add .
$ git commit -m "finish measurements exercise"

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