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to Armstrong Numbers in the Elixir Track

Published at Aug 18 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

An Armstrong number is a number that is the sum of its own digits each raised to the power of the number of digits.

For example:

  • 9 is an Armstrong number, because 9 = 9^1 = 9
  • 10 is not an Armstrong number, because 10 != 1^2 + 0^2 = 1
  • 153 is an Armstrong number, because: 153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153
  • 154 is not an Armstrong number, because: 154 != 1^3 + 5^3 + 4^3 = 1 + 125 + 64 = 190

Write some code to determine whether a number is an Armstrong number.

Running tests

Execute the tests with:

$ mix test

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.


Wikipedia https://en.wikipedia.org/wiki/Narcissistic_number

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


defmodule ArmstrongNumberTest do
  use ExUnit.Case

  test "One-digit numbers are Armstrong numbers" do
    assert ArmstrongNumber.valid?(1)
    assert ArmstrongNumber.valid?(6)
    assert ArmstrongNumber.valid?(9)

  @tag :pending
  test "Two-digit numbers aren't Armstrong numbers" do
    refute ArmstrongNumber.valid?(11)
    refute ArmstrongNumber.valid?(89)
    refute ArmstrongNumber.valid?(44)

  @tag :pending
  test "Three-digit numbers that are the Armstrong numbers" do
    assert ArmstrongNumber.valid?(153)
    assert ArmstrongNumber.valid?(370)

  @tag :pending
  test "Three-digit numbers that aren't the Armstrong numbers" do
    refute ArmstrongNumber.valid?(555)
    refute ArmstrongNumber.valid?(662)

  @tag :pending
  test "Four-digit number that is the Armstrong numbers" do
    assert ArmstrongNumber.valid?(1634)

  @tag :pending
  test "Four-digit number that is not the Armstrong numbers" do
    refute ArmstrongNumber.valid?(9989)

  @tag :pending
  test "Seven-digit number that is the Armstrong numbers" do
    assert ArmstrongNumber.valid?(9_926_315)

  @tag :pending
  test "Seven-digit number that is not the Armstrong numbers" do
    refute ArmstrongNumber.valid?(2_369_989)

  @tag :pending
  test "Ten-digit number that is the Armstrong numbers" do
    assert ArmstrongNumber.valid?(4_679_307_774)

  @tag :pending
  test "Fourteen-digit number that is the Armstrong numbers" do
    assert ArmstrongNumber.valid?(28_116_440_335_967)


ExUnit.configure(exclude: :pending, trace: true)
defmodule ArmstrongNumber do
  @moduledoc """
  Provides a way to validate whether or not a number is an Armstrong number

  @spec valid?(integer) :: boolean
  def valid?(number) do
    sum_of_powers =
      |> Integer.digits()
      |> sum_powers()

    number == sum_of_powers

  defp sum_powers(digits) do
    length = length(digits)
    Enum.reduce(digits, 0, &power(length, &1, &2))

  defp power(length, digit, acc), do: acc + :math.pow(digit, length)

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