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thekeele's solution

to Armstrong Numbers in the Elixir Track

Published at Jan 06 2020 · 1 comment
Instructions
Test suite
Solution

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!"
end

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.

Source

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.

armstrong_number_test.exs

defmodule ArmstrongNumberTest do
  use ExUnit.Case

  # @tag :pending
  test "Zero is an Armstrong number" do
    assert ArmstrongNumber.valid?(0)
  end

  test "Single digit numbers are Armstrong numbers" do
    assert ArmstrongNumber.valid?(5)
  end

  @tag :pending
  test "There are no 2 digit Armstrong Numbers" do
    refute ArmstrongNumber.valid?(10)
  end

  @tag :pending
  test "Three digit number that is an Armstrong number" do
    assert ArmstrongNumber.valid?(153)
  end

  @tag :pending
  test "Three digit number that is not an Armstrong number" do
    refute ArmstrongNumber.valid?(100)
  end

  @tag :pending
  test "Four digit number that is an Armstrong number" do
    assert ArmstrongNumber.valid?(9474)
  end

  @tag :pending
  test "Four digit number that is not an Armstrong number" do
    refute ArmstrongNumber.valid?(9475)
  end

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

  @tag :pending
  test "Seven digit number that is not an Armstrong number" do
    refute ArmstrongNumber.valid?(9_926_134)
  end
end

test_helper.exs

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule ArmstrongNumber do
  @spec valid?(integer) :: boolean
  def valid?(number) do
    digits = Integer.digits(number)
    num_digits = length(digits)
    sum_of_digits =
      for digit <- digits, reduce: 0 do
        sum -> sum + :math.pow(digit, num_digits)
      end

    number == sum_of_digits
  end
end

Community comments

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Avatar of thekeele

:math.pow/2 returns a float so the equality check here is 5.0 == 5 for example. I went with readability over strict equality here :O

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