 paulfioravanti's solution

to Armstrong Numbers in the Elixir Track

Published at Aug 18 2019 · 0 comments
Instructions
Test suite
Solution

Note:

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!"
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.

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

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

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

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

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

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

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

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

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

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

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

test_helper.exs

ExUnit.start()
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 =
number
|> Integer.digits()
|> sum_powers()

number == sum_of_powers
end

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

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