Published at Aug 18 2019
·
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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.

Execute the tests with:

```
$ mix test
```

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.

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

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)
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
```

```
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
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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