# paulfioravanti's solution

## to Prime Factors in the Ruby Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

## Example

What are the prime factors of 60?

• Our first divisor is 2. 2 goes into 60, leaving 30.
• 2 goes into 30, leaving 15.
• 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
• 3 goes cleanly into 15, leaving 5.
• 3 does not go cleanly into 5. The next possible factor is 4.
• 4 does not go cleanly into 5. The next possible factor is 5.
• 5 does go cleanly into 5.
• We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

• 2 * 2 * 3 * 5
• = 4 * 15
• = 60
• Success!

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

``````gem install minitest
``````

If you would like color output, you can `require 'minitest/pride'` in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

``````ruby prime_factors_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride prime_factors_test.rb
``````

## Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

## Submitting Incomplete Solutions

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

### prime_factors_test.rb

``````require 'minitest/autorun'
require_relative 'prime_factors'

class PrimeFactorsTest < Minitest::Test
def test_1
assert_equal [], PrimeFactors.for(1)
end

def test_2
skip
assert_equal [2], PrimeFactors.for(2)
end

def test_3
skip
assert_equal [3], PrimeFactors.for(3)
end

def test_4
skip
assert_equal [2, 2], PrimeFactors.for(4)
end

def test_6
skip
assert_equal [2, 3], PrimeFactors.for(6)
end

def test_8
skip
assert_equal [2, 2, 2], PrimeFactors.for(8)
end

def test_9
skip
assert_equal [3, 3], PrimeFactors.for(9)
end

def test_27
skip
assert_equal [3, 3, 3], PrimeFactors.for(27)
end

def test_625
skip
assert_equal [5, 5, 5, 5], PrimeFactors.for(625)
end

def test_901255
skip
assert_equal [5, 17, 23, 461], PrimeFactors.for(901_255)
end

def test_93819012551
skip
assert_equal [11, 9539, 894_119], PrimeFactors.for(93_819_012_551)
end
end``````
``````module PrimeFactors
MINIMUM_PRIME = 2
private_constant :MINIMUM_PRIME
FINAL_FACTOR = 1
private_constant :FINAL_FACTOR

module_function

def for(number)
[].tap do |prime_factors|
MINIMUM_PRIME.upto(number) do |n|
quotient, modulus = number.divmod(n)
next if modulus.nonzero?

prime_factors << n
break if quotient == FINAL_FACTOR

number = quotient
redo
end
end
end
end``````