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lujanfernaud's solution

to Prime Factors in the Ruby Track

Published at Jul 13 2018 · 1 comment
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 exercism help 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
require "prime"

class PrimeFactors
  def self.for(number)
    return [] if number == 1

    prime_numbers = Prime.each(number)

    [].tap do |prime_factors|
      prime_numbers.each do |prime_number|
        result = number.to_f / prime_number

        while (result % 1).zero?
          prime_factors << prime_number
          result /= prime_number
        end
      end
    end
  end
end

Community comments

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Avatar of lujanfernaud

As the goal is to find the prime factors, and not the prime numbers, I decided to use Prime.

What can you learn from this solution?

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?
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