Published at Jul 13 2018
·
2 comments

Instructions

Test suite

Solution

Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

The Sieve of Eratosthenes is a simple, ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite (i.e. not prime) the multiples of each prime, starting with the multiples of 2.

Create your range, starting at two and continuing up to and including the given limit. (i.e. [2, limit])

The algorithm consists of repeating the following over and over:

- take the next available unmarked number in your list (it is prime)
- mark all the multiples of that number (they are not prime)

Repeat until you have processed each number in your range.

When the algorithm terminates, all the numbers in the list that have not been marked are prime.

The wikipedia article has a useful graphic that explains the algorithm: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

Notice that this is a very specific algorithm, and the tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes.

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 sieve_test.rb
```

To include color from the command line:

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

Sieve of Eratosthenes at Wikipedia http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

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

```
require 'minitest/autorun'
require_relative 'sieve'
# Common test data version: 1.0.0 f2b2693
class SieveTest < Minitest::Test
def test_no_primes_under_two
# skip
expected = []
assert_equal expected, Sieve.new(1).primes
end
def test_find_first_prime
skip
expected = [2]
assert_equal expected, Sieve.new(2).primes
end
def test_find_primes_up_to_10
skip
expected = [2, 3, 5, 7]
assert_equal expected, Sieve.new(10).primes
end
def test_limit_is_prime
skip
expected = [2, 3, 5, 7, 11, 13]
assert_equal expected, Sieve.new(13).primes
end
def test_find_primes_up_to_1000
skip
expected = [
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59,
61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139,
149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233,
239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337,
347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439,
443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557,
563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653,
659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769,
773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883,
887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997
]
assert_equal expected, Sieve.new(1000).primes
end
# Problems in exercism evolve over time, as we find better ways to ask
# questions.
# The version number refers to the version of the problem you solved,
# not your solution.
#
# Define a constant named VERSION inside of the top level BookKeeping
# module, which may be placed near the end of your file.
#
# In your file, it will look like this:
#
# module BookKeeping
# VERSION = 1 # Where the version number matches the one in the test.
# end
#
# If you are curious, read more about constants on RubyDoc:
# http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html
def test_bookkeeping
skip
assert_equal 1, BookKeeping::VERSION
end
end
```

```
class Sieve
def self.cache
@cache ||= {}
end
def initialize(limit)
@limit = limit
@primes = self.class.cache[limit]
run unless @primes
self.class.cache[limit] = primes
end
def primes
@primes ||= (2..limit).select { |x| base_list[x] }
end
private
attr_reader :limit
def run
(2..limit).each do |i|
next unless base_list[i]
((i**2)..limit).step(i) do |j|
base_list[j] = false
end
end
end
def base_list
@base_list ||= Array.new(limit + 1) { true }
end
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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## Community comments

Attempted to cache so that we only have to run once per unique upper bound.

I find the code somewhat unintuitive to read. You are tightly mixing class with instance behaviour. The cache is a class level thing, but you also have an instance level cach. If you want primes to be accessible through the class, than I'd suggest also generating them at the class level. Point of warning: the class-level cache will have a duration of the entire life-time of the program, whereas the instance-level cache will only exist for the duration of the object.