implemented Sieve of Eratosthenes in cpp

[dibyam-jalan27]

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• ✨ New snippet
• 🛠 Improvement to an existing snippet
• 🐞 Bug fix
• 📖 Documentation update
• 🔧 Other (please describe):

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• I have tested my code and verified it works as expected.
• My code follows the style and contribution guidelines of this project.
• Comments are added where necessary for clarity.
• Documentation has been updated (if applicable).
• There are no new warnings or errors from my changes.

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[Mathys-Gasnier]

For me it looks good, let's wait on another reason before merging

[majvax]

Hello, Thanks for you contribution ! 🙌

However, I have few question/suggestion that I'm free to discuss to make this snippet better.

Instead of returning a std::vector<int> why don't you return std::vector<bool>.
It make it more efficient to find if a certain value is prime or not (O(1) for vec<bool> and O(log n) for vec<int>, using binary search.).

An int take up to 8 bytes in memory as a bool take only one. For primes up to n we can wrote this formula:

• vec<bool> = ~n
• vec<int> = ~(n/ln(n)) * sizeof(int)

For n = 100_000, vec<bool> uses 12500 bytes as vec<int> uses 38000 bytes. (used int=4bytes)

Consider adding const to your parameter and noexcept to your function.
Also, if you are still planning to use a vec<int>, consider reserving the memory using the prime number theorem to make less allocation.

• primes.reserve(n/std::log(n));
• or, for a more precise allocation:

  double ln_n = std::log(n);
  double estimate = (n / ln_n) * (1 + 1/ln_n + 1.8/(ln_n*ln_n));
  primes.reserve(estimate*1.005);
  

• Or, a for a bit precision allocation:

  size_t count = 0;
  for (int i = 2; i <= n; ++i) {
      if (is_prime[i]) count++;
  }   
  std::vector<int> primes;
  primes.reserve(count);
  

Letting the vector grow itself seems a bad idea, when benchmarking, for n = 1000000 I got:

For n = 1000000:
there should be 78695.8 primes
Number of primes: 78498
Vector<int> size in bytes: 313992
Vector<int> capacity in bytes: 524288
Unused capacity in bytes: 210296

Even for smaller n, I always got 30-70% byte loss and for bigger n, the problem is worse

[dibyam-jalan27]

I am using windows and unable to commit due to prettier rules. How can i fix that?

[majvax]

Can you please run

• npm install
• npm run snippets:check

If you are sure your commit is well formatted you can skip GitHub hook

It is strange that it doesn't allow you to commit since pre-commit hook doesn't contain any prettier related formatting.

Lastly I can only recommend to hard reset branch and open a new PR

[majvax]

We won't help you if you close your PR definitely. Please open it again if you want to seek support.

[Mathys-Gasnier]

(Eslint runs prettier so it's probably why it fails, but I have no clue why it fails above that)