Is it a valid code for Sieve of Eratosthenes?

Question

I recently learned Sieve of Eratosthenes to find prime numbers. After knowing the method i wrote this code for it.
Would it be a valid C++ code for Sieve of Eratosthenes?

#include <iostream>
using namespace std;

int main()
{
int n;
cin>>n;
bool array[n];

for(int i=2;i<=n;i++)
{
    array[i]=true;
}

   for(int i=2;i<=n/2;i++)
   {
      for(int j=i+1;j<=n;j++)
      {
         if(array[j]==true)
         {
            if(j%i==0)
            array[j]=false;
         }
      }
   }

  for(int k=2;k<=n;k++)
  {
    if(array[k]==true)
    cout<<k<<" ";
  }
}

Thanks for help!

Does the code give the correct result? That would be an important first test. — Bart, Jul 17 '18 at 15:34
bool array[n] is not something valid in cpp, and even if it would be, your loops are <= n which means you will read/write memory outside the boundaries of array[n]. Get a good book and an environment that lets you compile and use a debugger. — AlexG, Jul 17 '18 at 15:34
No, arrays and vectors use zero based indexing. Did you try to run it? — JVApen, Jul 17 '18 at 15:35
No. If you compile it with warnings you'll see that the declaration of `array` isn't valid C++ because C++ doesn't have variable-length arrays and `n` is a variable. If you fix that (or set your compiler to allow VLAs), you run into undefined behavior because you access `array[n]`. Otherwise, I think you get the right answer but the algorithm is still wrong. You waste time crossing off the multiples of non-prime, checking `j` even though it doesn't matter (which doesn't affect the complexity but does slow you down), and incrementing `j` by 1 instead of by `i`, which does affect the complexity. — Daniel H, Jul 17 '18 at 15:41
***it gives correct output*** Unfortunately some times Undefined Behavior appears to work only to fail at random. — drescherjm, Jul 17 '18 at 16:09

score 2 · Accepted Answer · edited Jul 17 '18 at 19:48

2

No. This is not the Sieve of Eratosthenes. There is no division (j % i == 0) in the Sieve of Eratosthenes.

edited Jul 17 '18 at 19:48

answered Jul 17 '18 at 15:33

user448810

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Can you provide reference to a valid code, which can be easily understood by a beginner? – Suyash Jul 17 '18 at 15:35
Just search the Internet (e.g. Google) if you just need working code and don't want to develop it by yourself – anatolyg Jul 17 '18 at 15:37
1

See https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_and_variants – Fred Larson Jul 17 '18 at 15:38
@Suyashky: I recommend [this essay](https://programmingpraxis.files.wordpress.com/2012/09/primenumbers.pdf) at my blog. – user448810 Jul 17 '18 at 15:46

Is it a valid code for Sieve of Eratosthenes?

1 Answers1