prime generator using bitmap

[DJWS]

I tried to generate a prime generator using bitmap. Here is the reference:
http://www.algorithmist.com/index.php/Prime_Sieve_of_Eratosthenes.c
But I failed. My program generates wrong number of primes.

I efforted to find out the missing primes, and I got two values: 203897 and 643649.
This two values are unusual and I have no idea about it.

Sincerely,
Please help correct the code.

Code: Select all

#include <iostream>
#include <vector>
using namespace std;

   unsigned sieve[1000000/64+1];
   vector<int> v1;

void make_prime() {
   v1.push_back(2);

   register unsigned i, j, k;
   for (i = 1; i < 1000000/2; i++) {
      if (((sieve[i>>5]>>(i&31))&1)==0) {
         for (j=(i+1)*i*2, k=2*i+1; j < 1000000/2; j+=k)
            sieve[j>>5] |= 1<<(j&31);

         v1.push_back(i*2+1);
      }
   }
}

   bool sieve2[1000000];
   vector<int> v2;

void make_prime2() {
   for (int p=0;p<1000000;p++) sieve2[p]=true;

   for (int i=2;i<1000000;i++)
      if (sieve2[i]) {
         for (int j=i+i;j<1000000;j+=i)
            sieve2[j]=false;

         v2.push_back(i);
      }
}

int main() {
   make_prime();
   make_prime2();

   cout << "size of v1 = " << v1.size() << endl;
   cout << "size of v2 = " << v2.size() << endl;

   // the missing prime are 203897 and 643649

   for (int i=0;i<v2.size();i++) {
      if (v2[i] == 203897) cout << "find 203897 at position " << i << endl;
      if (v2[i] == 643649) cout << "find 643649 at position " << i << endl;
   }

   return 0;
}

--
DJWS, a newbie in programming

[mf]

You modified the code, and it caused integer overflow. Try this version of make_prime():

Code: Select all

void make_prime() {
   v1.push_back(2);

   register unsigned i, j, k;
   for (i = 1; i < 1000000/2; i++) {
      if (((sieve[i>>5]>>(i&31))&1)==0) {
         if (i < 500) {    /* 500 = sqrt(1000000)/2 */
            for (j=(i+1)*i*2, k=2*i+1; j < 1000000/2; j+=k)
               sieve[j>>5] |= 1<<(j&31);
         }

         v1.push_back(i*2+1);
      }
   }
}


[DJWS]

Sharp eyes!
Thank you

PS: bitmap approach is truly efficient
--
DJWS, a newbie in programming

[edeferaxy]

How do you change massive amounts of BITMAP images to JPEG images? I have folders of scanned photos in Bitmap format that I want to change to Jpeg images at one time. What is a good program to use for this?
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