## prime generator using bitmap

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DJWS
Learning poster
Posts: 100
Joined: Sat Oct 11, 2003 3:30 pm
Location: Taiwan
Contact:

### prime generator using bitmap

I tried to generate a prime generator using bitmap. Here is the reference:
http://www.algorithmist.com/index.php/P ... osthenes.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,

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
Guru
Posts: 1244
Joined: Mon Feb 28, 2005 4:51 am
Location: Zürich, Switzerland
Contact:
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
Learning poster
Posts: 100
Joined: Sat Oct 11, 2003 3:30 pm
Location: Taiwan
Contact:
Sharp eyes!
Thank you

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

edeferaxy
New poster
Posts: 2
Joined: Fri Jan 20, 2012 8:23 am

### Re: prime generator using bitmap

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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