UGN Security Forums
My ProfileMember DirectoryLogin
Search our ForumsView our FAQView our Site Rules
View our CalendarView our Active TopicsGo to our Main Page

UGN Security Store
 

Network Sites UGN Security, The GoNix Initiative, Elite Web Gamers, Back of the Web, EveryDay Helper, VNC Web Design & Development
September
Su M Tu W Th F Sa
1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30
Sponsored Links
Latest Postings
The History Thread...
by Gremelin
09/18/14 09:42 AM
Doom 3
by Cyrez
09/11/14 08:58 PM
Latest Reviews
Topic Options
Rate This Topic
#17181 - 04/18/04 03:37 AM c prime # generator
geodesic Offline
Junior Member

Registered: 04/18/04
Posts: 6
Loc: The Riemann Manifold
hello.

so far i have this program to generate the prime numbers (numbers whose only factors are 1 and themselves) between 1 and 100.

what's wrong with this picture?
Code:
#include <stdio.h>

main
{
  int i,j,k,l=0;
	
	for i=2; i<=20; ++i
	{
	   for j=1; j<=i ; ++j
	   {
	     k = i%j;
	    
            if k == 0 
	    { l = ++l; 
	    }  
	    if l == 2
            {printf "%d is a prime \n",i
            }       
           }
        } 
	
	return 0;
}
thanks for the tips / suggestions

G

Top
Sponsored Links
      
#17182 - 04/18/04 04:16 AM Re: c prime # generator
geodesic Offline
Junior Member

Registered: 04/18/04
Posts: 6
Loc: The Riemann Manifold
well i solved it in this manner
Code:
#include <stdio.h>

int main(void)
{
	int i,j,l=0;
	
	while i <= 100
	{
	   l = 0;
	   
	   for j=1; j<=i; j++
	   {
	    if i%j == 0
	    {
	     l++;
	    }
	   
	   }
	   
	   if l == 2
	   {
	    printf "%d is a prime number\n" , i;
	   } 
	   
	   i++;
	}  
	
	return 0;
}
rather brutish etc. is there any way to stremline it .. like any way to use for loops instead of the while ...

thanks

G

Top
#17183 - 04/18/04 06:29 AM Re: c prime # generator
SilentRage Offline
DollarDNS Owner

Registered: 03/04/02
Posts: 1273
Loc: OH, USA
I've changed the while into a for like you asked, and also made the algorithm twice as fast by skipping the evaluation of even numbers which are all non-prime. Actually it's more than twice as fast with several other optimizations I made.

Code:
#include <stdio.h>

int main(void)
{
    int i=0,j=0,l=0;

    printf("1 is a prime number\n");
    printf("2 is a prime number\n");

    for (i=3; i < 101; i=i+2)
    {
        l = 0;

        // skip 1, don't go as high as i
        for (j=2; j<i; j++)
        {
            if (i % j == 0)
            {
                l++;
                // we already know it isn't prime
                break; 
            }
        }

        if (l == 0)
        {
            printf("%d is a prime number\n", i);
        }
    }

    return 0;
}
_________________________
Domain Registration, Hosting, Management
http://www.dollardns.net

Top
#17184 - 04/18/04 08:12 AM Re: c prime # generator
geodesic Offline
Junior Member

Registered: 04/18/04
Posts: 6
Loc: The Riemann Manifold
outstanding silentrage !!

that is MUCH faster and more elegant. thank you very much !!

G

Top
#17185 - 04/18/04 08:51 AM Re: c prime # generator
Gremelin Offline

Community Owner
*****

Registered: 02/28/02
Posts: 7193
Loc: Portland, OR; USA
I edited the original posts, mainly to add the code tags so that formatting was done properly.

Thanks for the code though, we were actually toying with some encryption a while ago ...
_________________________
Donate to UGN Security here.
UGN Security, Back of the Web, Elite Web Gamers & VNC Web Design Owner

Top
#17186 - 04/28/04 10:38 PM Re: c prime # generator
ninjaneo Offline
UGN Security Staff

Registered: 03/06/02
Posts: 256
Loc: CA, USA
When I made my prime generator, I did it a different way...

If you take your range of numbers... lets say 20, starting with one. go like so...

Well, we know 1 is a prime. so skip that.
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20

(things that we've called primes are in bold, composites are in italics)

now, we will eliminate every multiple of 2.

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20

ok, now we see that three is the next prime (because its the next not elimanated number).
So, lets kill multiples of 3.

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20

mmk, Most of those we already eliminated - but so what.

5 is next

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20

ok, well that didn't do much.

Well, now that its 11's turn and that is more than half of the range of numbers (20/2==10) there is no reason to check the rest.

Any non-eliminated number is prime.

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20

hmmm... those italics didn't show up very well... oh well.

Top
#17187 - 03/30/05 07:05 PM Re: c prime # generator
fortytwo Offline
Junior Member

Registered: 03/30/05
Posts: 1
Loc: Boston
I just stored the primes in an array as I found them. To find them you just see if the number can be divided by any of the primes you have already found.

Top

Featured Member
Registered: 03/02/02
Posts: 136
Forum Stats
2148 Members
46 Forums
34540 Topics
69710 Posts

Max Online: 1567 @ 04/25/10 02:20 AM
Top Posters
UGN Security 27702
Gremelin 7193
§intå× 3255
SilentRage 1273
Ice 1146
pergesu 1136
Infinite 1041
jonconley 955
Girlie 908
unreal 860
Newest Members
Tim050, Gecko666, defghi795767, Devo60, ali
2148 Registered Users
Who's Online
0 registered (), 381 Guests and 289 Spiders online.
Key: Admin, Global Mod, Mod
Latest News


Donate
  Get Firefox!
Get FireFox!