error in pow.
as long as n < 2^31, x*x fits into int64_t, since x<n.
On 2012-1-18 20:51, Sourabh Singh wrote:
@ Terence
I belive nw its giving wrong answer for n>41 onwards
due to error's in pow and x*x over flow as u already stated...
i guess algo is right nw.. ;-)
thanx again..
On 1/18/12, Sourabh Singh<singhsourab...@gmail.com> wrote:
@ thanx got it.. silly mistakes ;-) . missed thought it ws + between s and
d.
//suggested corrections made.. still not working..
#include<iostream>
#include<conio.h>
#include<math.h>
using namespace std;
int is_prime(int n)
{
if(n==2 | n==3)
return 1;
if(((n-1)%6!=0& (n+1)%6!=0) || n<2)
return 0;
int j,s,d=n-1; while((d&1)==0) d>>=1;
s=n/d;for(j=0;1<<j<s;j++); s=j;
int primes[6]={2,3,7,31,61,73},i,a,flag;
uint64_t x;
for(i=0;i<6;i++)
{ if(primes[i]>=n)
break;
a=primes[i];
x=uint64_t(pow(a,d))%n;
if(x==1 || x==n-1)
continue;
int r;
for(r=1;r<s;r++)
{ x=(x*x)%n;
if(x==1)
return 0;
if(x==n-1)
break;
}
if(x!=n-1)
return 0;
}
return 1;
}
main()
{for(int k=1;k<2000;k++)
if(is_prime(k))
printf("%d is %d\n",k,is_prime(k));
getch();
}
On 1/18/12, Terence<technic....@gmail.com> wrote:
@Sourabh
m=2^s***d
primes[i]*<*n
On 2012-1-18 19:39, Sourabh Singh wrote:
@Terrence
sry i didn't explain what s,d were they were also wrong i ws
calculating for n not n-1
earlier n=2^s+d
nw m=n-1; for odd n
m=2^s+d;
//suggested corrections made.. still not working..
#include<iostream>
#include<conio.h>
#include<math.h>
using namespace std;
int is_prime(int n)
{
if(n==2 | n==3)
return 1;
if(((n-1)%6!=0& (n+1)%6!=0) || n<2)
return 0;
int m=n-1;
int s,d;
for(s=0;1<<s<m;++s); s--;d=(m%(1<<s));
int primes[6]={2,3,7,31,61,73},i,a,flag;
uint64_t x;
for(i=0;i<6;i++)
{ if(primes[i]>n)
break;
a=primes[i];
x=uint64_t(pow(a,d))%n;
if(x==1 || x==n-1)
continue;
for(int r=1;r<s;r++)
{ x=(x*x)%n;
if(x==1)
return 0;
if(x==n-1)
break;
}
if(x!=n-1)
return 0;
}
return 1;
}
main()
{for(int k=1;k<20;k++) printf("%d is %d\n",k,is_prime(k)); getch();}
On 1/18/12, Sourabh Singh<singhsourab...@gmail.com> wrote:
@ sunny agrawal
you are right. but i have used check a>n also . no improvement .
i think algo is wrong in later part.. somewhere..
@ Terence
1) pow may not work for big n but , m just checking for n=1..200
just to check wether algo is right.. and it's not working even for
n=7,19...
2) d,s are also coming fine for small values.. of n
3) for x i have used... 64bit integer.. uint64_t in it's declaration.
i just want to get algo right first then bother about big n ;-)
On 1/18/12, Terence<technic....@gmail.com> wrote:
1. the computing of d is incorrect.
d = n-1;
while((d&1)==0) d>>=1;
2. the accuracy of pow is far from enough. you should implement your
own
pow-modulo-n method.
3. for big n, (exceed 32bit), x=(x*x)%n can be overflow also. you may
need to implement your own multiply method in this case.
On 2012-1-18 18:15, Sourabh Singh wrote:
@all output's to above code are just random.. some prime's. found
correctly while some are not
why i used certain primes to check ie.(2,3,31,73,61,7) coz.. its
given
n wiki for about 10^15 checking with these is enough..
On 1/18/12, Sourabh Singh<singhsourab...@gmail.com> wrote:
@ALL hi everyone m trying to make prime checker based on
miller-rabin
test . can some1 point out . wat's wrong with the code. thank's alot
in advance...
//prime checker based on miller-rabin test
#include<iostream>
#include<conio.h>
#include<math.h>
int is_prime(int n)
{
if(n==2 | n==3)
return 1;
if(((n-1)%6!=0& (n+1)%6!=0) || n<2)
return 0;
int s,d;
for(s=0;1<<s<n;++s); s--;d=(n%(1<<s));
int primes[6]={2,3,7,31,61,73},i,a,flag;
uint64_t x;
for(i=0;i<6;i++)
{ flag=0;
a=primes[i];
x=uint64_t(pow(a,d))%n;
if(x==1 | x==n-1)
continue;
for(int r=1;r<s;r++)
{ x=(x*x)%n;
printf("x is %llu\n",x);
if(x==1)
return 0;
else
flag=1;
}
if(flag)
continue;
return 0;
}
return 1;
}
main()
{
for(int k=1;k<100;k++)
{
printf("%d is %d\n",k,is_prime(k));
}
getch();
}
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