Prime No.

[Pawan Arora]

Hi, I want to find out 10001st prime number of the site Projecteuler, and here is my programme

class Prime_Number
{
 
public static boolean Isprime(int i)
{
 
if(i<2)
return false;
if(i<4)
return true;
if((i%2==0)||(i%3==0))
return false;
for(int k=6;k<=(int)Math.sqrt(i);k+=6)
{
if(i%(k-1)==0||i%(k+1)==0)
return false;
}
return true;
}
 
public static void main(String[] args)
{
int number=0;
int i=1;
 

for(int j=2;j<=10000000;j++)
{
if(Isprime(j))
{
++number;
if(number==10001)
{
System.out.println(j);
 
break;
}
 
}
}
System.out.println(" finish");
}
}

and I'm getting the answer as 104149. I thougth I've written my code correctly, but yet I'm getting wrong answer. Could anyone please tell me the reason of not having a right result? A small hint is enougth.

[Ulf Dittmer]

What does isPrime(25) return?

[Campbell Ritchie]

When you get prime(25) to return true, get a pencil and paper, and write 1 2 3 4 . . . 24 25. Then go through your method (a complicated bit of programming, there) and work out which numbers it will reject. You need to count the iterations of the loop carefully.

Then find out how the "Sieve of Eratosthenes" works. Far better way of working out prime numbers.