need help in simple prime number code

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C#

int n;
            bool prime = true;
            Console.Read();
            for(int i=2;i<n-1;i++)>
            {
                if((n/i)==0)
                {prime=false;
                }
                break;}
            if(prime==true){
            Console.WriteLine("Yes the no. is prime");}
            else{
            Console.WriteLine("NO this is not prime");
            }
            }


    }
}

i want that whenever i write a prime number it show me message but i am getting problem in this code is right or not.how can i implement.

Posted 20-Aug-11 11:06am

M Ali Qadir

Updated 20-Aug-11 19:17pm

thatraja

v2

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Simon Bang Terkildsen · Accepted Answer · 2011-08-20T12:07:00

~~Your code will determine if a integer is a prime number or not, but not very efficiently, which might not matter if the prime numbers you're going to test are small.~~

You can easely optimize your code if you realize that N = Sqrt(N)*Sqrt(N) which means your loop condition would be i<=sqrt(n)
While this optimizes your code it's still not a good solution when you're gonna test big integers. The best algorithm, I know of, for determining whether an integer is a prime or not is the Miller-Rabin test[^]

Edit:
I just saw that your code actually isn't correct.
you'll need to initialize n ofcourse. and you're saying that n/i == 0 means that n is not a prime, that is not correct. If n%i == 0 then n is not a prime. You also have your break outside your if-block so your loop will stop after the first run through.