# Help: How to write a prime number greater than one billion

• 12-18-2008
kenryuakuma
Help: How to write a prime number greater than one billion
My question is like I am clueless. And how I can write such as program that the first prime number is greater than one billion?
• 12-18-2008
laserlight
Basically, you are supposed to implement a program that finds primes greater than one billion? What ideas do you have? What do you know about primality testing?
• 12-18-2008
kenryuakuma
As far as I know...Finding a prime needs to use the if-statement with a comparison operator == with the divisor of remainder. That's, if(something % something == 0) is not prime. So there is must be a remainder. I am just clueless as my level is not up to that or maybe I am just at the end of my wit.
• 12-18-2008
tabstop
You've got prime number testing code, right? You've posted it before. Why do you expect this to be in any way, shape, or form different?
• 12-19-2008
EVOEx
I assume he has encountered the problem of integer overflows ;).

I think you used a simple int in your code? It's maximum value is around 2 billion. An unsigned int would increase it to 4 billion. You could also use a "long long int"; or an "unsigned long long int", which have way higher maximums. If these limits are still too low, you'll have to use a big integer class.
• 12-19-2008
anon
If I'm not mistaken long long is currently non-standard (but is going to make it in soon), so if the compiler supports it, go ahead.
• 12-19-2008
tabstop
long long is "merely" C99.