Hi,
For a project, I've to write a program that returns the first 10 Mersenne primes. For the uninitiated: A Mersenne Prime can be defined as this: If 2^n-1 is a prime, then this number is Mersenne prime.
E.G.: 2^3 - 1 = 7 is a Mersenne prime.
2^6 - 1 = 63 is not a Mersenne prime since 63 % 3 = 0
Making the program was no problem, but one small error occurs: When it reaches 2^31 - 1, it finds that 2 is a divisor (hence not a prime), and then it loops infinitely, calculating (2^31 - 1) % 2 all of the time without stopping.
Other numbers don't show this behaviour when the program finds out it's not a prime.
In essence, the program detects if it's a prime. If it is, it stops, outputs the prime, increments the exponent and starts anew. If it isn't, it stops, increments the exponent and starts anew. Except with that number. It always occurs at that particular number. Even if I change the exponent to >32, it calculates (2^31 - 1) % 2. Here's the code:
PHP Code:
public class CollectionB {
private static int numberOfPrimes = 10;
public static void main(String[] args) {
calculate();
}
private static void calculate() {
int primesFound = 0;
int exponentToCheck = 0;
while(primesFound < numberOfPrimes) {
exponentToCheck++;
//Yeah, I accidentally calculated this twice.
int maxDivisor = (int)(Math.pow(2, exponentToCheck)) - 1;
int numberToCheck = (int)(Math.pow(2, exponentToCheck)) - 1;
int divisor = 2;
boolean isPrime = true;
while(divisor < maxDivisor && isPrime) {
isPrime = numberToCheck % divisor > 0;
divisor++;
}
if(isPrime) {
primesFound++;
System.out.println("Prime #" + primesFound + ": " + numberToCheck);
}
}
}
}
I'm not sure where the problem lies, to be honest. Any ideas would be welcome.
Thanks in advance!
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September 28th, 2006, 10:38 AM
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