Thread: Finding primes...

  1. #1

    Unhappy Finding primes...

    void determinePrimes(int maxPrime)
    	int numPrimes = 0, *primes = new int[1.01 * (maxPrime / (log(maxPrime) - 1))];
    	for (int j = 3; j <= maxPrime; j += 2)
    		if (isPrime(j, primes))
    			primes[++numPrimes] = j;
    	delete[] primes;
    inline bool isPrime(int number, int *primes)
    	int biggestTest = sqrt(number) + 1;
    	for (int j = 0; primes[j] <= biggestTest; ++j)
    		if (number % primes[j] == 0) return 0;
    	return 1;
    When I put in a number greater than around 100,000, the program performs an illegal operation and gets shut down. Is there any way I can allocate more memory for the array that seems to be causing the problem?

  2. #2
    geek SilentStrike's Avatar
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    Aug 2001
    You sure it's not just overflowing the number of primes you allocated?

    I don't know about the mathematical frequency of primes in a list of in a general continous sequence of N integers (I've actually thought about it a bit though).

    It's possible, however, that there are more primes than

    1.01 * (maxPrime / (log(maxPrime) - 1)).

    Try making that 1.01 a 1.5 and seeing if that works.
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  3. #3
    Nope, that's not it...I tested it with a mathematical reference book. My algorithim allocates 78800 spaces in the array for 78498 primes, which is enough efficiency for me.

    What else could be the problem?

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