Prime number

Question

Prime number

KA · Accepted Answer

boolean isPrime(int n) {

if (n%2==0) return false;
for(int i=3;i*i<=n;i+=2) {
        if(n%i==0)
           return false;
        }
	return true;
}

Peter · Answer

// n > 0
bool IsPrime(int n)
{
    if (n  1
void PrimeList(int n)
{
    bool* primalities = new bool[n+1];

    // 0 and 1 are not prime numbers
    // all others can be set to true
    memset(primalities + 2, true, n-1);

    int p = 2; // Smallest prime number

    while (p*p <= n) // '=' is necessary, for example p = 11
    {
        // Set multiples of p as non-prime numbers
        // Here you don't have to start with 'int x = p+1'
        // Instead, 'int x = p*p' is enough
        for (int x = p*p; x <= n; x += p)
            primalities[x] = false;

        // Move to the next prime number;
        while(!primalities[++p]);
    }

    for (int c = 0, p = 2; p <= n; ++p)
        if (primalities[p])
            printf("%4d%c", p, (++c)%10 ? ' ' : '
');
    printf("
");

    delete[] primalities;
}

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