Given a number n, print all primes smaller than or equal to n.

Examples:

Input: n = 10 Output: 2, 3, 5, 7

Input: n = 20 Output: 2, 3, 5, 7, 11, 13, 17, 19

// Prints all prime numbers smaller
int SieveOfSundaram(int n)
{
    // In general Sieve of Sundaram, produces primes smaller
    // than (2*x + 2) for a number given number x.
    // Since we want primes smaller than n, we reduce n to half
    int nNew = (n-2)/2;

    // This array is used to separate numbers of the form i+j+2ij
    // from others where  1 <= i <= j
    bool marked[nNew + 1];

    // Initalize all elements as not marked
    memset(marked, false, sizeof(marked));

    // Main logic of Sundaram.  Mark all numbers of the
    // form i + j + 2ij as true where 1 <= i <= j
    for (int i=1; i<=nNew; i++)
        for (int j=i; (i + j + 2*i*j) <= nNew; j++)
            marked[i + j + 2*i*j] = true;

    // Since 2 is a prime number
    if (n > 2)
        cout << 2 << " ";

    // Print other primes. Remaining primes are of the form
    // 2*i + 1 such that marked[i] is false.
    for (int i=1; i<=nNew; i++)
        if (marked[i] == false)
            cout << 2*i + 1 << " ";
}

REF:http://www.geeksforgeeks.org/sieve-sundaram-print-primes-smaller-n/