Question

I am trying to make a function that can return the prime factors of a given number in an array (or multi-set, but I’m trying to use an array).

For example, if I put in 12, I want to get 2, 2, and 3, not 2, and 3 like with a set. This is so that I can use these to see if it is a Smith number or not, so I need the numbers seperately.

Also, I am taking a recursive approach.

I have tried (to no avail) to return the array many ways, including passing an initial pointer into the code which points to a space to store the array.

I’ve tried just initializing the array in the function and then returning it.

From what I can tell, I can get the array back from the base case iteration and then when trying to construct a new array with size oldArray+1 to copy values to, things get messy. This is where I get lost.

From what I’ve read, although this isn’t the most efficient implementation, I should be able to make it work.

I have a function, nextPrime(int n), which given n will give back the next prime up from that number.

See source below:

int* find(int n, int p) {

int root = (int) floor(sqrt(n));
if (p > root) {
    // Base case, array gets initialized and returned
    // depending on value of n and p.
    if (n > 1) {
        factors = new int[1];
        factors[0] = n;
        return factors;
    }
    else {
        factors = new int[0];
        return factors;
    }
}
else
    if (n%p == 0){
        // Inductive step if p is a factor
        int newFloor = (int) floor(n/p);
        factors = find(newFloor, p);

        // Initialize new array.
        int* newFactors;
        newFactors = new int[(sizeof(factors) / sizeof(int)) + 1];

        // Add p to first slot, fill rest with contents of factors.
        factors[0] = p;
        for (int i = 0; i < (sizeof(factors) / sizeof(int)); i++) {
            newFactors[i+1] = factors[i];
        }

        return newFactors;
    }
    else {
        // Inductive step p isn't a factor of n
        factors = find(n, factors, nextPrime(p));
        return factors;
    }
}

As I say, the error is with returning the array and using its value, but why does it seem to return OK from the first iteration?

Answer 1

Something like this could work. Not terribly efficient !!

void FindFactors( int number , std::vector<int>&  factors )
{
    for ( int i = 2; i <= number; ++i )
    {
        if ( number % i == 0 )
        {
            factors.push_back( i );
            FindFactors( number / i , factors);
            break;
        }
    }
}

int main()
{

    std::vector<int> factors;
    FindFactors( 121 , factors );
    return 0;
}

After you call the function factors will contain only the prime factors.