Polynomial Class: Polynomial Multiplication

Question

I have a Polynomial class that has a get_vect member function which stores integers in a vector that is to be the representation of the coefficients of the polynomial. Now, I am trying to multiply two polynomials together using a Multiply non-member function, but I get stuck when it comes to the actual multiplication of the vectors. So far, what I have is what is shown below:

Polynomial Multiply(const Polynomial & poly1, const Polynomial & poly2)
{
    vector<int> Poly1 = poly1.get_vect();
    vector<int> Poly2 = poly2.get_vect();
    vector<int> Poly3;

    if( Poly1.size() < Poly2.size() )
    {
        for(size_t i = 0 ; Poly2.size()-Poly1.size() ; ++i )
        {
            Poly2.push_back(0);
        }
    }
    else if( Poly1.size() > Poly2.size() )
    {
        for(size_t i = 0 ; Poly1.size()-Poly2.size() ; ++i )
        {
            Poly1.push_back(0);
        }
    }

    return Poly3;
}

I see that it some how has to follow the below pattern:

enter image description here

I'm not sure why any pattern is needed. When you multiply the same base, the exponents are added. — chris, Aug 10 '13 at 05:34
This photo is currently unavailable. (your image link is no good) — Benjamin Lindley, Aug 10 '13 at 05:37
@BenjaminLindley, It worked fine for me. In any case, I forgot to add it to the question before. — chris, Aug 10 '13 at 05:50
@LoieBenedicte: I'm pretty certain my nested loop below directly corresponds to the red/orange diagonals in the diagram you posted. — Joe Z, Aug 10 '13 at 06:03

Joe Z · Accepted Answer · 2013-08-10T06:01:58.720

Ok, so if I understand the problem correctly, you want Poly3 to be a vector<int> that holds the coefficients that result from a polynomial multiplication between the polynomials represented by Poly1 and Poly2.

Tacit in this request is that all three polynomials are polynomials in a single variable, with each coefficient representing the coefficient in front of an increasing power of that variable. ie. that { 4, 5, 6, 7 } corresponds to 4 + 5x + 6x² + 7x³.

If so, then the actual multiplication shouldn't be that difficult at all, as long as your polynomials aren't terribly huge. You need code that looks approximately like this:

    Poly3.resize(Poly1.size() + Poly2.size() - 1, 0);  // Make the output big enough; init to 0

    for (size_t i = 0; i != Poly1.size(); i++)
        for (size_t j = 0; j != Poly2.size(); j++)
            Poly3[i+j] += Poly1[i] * Poly2[j];

Now the result in Poly3 should be the product of Poly1 and Poly2.

It's entirely possible I forgot an edge condition; I'll watch for comments here to point out where I did. In the meantime, though, I did a few tests and it appears this gives the correct output.

If you have rather large polynomials, then you might want to look into math libraries to handle the multiplication. But for anything under about 20 - 30 terms? Unless your code leans very hard on this polynomial evaluation, I suspect this won't be your bottleneck.

If so, then do tag this as the answer, so it's no longer listed among the unanswered questions. And, glad to help. You're welcome! :-) — Joe Z, Aug 10 '13 at 06:26

Polynomial Class: Polynomial Multiplication

1 Answers1