Polynomial Multiplication with Complex Numbers Class

Question

Gist link to my code. The problem I'm having uses the class Polynomial, method multiply, lines 136-172.

Here is the method:

public Polynomial multiply(Polynomial right){
    int size = right.length + length -1;
    int i;
    int r;
    Complex productCoeff;

    Complex coeffP = new Complex();
    Complex coeffQ = new Complex();
    Complex currentValue;

    Polynomial temp = new Polynomial(size);

        for (i = 0; i < length; i++)
        {
            for (r = 0; r < right.length; r++) {
                coeffP = (retrieveAt(i));
                coeffQ = (right.retrieveAt(r));

                productCoeff = coeffP.multiplyComplex(coeffQ);


                if (temp.retrieveAt(i+r) == null)
                    currentValue = productCoeff;

                else

                    currentValue = (temp.retrieveAt(i+r));
                    currentValue = currentValue.addComplex(productCoeff);

                temp.replaceAt(i+r, currentValue);

            }
        }

    return temp;
}

I was given the class Polynomial and am trying to implement complex numbers for addition, subtraction, and multiplication. The class polynomial works by storing the coefficients into an array. [x^0, x^1, x^2, x^3 ...] I got the addition and subtraction to work with complex numbers, but I am having trouble getting the multiplication to work correctly.

My thought process for multiplying complex numbers: For each item that is being looped through in the first array, I want to loop through all the items in the second array and multiply. After each series of multiplication, I want to store this value into the temporary array. If the temporary array has a value at that location, I want to add the multiplied value to the value stored at that position in the temporary array. If there is no value at that location in the temporary array, I can simply replace it.

The method worked for regular polynomials, but when using complex numbers I am getting incorrect answers. For example:

((1+2i)+(3+4i)x) ((2+7i)+(4+3i)x) should equal (-12+11i) + (-24+40i)x + (25i)x^2 but when I run the program my answer is (-24+22i) + (-26+51i)x + (50i)x^2. So, it looks like some things are being doubled, but I can't figure out why.

Can anyone figure out why the multiplication is not correct?

Post your relevant code here with your question and not in a link. If it's too much code to post here, then it's too much code to ask a volunteer to go through. — Hovercraft Full Of Eels, Jul 08 '15 at 01:28

score 2 · Accepted Answer · answered Jul 08 '15 at 14:25

As saka1029 already mentioned: Your code's indentation does not match its logical structure. Your if-else construct

if (temp.retrieveAt(i+r) == null)
    currentValue = productCoeff;

else

    currentValue = (temp.retrieveAt(i+r));
    currentValue = currentValue.addComplex(productCoeff);

will actually be interpreted as

if (temp.retrieveAt(i+r) == null) {
    currentValue = productCoeff;
} else {
    currentValue = (temp.retrieveAt(i+r));
}

currentValue = currentValue.addComplex(productCoeff);

meaning that the last line will be executed on every iteration of the for loop, no matter what the above condition yields. Even if it looks pedantic, I always write braces to avoid hard to track errors like this. See Is it ok if I omit curly braces in Java?. And if Jon Skeet does it, so should you!

Polynomial Multiplication with Complex Numbers Class

1 Answers1