Writing out a function to multiply complex numbers

Question

I keep running into problems when trying to write code to multiply two complex numbers. I first created a simple class called Complex:

class Complex:
    def __init__(self,real,imag):
        self.real = real   
        self.imag = imag


    def multiply(self, d):
        self.real = (self.real * d.real) - (self.imag * d.imag) 
        self.imag = (self.imag * d.real) + (self.real * d.imag)

    def __str__(self): 
        if self.imag > 0:
            return str(self.real) + " + " + str(self.imag) + "i"
        elif self.imag == 0:
            return str(self.real)
        else:
            return str(self.real) + " - " + str(-self.imag) + "i"

Where self.real is the real part and self.imag is the imaginary part. When I run

z = Complex(3,4)
x = Complex(2,3)
z.multiply(x)
print(z,x)

The result should be -6 + 17i, but instead it outputs -6 - 10i. What's happening? Is it something to do with the self.imagpart in the multiply function?

I tried doing it by hand by reading my code, and I got the correct answer. Is there something obvious that I'm missing?

Answer 1

You modify self.real in the first line of your multiply method, then use the modified value in the second line. Do both assignments in a single line, i.e.:

self.real, self.imag = (self.real * d.real) - (self.imag * d.imag), (self.imag * d.real) + (self.real * d.imag)