Complex Matrix Multiplication

Dear all,

Im trying to do a matrix multiplication with complex number. I have 2 structures:

/* Structures */
typedef struct complex { // Complex structure = real and imaginary part.
	double re;
	double im;
} Complex;

typedef struct complex_matrix {
	int rows;
	int cols;
	Complex **mat;
} Complex_Matrix;

I then try do to my complex matrix multiplication, but in "sum" it gives me an error that says I can't use sum that way. Maybe I need to define sum as a Complex_Matrix as well? Or how do I solve this?

Here is my complex matrix multiplication C code:

Complex_Matrix complex_multiply(Complex_Matrix a, Complex_Matrix b){
	Complex_Matrix m;
	int i,j,k;
	float sum;
	
	m = complex_allocate(a.rows, a.cols);
	
	for (i=0; i<a.rows; i++) {
		printf("\n");
		for (j=0; j<b.cols; j++) { // columns
			for (k=0; k<a.cols; k++) { // k=a.cols or b.rows.
				sum += multiplication(a.mat[i][k], b.mat[k][j]);
			}
			m.mat[i][j] = sum; // puts the sum in matrix array.
		}
	}
	return m;
}

Where multiplication is:

Complex multiplication(Complex x, Complex y){ // multiply the two inputs together.
	Complex z;
	
	z.re = (x.re*y.re) - (x.im*y.im);
	z.im = (x.re*y.im)+(x.im*y.re);
	
	return z;
}

Thank you guys so much!