Question

I wrote an IntegerMatrix class to add my own methods to work with matrices. Now I’ve written a function like this:

IntegerMatrix** IntegerMatrix::multiplyMatrix(IntegerMatrix** table2)

(It’s a double pointer because I’m holding a huge array of pointers to 4×4 2D arrays.) so I simply could do this:

matrix1.multplyMatrix(matrix2)

One little problem is the * isn’t defined for my own class. So I thought to overload this operator that I could do something like this:

sum += this->table[i][k] * table2[k][j];

But how can I get the right i and k in the overloaded operator, which is defined like this:

IntegerMatrix IntegerMatrix::operator*(const IntegerMatrix & k);

The only problem I can’t figure out right now is how to get the right values ?

EDIT:

I’ve rewrote this and now I have:

IntegerMatrix IntegerMatrix::operator*(const IntegerMatrix & table2)
{
    int i, j, k;
    int sum;
    IntegerMatrix * result = new IntegerMatrix(SIZE);

    for (i = 0; i < SIZE; i++) {
        for (j = 0; j < SIZE; j++) {
            sum = 0;
            for (k = 0; k < SIZE; k++) {
                sum += this->table[i][k] * table2[k][j];
            }
            result[i][j] = sum;
        }
    }
    return *result;

}

That gives me just an error on the [] :

Binary '[' : 'IntegerMatrix' does not define this operator or a conversiont o a type acceptable to the predefined operator.

Answer 1

I don’t understand your question, but here’s a brief demo of how matrix multiplication normall works:

class IntegerMatrix {
    int table[3][3];

public:
      IntegerMatrix& operator*=(const IntegerMatrix& rhs) {
           //multiply table by rhs.table, store in data.
           return *this;
      }
};
IntegerMatrix operator*(IntegerMatrix lhs, const IntegerMatrix& rhs)
{return lhs*=rhs;} //lhs is a copy, so we can alter and return it

FOR YOUR EDIT

You have the code

IntegerMatrix * result = new IntegerMatrix(SIZE); //pointer to an IntegerMatrix
...
result[i][j] = sum; //assign sum to the jth index of matrix # i

when in actuality, I presume you wanted

result->table[i][j] = sum; //sum to the ixj index of the result matrix.

Also, your function is leaky, because you have a new, but no delete. This is easy to fix in your case, since you don’t need the new. (Are you from a Java or C# background?)

IntegerMatrix result(SIZE);
...
        result[i][j] = sum;
...
return result;

Unrelated to all of the above, you might actually want to provide a [] operator for your Integer Matrix.

class row {
    int* data;
    int size;
public:
    row(int* d, int s) :data(d), size(s) {}
    int& operator[](int offset) {
        assert(offset<size);
        return data[offset];
    }
};

row operator[](int column) {
    assert(column<SIZE); 
    return row(table[column], SIZE);
}

And this would allow you to write:

IntegerMatrix result;
result[i][j] = sum;