Incorrect output from overloaded [] function

Question

The spec says that the function must return the row of the matrix specified by the "row number" in []

Class Definition:

class Matrix
{
 public:

 //functions taken out 

 private:
  double ** matrix; // the matrix array
  unsigned rows; // # rows
  unsigned cols; // # columns
};

The brief main:

 cout << "Test []: " << endl;
  try {
   Matrix row = m0[0]; //**m0 is Matrix m0(1,1); where the constructor creates the appropriate array** 
    cout << row << endl;
    row = m0[1];
    cout << row << endl;
    row = m0[100];  // should throw an exception
  } catch (const char * err) {
    cout << err << endl;
  }

The Function implementation:

 double& Matrix::operator [](const unsigned int &sub)
{   
     if( sub >= rows)
    {
        const char * error = "Error: invalid row index";
        throw error;

    } else
        {       
            return *matrix[sub];

        }


}

Overloaded << operator for display:

//This is working for my other displays so this shouldn't be the problem
ostream &operator << (ostream &ostrm, const Matrix &obj)
{
    //Loop through to display
    for(unsigned int i = 0; i < obj.rows; i++)
    {       
        for(unsigned int j = 0; j< obj.cols; j++)
        {
            ostrm << setw(10)  << setprecision(3) << obj.matrix[i][j]; 
        }

        ostrm << endl;
    }

    return ostrm;
}

Overloaded = operator:

//Again this works for everything else 
Matrix& Matrix::operator=(const Matrix &rhs)
{
    //Copy Rows and Cols
    rows = rhs.rows;
    cols = rhs.cols;

    //If statement to check for self assignment
    if(&rhs == this)
    {
        return *this;
    }
    else 
    {
        delete [] matrix;

        matrix = new double*[rows]; //Allocate Dynamic Array

        //Deep copy elements by looping and copying each element
        for(unsigned int i = 0; i < rows; i++)
        {
            matrix[i] = new double[cols];
            for(unsigned int j = 0; j < cols; j++)
            {
                matrix[i][j] = rhs.matrix[i][j];
            }

        }

        return *this;

    }


}

My Output:

Test []: 


Error: invalid row index

Expected Output:

Test []: 
      17.2        -3      -0.5         6

       8.2         4         3         1

Error: invalid row index

I am uncertain as to why the rows are not displaying or possibly not even being stored.

Thanks in advance

Answer 1

Aside comment: your assignment operator is leaking memory: you delete matrix, but you need to delete also the individual rows (using the original value of rows) for(unsigned int i = 0; i < rows; i++) delete[] matrix[i];

Your operator[] should return a double[] or double *, not a double - you want to return a whole row, not a single value.

Your "test []" code should not even compile... Matrix row = m0[0]; assigns a double to a Matrix object.

Etc.

Bottom line: just use Eigen.

Answer 2

I managed to find a solution that worked for my question. Below is what I implemented in case someone else has a similar problem.

Matrix temp(1, cols); //Calls my constructor here

        for(unsigned int i = 0; i < 1; i++)
        {   

            for(unsigned int j = 0; j < cols; j++)
            {
                temp.matrix[i][j] = matrix[sub][j]; //Accessed temp's matrix and assigned it with what is in the matrix that called the function
            }


        }

        return temp;

Thank you to everyone who helped and added some input. Much appreciated

Answer 3

The first line is wrong. operator[] is returning a double. You assign it to a Matrix. The Matrix is initialized with one value. You've taken out your Constructors. Which one is called? I assume, the called constructor initializes the rows and cols members with zero. When they're zero, the output stream operator does nothing.