What is the proper way to assign to an array inside a C++ class?

Question

I have this C++ class Matrix (see code snipet below).

In my randomize method, I set all the values of matr(matr is a 2x2 matrix).

When i call print_matrix it is duplicating elements (1,0) and (1,1) and printing them both twice, as well a not printing (0,0), or (0,1).

What am I doing incorrectly?

See the output below.

class Matrix {

public:


    int rows;
    int cols;
    double rnd;

    double* matr;

    Matrix(int a, int b) {

        printf(" vals %d %d \n", a, b);

        rows = a;
        cols = b;

        matr = new double[a, b];
        //this->print_matrix();
        //clear_matrix();
        //this->print_matrix();
        //this->randomize();
        //this->print_matrix();

    }



    double &at(int pos1, int pos2) {


        return matr[pos1, pos2];
    };

    void setPos(int pos1, int pos2, double value) {
        matr[pos1, pos2] = value;
    };

    void randomize() {

        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                rnd = double(rand()) / double((RAND_MAX)+1.0);
                printf("Rand : %d, C: %d, Val: %f \n",r,c, rnd);
                this->setPos(r, c, rnd);
                //matr[r, c] = rnd;
                printf("New value R: %d C: %d Val: %f \n", r, c, matr[r,c]);
                //rnd = 0;
            }
        }
    };

    void subtract_Scalar(double val) {
        double curr_Val = 0;
        double result = 0;
        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                curr_Val = this->at(r, c);
                result = curr_Val - val;
                this->setPos(r, c, 0);
                this->setPos(r, c, (float)result);
                //this->setPos(r, c, 5);
                //printf("SS CV : %f, Re: %f  \n", curr_Val, result);
                curr_Val = 0;
                result = 0;
            }
        }
    };

    void print_matrix() {
        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                printf("PM R : %d, C: %d Val: %f \n", r, c, matr[r,c]);
                //printf("%f", this->at(r, c));
            }
            //printf("\n");
        }
    }



    void clear_matrix() {
        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {

                this->setPos(r, c, 0.0);
            }
        }
    }

    };

Answer 1

Your program has undefined behavior because of the following line:

printf("PM R : %d, C: %d Val: %f \n", r, c, matr[r,c]);

matr[r,c] does not access an element of the matrix. Due to the comma operator, it is simply matr[c], which evaluates to a pointer. You are printing the pointer using %f. That's the undefined behavior part. You need to use matr[r][c] to access the element of the matrix.

printf("PM R : %d, C: %d Val: %f \n", r, c, matr[r][c]);

Use of matr[r, c] is not right.

See the documentation of the comma operator for further details.

---

Update

Thanks to the keen eyes of @PeteBecker, the problem is different than what I initially thought.

It turns out, matr is of type double*. Hence matr[c] evaluates to a double. Hence, the program does not have undefined behavior. It just accesses the c-the element of matr all the time regardless of the value of r.

The problem starts with:

matr = new double[a, b];

It needs to be

matr = new double[a * b];

Anywhere the element of the matrix is accessed using matr[r, c], it needs to be matr[r*cols + c].

In at, you need to use:

return matr[pos1 * cols + pos2];

In setPos, you need to use:

matr[pos1 * cols + pos2] = value;

In randomize, you need to use:

printf("New value R: %d C: %d Val: %f \n", r, c, matr[r*cols + c]);

In print_matrix, you need to use:

printf("PM R : %d, C: %d Val: %f \n", r, c, matr[r*cols + c]);

You can simplify the code for accessing the elements by providing an overload of at for const non-const objcts.

double& at(int r, int c) { return matr[r*cols + c]; }
double at(int r, int c) const { return matr[r*cols + c]; }

Then, setPos can be implemented as:

  void setPos(int pos1, int pos2, double value) {
     at(pos1, pos2) = value;
  };

The printf lines can be updated to:

printf("New value R: %d C: %d Val: %f \n", r, c, at(r, c));
printf("PM R : %d, C: %d Val: %f \n", r, c, at(r, c));