How to create a null matrix and show it on the screen?

Question

How to create a null matrix and show it on the screen?

I'm trying to create a null matrix with the method zeros (); and then show it on the screen. Nothing is shown.

double * zeros(int m,int n){
    double matrix[3][3] ={0};
    return matrix;
}

void printMatrice(int row,int column, double matrix [3][3]) {
     for (row=0; row<3; row++)
     {
        for(column=0; column<4; column++)
            {printf("%f     ", matrix[row][column]);}
            printf("\n");
     }
}


MAIN:

int main () {

    printMatrice(3,3,zeros(3,3));

   return 0;
}

Show this on screen:

Process returned -1073741819 (0xC0000005)

These are the warnings

Of method printMatrice():

--warning: passing argument 3 of 'printMatrice' from incompatible pointer type [-Wincompatible-pointer-types]|

Of method zeros():

-- warning: returning 'double (*)[3]' from a function with incompatible return type 'double *' [-Wincompatible-pointer-types]

--warning: function returns address of local variable [-Wreturn-local-addr]|

c

matrix

null

asked on Stack Overflow Apr 11, 2019 by

Diego R • edited Apr 11, 2019 by

Diego R

1 Answer

Working with multidimensional arrays in c++ is a pain in the ass. One of the easiest ways is to use a simple array and convert coordinates into linear index manually.

This way, you do not have to hard-wire matrix dimensions into functions, you do not have to alloc/free memory, you can reuse and reinterpret data, etc. Of course, it would be better to encapsulate it in a class, but this is just an example.

void reset(double matrix[], int rows, int cols, double value) {
    for (int i = 0; i < rows * cols; i++)
        matrix[i] = value;
}

void print(double matrix[], int rows, int cols) {
    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < cols; c++)
            printf("%f     ", matrix[r * cols + c]);
        printf("\n");
    }
}

int main() {

    double m[3 * 3];
    reset(m, 3, 3, 0.0);
    print(m, 3, 3);

    printf("\n");

    double n[2*5];
    reset(n, 2, 5, 1.0);
    print(n, 2, 5);

    return 0;
}

answered on Stack Overflow Apr 11, 2019 by

Jiri Volejnik • edited Apr 11, 2019 by

Jiri Volejnik

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