Problem with pointer to a pointer variable

**Rodri** · 11-20-2011

Hi all.

I am using a program called HPGCC which converts C files into HP files for the HP programming calculator. However, I think my problem has to do with C programming, not HP.

I have the following main:

Code:

int main(void){

    double m[2][2];
    
    m[0][0]=1;
    m[0][1]=2;
    m[1][0]=3;
    m[1][1]=4;
    
    sat_push_double_matrix(2,2,m);
    

    
    return 0;

}

"sat_push_double_matrix" is a function to push a matrix to the stack of the HP calculator. Its format is:

Code:

BOOL sat_push_double_matrix(int rows,int columns,double **array)

I'm not interested in the BOOL output and I am not familiar with pointer to pointer variables, which is my problem. If I leave it like this I get the following warning in my compiler:

warning: passing argument 3 of 'sat_push_double_matrix' from incompatible pointer type

On my HP calculator it does give back a 2x2 matrix, but its content is not what i expected, which should be [[1,2],[3,4]] as i wrote in the main.

So my question is what should I do in the main function in order to avoid this variable format incompatibility so the expected matrix gets pushed into the stack? I would be very grateful if you could post me a solution to this problem.

Regards,
Rodri

**MK27** · 11-20-2011

Originally Posted by Rodri

So my question is what should I do in the main function in order to avoid this variable format incompatibility so the expected matrix gets pushed into the stack? I would be very grateful if you could post me a solution to this problem.

A double** is a pointer to an array of pointers to double (I think you realize this). Kind of a hassle, but this is the only way to have a parameter for a variable sized matrix in C. So, what you need is an array of pointers to double (and as with all arrays in C, you can use the variable name as a pointer to itself):

Code:

	double *m[2];
	int i;
	for (i=0; i<2 ;i++) {
		m[i] = malloc(2*sizeof(double));
	}
	m[0][0]=1;
	m[0][1]=2;
	m[1][0]=3;
	m[1][1]=4;

Alternately, you could do this:

Code:

	double m[2][2], *p[2];
	m[0][0]=1;
	m[0][1]=2;
	m[1][0]=3;
	m[1][1]=4;
	int i;
	for (i=0; i<2 ;i++) {
		p[i] = m[i];
	}

And use "p" for the function call.

If you use the first version, with malloc(), you must free() the matrix when done with it:

Code:

	for (i=0; i<2 ;i++) {
		free(m[i]);
	}

**Rodri** · 11-20-2011

Works nicely now, thanks a lot!

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