problem with variable assignment to arrays using structs in c - c

Question

So I'm trying to multiply matrices in c. However when I try to multiply the numbers in the two arrays, and put them in an answer array its always zero. heres the code for the method, thanks.
My matrix struct:
typedef struct matrix {
int r;
int c;
double **mat;
} *matrix_t;
My matrix multiplying method:
matrix_t mat_mult(matrix_t a, matrix_t b)
{
int i, j, k;
double x, temp1, temp2;
double tempsol = 0.0;
x = temp1 = temp2 = 0;
matrix_t answer;
if(a -> c == b -> r)
{
answer = mat_new(a -> r, b -> c);
for(i = 0; i < a -> r; i++)
for( j = 0; j < b -> c; j++)
{
for( k = 0; k < a -> c; k++)
{
tempsol += a->mat[i][k] * b->mat[k][j];
answer-> mat[i][j] = tempsol;
}
}
return answer;
}
else if(a -> r == b -> c)
{
answer = mat_new(a -> c, b -> r);
return answer;
}
else
{
printf("Matrices could not be multiplied");
exit(1);
return;
}
}
heres the code for my mat_new as well
matrix_t mat_new(int r,int c)
{
int i = 0;
double **a;
matrix_t matrix_a;
a = (double**)malloc(r *sizeof(double *));
for(i = 0; i < r; i++)
{
a[i] = (double*)malloc(c *sizeof(double));
}
matrix_a = (matrix_t) malloc ( sizeof(struct matrix));
matrix_a -> mat = a;
matrix_a -> r = r;
matrix_a -> c = c;
return matrix_a;
}

Answer 1

You need to free your objects. You need to reset tempsol. But most importantly, you need to review your mat_mult().
matrix_t mat_mult(matrix_t a, matrix_t b)
{
/* ... */
if(a -> c == b -> r)
{
/* ... */
}
else if(a -> r == b -> c)
{
/* BZZZZT! */
answer = mat_new(a -> c, b -> r); /* BZZZZT! mat_mult(b, a); */
/* BZZZZT! */
return answer;
}
else
{
/* ... */
}
}

Answer 2

Seems like all your issues stem from reading in matrix values as integers rather than doubles. Everything works fine if you change temp in read_mat() to an int, then cast it to a double when you're putting it in the matrix.

Answer 3

This should work for your example:
matrix_t mat_new(int r,int c)
{
matrix_t new = malloc(sizeof*new);
new->r = r;
new->c = c;
new->mat = malloc( r*c*sizeof(double) );
return new;
}

Answer 4

Your code doesn't contain any obvious errors. Perhaps the problem lies in your mat_new(). The way you defined mat in your matrix structure as double **mat;, which I wouldn't recommend, may be causing some problems.
To allocate a 2x2 matrix to mat, you would need to do:
mat = new (double*)[2];
mat[0] = new double[2];
mat[1] = new double[2];
or a n by m matrix:
mat = new (double*)[n];
for (int i=0;i<n;i++) {
mat[i] = new double[m];
}
Is this what you are doing?