Matrix memory allocation through a function - c

Question

I am using these lines to create variable size matrices:
Temp_Mat_0 = (double **)malloc((M)*sizeof(double ));
for (i=0;i<M;i++)
Temp_Mat_0[i] = (double *)malloc((N)*sizeof(double ));
They are working fine but I keep using them repeatedly in my code. I need to convert them to a function where I pass the pointer and the size. I was not able to do it due to pointers mess.
matrixAllocate(Matrix Pointer,rows,colms)
Can you help!

Answer 1

I think you'd be better off with a very simple scheme that allocates your matrix as a single contiguous block.
double **matrix_alloc(int rows, int cols)
{
/* Allocate array of row pointers */
double ** m = malloc(rows * sizeof(double*));
if (!m) return NULL;
/* Allocate block for data */
m[0] = malloc(rows * cols * sizeof(double));
if (!m[0]) {
free(m);
return NULL;
}
/* Assign row pointers */
for(int r = 1; r < rows; r++) {
m[r] = m[r-1]+cols;
}
return m;
}
This has the added bonus that when you free the matrix you don't need to remember how big it was:
matrix_free( double** m )
{
if (m) free(m[0]);
free(m);
}
As an extension to this, you might declare a struct that also keeps track of the number of rows and columns it has. e.g.
struct matrix {
int rows, cols;
double **m;
};
That makes your matrix functions look a little nicer (i.e. you can pass around struct matrix* instead of double**). It has the added bonus that the matrix dimension travels around with the associated data.
Using one contiguous block for your matrix data is generally preferable unless you have huge matrices. And it's very nice if your matrices are small, because you'll get the benefit of better memory locality in your CPU cache -- that means potential for faster code.

Answer 2

they are working fine
They do not seem really fine. Code should be changed to :
Temp_Mat_0 = malloc((M)*sizeof(double*)); //double* instead of double
if (Temp_Mat_0 == NULL)
return;
for (i = 0; i < M; i++){
Temp_Mat_0[i] = malloc((N)*sizeof(double));
if (Temp_Mat_0[i] == NULL){
free(Temp_Mat_0);
return;
}
}
Then, you can use a function like this :
double ** matrix_pointer = matrixAllocate(rows,colms);
where function matrixAllocate returns the pointer it allocated. For example :
matrixAllocate(rows,colms){
Temp_Mat_0 = malloc((rows)*sizeof(double*));
if (Temp_Mat_0 == NULL)
return NULL;
for (i = 0; i < rows; i++){
Temp_Mat_0[i] = malloc((colms)*sizeof(double));
if (Temp_Mat_0[i] == NULL){
free(Temp_Mat_0);
return NULL;
}
}
return Temp_Mat_0;
}
and call it like :
double **matrix pointer;
matrix pointer = matrixAllocate(rows, colms);
Don't forget to free the malloced memory afterwards.
for (i = 0; i < M; i++){
free(Temp_Mat_0[i]);
}
free(Temp_Mat_0);
Note that you should not cast the result of malloc, and you should also check if malloc was successful.