Question

I am trying to define a matrix like so
I have a structure

typedef struct _struct {
  int name;
  int data;
} myDataType;

Afterwards i am defining a matrix

int **myMatrix = calloc(size,sizeof(int*));
for()
   // allocate rows except last index
myMatrix[last_index_in_matrix] = calloc(1,sizeof(myDataType));

The problem is that I cannot access myMatrix[last_index].data it says, also tried with -> (i really don’t know when to use what)

request for member ‘data’ in something not a structure or union

What am i doing wrong ? Should i post actual code ? If this method is not possible can i get a different suggestion ?

UPDATE: I’ll say it again, the matrix is all int, i just want the last row to point to that structure, some of the comments have not taken this into consideration. That is way i declared it the way i did in my example.

Answer 1

Let’s start simple:

You want to create a matrix of myDataType. If you know the number of rows and columns at compile time, you can simply declare it as

myDataType matrix[ROWS][COLS];

If you need to allocate it dynamically, you would do something like this:

myDataType **matrix;

matrix = calloc(rows, sizeof *matrix);
if (matrix)
{
  size_t r;
  for (r = 0; r < rows; r++)
  {
    matrix[r] = calloc(cols, sizeof *matrix[r]);
  }
}

Either way, to access the struct members at the i‘th and j‘th elements, you’d write:

matrix[i][j].name = ...;
matrix[i][j].data = ...;

Edit

Ah, now I get it.

Don’t do that.

There’s no guarantee that pointers to struct types have the same size and representation as pointers to int. They do on most common architectures, but it’s not something you can rely on. If they don’t, then you’re going to have runtime problems.

From a design point of view this just makes me itchy; if you need to associate that struct with a matrix, create a new aggregate type to explicitly do so:

struct composite
{
  int **matrix;
  struct myData data;
};

This will make life easier when you need to free up the matrix as well.

FWIW, to do what you wanted, you’d need to engage in some casting gymnastics such as

(struct myData *) myMatrix[last_index] = malloc(sizeof (struct myData));

and

((struct myData *) myMatrix[last_index])->data = ...;

but as I said above, if the pointer types are not compatible, the conversion could lead to runtime errors. Don’t do it. Bad juju.