C structures

Structures in C are collection of variables in the computer memory, for instance:

struct matrix
{
	double* x;
	int m;
	int n;
};

struct matrix A;

In computers memmory:

   +--------+--------+
   | m      | n      |
   +--------+--------+
   | x               |
   +-----------------+

Assignment

A.m = 2;
A.n = 3;
A.x = (double*) malloc(A.m*A.n*sizeof(double));

In computers memmory:

   +--------+--------+
   | m: 2   | n: 3   |
   +--------+--------+
   | x: 0x03da8cd6   |
   +-----------------+

Structures are ussed to group related values both conceptually and in memory. They are very useful in return multiple results from functions. For instance:

struct min_and_max
{
	int min;
	int max;
};

struct min_and_max find_min_and_max(int* A, int lenght)
{
	struct min_and_max s;
	
	s.min = A[0];
	s.max = A[0];
	for (int i = 1; i < length; ++i) {
		if (s.min > A[i]) {
			s.min = A[i];
		} else if (s.max < A[i]) {
			s.max = A[i];
		}
	}
	
	return s;
}

Assignment shortcut

Very often pointers to structures are used. In such cases the following notation shorthand is very handy:

   A->m == *A.m [ == (*A).m ]

For instance, we write,

struct matrix* A = (struct matrix*) malloc(sizeof(struct matrix));

A->m = 2;
A->n = 2;
A->x = (double*) malloc(A->m*A->n*sizeof(double));

instead of:

struct matrix* A = (struct matrix*) malloc(sizeof(struct matrix));

*A.m = 2;
*A.n = 2;
*A.x = (double*) malloc((*A.m)*(*A.n)*sizeof(double));

Allignement

The order of definition matters!

struct matrix
{
	int m;
	double* x;
	int n;
};

struct matrix A;

In computers memmory:

   +--------+--------+
   | m      |        |
   +--------+--------+
   | x               |
   +-----------------+
   | n      |        |
   +--------+--------+

• 3 bytes instead of 2 due to 64bit allignment for pointer tyes.
• If you have a large array of structures, it needs 50% more space!

For instace, when storing many 3x3 rotation matrices the size of the matrix structure is significant:

struct matrix* A = (struct matrix*) malloc(500*sizeof(struct matrix))

Structures/Classes in C++

Same as C in the computer memory, but they are also assigned conceptually a set of fuctions that can operate on instances of the structure.