How to get an array size

Question

I'd like to know how to get an array rows & columns size. For instance it would be something like this:

int matrix[][] = { { 2, 3 , 4}, { 1, 5, 3 } }

The size of this one would be 2 x 3. How can I calculate this without including other libraries but stdio or stdlib?

Answer 1

This has some fairly limited use, but it's possible to do so with sizeof.

sizeof(matrix) = 24  // 2 * 3 ints (each int is sizeof 4)
sizeof(matrix[0]) = 12  // 3 ints
sizeof(matrix[0][0]) = 4  // 1 int

So,

int num_rows = sizeof(matrix) / sizeof(matrix[0]);
int num_cols = sizeof(matrix[0]) / sizeof(matrix[0][0]);

Or define your own macro:

#define ARRAYSIZE(a) (sizeof(a) / sizeof(a[0]))

int num_rows = ARRAYSIZE(matrix);
int num_cols = ARRAYSIZE(matrix[0]);

Or even:

#define NUM_ROWS(a) ARRAYSIZE(a)
int num_rows = NUM_ROWS(matrix);
#define NUM_COLS(a) ARRAYSIZE(a[0])
int num_cols = NUM_COLS(matrix);

But, be aware that you can't pass around this int[][] matrix and then use the macro trick. Unfortunately, unlike higher level languages (java, python), this extra information isn't passed around with the array (you would need a struct, or a class in c++). The matrix variable simply points to a block of memory where the matrix lives.