How to dynamically allocate a contiguous block of memory for a 2D array

Question

If I allocate a 2D array like this int a[N][N]; it will allocate a contiguous block of memory.

But if I try to do it dynamically like this :

<

Answer 1

In fact, n-dimensional arrays (allocated on the stack) are really just 1-dimension vectors. The multiple indexing is just syntactic sugar. But you can write an accessor function to emulate something like what you want:

int index_array(int *arr, size_t width, int x, int y)
{
    return arr[x * width + y];
}

const size_t width = 3;
const size_t height = 2;
int *arr = malloc(width * height * sizeof(*arr));

// ... fill it with values, then access it:

int arr_1_1 = index_array(arr, width, 1, 1);

However, if you have C99 support, then declaring a pointer to an array is possible, and you can even use the syntactic sugar:

int (*arr)[width] = malloc(sizeof((*arr) * height);
arr[x][y] = 42;