How do I work with dynamic multi-dimensional arrays in C?

Question

Does someone know how I can use dynamically allocated multi-dimensional arrays using C? Is that possible?

Answer 1

If you know the number of columns at compile time, it's pretty simple:

#define COLS ...
...
size_t rows;
// get number of rows
T (*ap)[COLS] = malloc(sizeof *ap * rows); // ap is a *pointer to an array* of T

You can treat ap like any 2D array:

ap[i][j] = x;

When you're done you deallocate it as

free(ap);

If you don't know the number of columns at compile time, but you're working with a C99 compiler or a C2011 compiler that supports variable-length arrays, it's still pretty simple:

size_t rows;
size_t cols;
// get rows and cols
T (*ap)[cols] = malloc(sizeof *ap * rows);
...
ap[i][j] = x;
...
free(ap);

If you don't know the number of columns at compile time and you're working with a version of C that doesn't support variable-length arrays, then you'll need to do something different. If you need all of the elements to be allocated in a contiguous chunk (like a regular array), then you can allocate the memory as a 1D array, and compute a 1D offset:

size_t rows, cols;
// get rows and columns
T *ap = malloc(sizeof *ap * rows * cols);
...
ap[i * rows + j] = x;
...
free(ap);

If you don't need the memory to be contiguous, you can follow a two-step allocation method:

size_t rows, cols;
// get rows and cols
T **ap = malloc(sizeof *ap * rows);
if (ap)
{
  size_t i = 0;
  for (i = 0; i < cols; i++)
  {
    ap[i] = malloc(sizeof *ap[i] * cols);
  }
}

ap[i][j] = x;

Since allocation was a two-step process, deallocation also needs to be a two-step process:

for (i = 0; i < cols; i++)
  free(ap[i]);
free(ap);