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

There's no way to allocate the whole thing in one go. Instead, create an array of pointers, then, for each pointer, create the memory for it. For example:

int** array;
array = (int**)malloc(sizeof(int*) * 50);
for(int i = 0; i < 50; i++)
    array[i] = (int*)malloc(sizeof(int) * 50);

Of course, you can also declare the array as int* array[50] and skip the first malloc, but the second set is needed in order to dynamically allocate the required storage.

It is possible to hack a way to allocate it in a single step, but it would require a custom lookup function, but writing that in such a way that it will always work can be annoying. An example could be L(arr,x,y,max_x) arr[(y)*(max_x) + (x)], then malloc a block of 50*50 ints or whatever and access using that L macro, e.g.

#define L(arr,x,y,max_x) arr[(y)*(max_x) + (x)]

int dim_x = 50;
int dim_y = 50;

int* array = malloc(dim_x*dim_y*sizeof(int));

int foo = L(array, 4, 6, dim_x);

But that's much nastier unless you know the effects of what you're doing with the preprocessor macro.