Question

I’m looking for a shortcut for 2 and 3 dimensional indexing of arbitrary values, and I know this works for a given array a[]

#define a(i,j,k) a[(i)*span*span+(j)*span+(k)]
#define b(i,j) b[(i)*span+(j)]

But I don’t understand how to allow these macros to operate on arbitrary arrays eg;

x(i,j,k)

Anyone care to clue me in?

Answer 1

Don’t define the macro name to a or b, instead, give the array as parameter:

#define arr3d(a,i,j,k) a[(i)*span*span+(j)*span+(k)]
#define arr2d(b,i,j) b[(i)*span+(j)]

Then a and b will be parameters and you’ll be able to use it for different arrays.

Edit

For example:
char newarr[5][5][5];
arr3d(newarr, 3, 3, 3);

also, if span is not a variable defined in all scopes where you need to use this macro, it will not work, and you’ll need to add another parameter to the macro – span, so the macro definition will look like:

#define arr3d(a,i,j,k,span) a[(i)*(span)*(span)+(j)*(span)+(k)]

And yes, I would usually avoid this.