C Macros for 2 and 3 dimensional indexing

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 开发者_如何学JAVAallow these macros to operate on arbitrary arrays eg;

x(i,j,k)

Anyone care to clue me in?

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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.

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get your compiler to preprocess the macros without compiling them to debug macros.

gcc -E -P for gcc.

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This macro will expand anything of the form a(i,j,k) into the form a[(i)*span*span+(j)*span+(k)]. It doesn't specifically use those values (by expecting them to be variables), however. Thus, if you use x(i,j,k), it will replace all as in the macro with xs.

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As it looks your dimension span seems to be a compile time integer constant? Then you just shouldn't use a macro. If you use

double A[span][span][span] = { 0 };

or similar, where

#define span 42

or

enum { span = 42 };

indexing in C with A[i][j][k] will work out of the box and do the right thing. Don't complicate your life with an opaque macro for such cases.