Array macro definition

Question

I'm trying to define a macro to convert 2D array syntax to 1D vector. i.e., for a 12x12 array, matrix[i,j] should evaluate to matrix[12*i + j]. So far I have tried

#define matrix[i,j] matrix[12*i+j]

#define (matrix[i,j]) matrix[12*i+j]

#define matrix[(i,j)] matrix[12*i+j]

#define matrix([i,j]) matrix[12*i+j]`

The reason I am using matrix[i,j] syntax is because it will be an extension to be called from R code, and I would like the other authors of the project to understand exactly what is being done. Is there a way to do this with a macro?

Why a macro? I would think this is function territory, esp. if you're talking about exposing it to something else. Remember macros are pre-processor time only. — C. Ross
– C. Ross, Commented Feb 8, 2013 at 15:16
Why are you even doing that? What's wrong with the normal [][] syntax? — Patrick Schlüter
– Patrick Schlüter, Commented Feb 8, 2013 at 16:08
I rather suspect that "other authors" will be more, not less, confused if you take this approach. Why aren't you creating your data objects as matrices in the first place? — Carl Witthoft
– Carl Witthoft, Commented Feb 8, 2013 at 16:54

Jack · Accepted Answer · 2013-02-08 15:21:01Z

1

Something like this may work:

#define TO_1D(m,x,y) (m[x*12+y])

int matrix[100];
TO_1D(matrix, 0, 4) = 13;

but I wonder why you don't prefer something safer like:

int data[100];
inline int& matrix(int x, int y)
{
   return data[x*12+y];
}

answered Feb 8, 2013 at 15:21

Jack

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MOHAMED · Accepted Answer · 2013-02-08 15:23:40Z

1

The only way that you can do is with ( ) and not with []

#define matrix(i,j) matrix[12*i+j]

and in the code, you define matrix in this way

int matrix[12*12];
matrix(i,j) = 5;

edited Feb 8, 2013 at 15:23

answered Feb 8, 2013 at 15:16

MOHAMED

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3 Comments

David Heffernan Over a year ago

How can that work. The macro will expand both instances of matrix.

MOHAMED Over a year ago

I checked with gcc -E and no problemwith that

MOHAMED Over a year ago

Code generated with gcc -E: int main() { int matrix[12*12]; matrix[12*1 +1] = 15; }

Patrick Schlüter · Accepted Answer · 2013-02-08 16:06:55Z

1

It would be better to only define a macro for the calculation, not the dereferencing.

#define X_DIM 12
#define Y_DIM 12
#define D21(x,y) (X_DIM*(x)+(y))

int my_matrix1[X_DIM*Y_DIM];
double my_matrix2[X_DIM*Y_DIM];

my_matrix1[D21(4, 3)];
my_matrix2[D21(2, 1)];

So you are not limited to the unique matrix defined in your macro.

More fundamentally, why are you even trying to do that? The normal multidimensional array syntax is already there to do exactly that.

answered Feb 8, 2013 at 16:06

Patrick Schlüter

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