How to loop over variable names?

Question

I need to do some assigns as follows.

In struct REGS, tiles are arranged by names;

while on outside, tiles are arranged by indices.

#include <stdio.h>

typedef struct TILE {} TILE;

typedef struct REGS {
    TILE tile00, tile01, tile02,
         tile10, tile11, tile12,
         tile20, tile21, tile22;
} REGS;

int main() {
    TILE tiles[3][3];
    REGS regs;

    regs.tile00 = tiles[0][0];
    regs.tile01 = tiles[0][1];
    regs.tile02 = tiles[0][2];
    regs.tile10 = tiles[1][0];
    regs.tile11 = tiles[1][1];
    regs.tile12 = tiles[1][2];
    regs.tile20 = tiles[2][0];
    regs.tile21 = tiles[2][1];
    regs.tile22 = tiles[2][2];

    return 0;
}

so I suppose I need a loop like:

for (int i = 0; i < 3; ++i) {
    for (int j = 0; j < 3; ++j) {
        // not real code
        regs.tile##i##j = tiles[i][j];
    }
}

for doing that, I defined some macros:

#define TILE_MEM(prefix, i, j) prefix##i##j

#define WRITE_TILE(base, mem, value) (base.mem = value;)

#define MACRO_LOOP(i,j) (WRITE_TILE(regs, TILE_MEM(tile, i, j), tiles[i][j]))

then I did this:

for (int i = 0; i < 3; ++i) {
    for (int j = 0; j < 3; ++j) {
        MACRO_LOOP(i, j);
    }
}

but it doesn't compile.

PLEASE HELP!

The C preprocessor does not work like that. Your MACRO_LOOP will only be replaced once at compile time. You migh try to put REGS regs into a union that holds an array besides REGS content. — Gerhardh
– Gerhardh, Commented Jan 11, 2023 at 6:31
@Chris because they are hardware registers, they are different register with different names... — Xingx1
– Xingx1, Commented Jan 11, 2023 at 7:08
Names are irrelevant. Are they in sequence, no gaps, same size? Do I understand correctly that we are talking memory-mapped registers? I am used to that in embedded environments. Please provide some examples of how they are accessed; because the shown code does not access hardware. — Yunnosch
– Yunnosch, Commented Jan 11, 2023 at 8:24
What's typedef struct TILE {} TILE; supposed to be? That's invalid C and doesn't make any sense. — Lundin
– Lundin, Commented Jan 11, 2023 at 10:39

Lundin · Accepted Answer · 2023-01-11 10:46:04Z

2

Macros are the wrong solution and they only exist at compile-time, so you can't use them together with run-time values.

The standard way of creating a type where you can either access items by individual names or as an array, is to use a union:

typedef union {
  struct  // anonymous struct, must be compiled with a standard C compiler
  {
    TILE tile00, tile01, tile02,
         tile10, tile11, tile12,
         tile20, tile21, tile22;
  };
  TILE tile [3][3];
} REGS;

Now you can either use individual names:

regs.tile00 = tiles[0][0];
regs.tile01 = tiles[0][1];

Or the array type:

for(size_t i=0; i<3; i++)
{
  for(size_t j=0; j<3; j++)
  {
    regs.tile[i][j] = tiles[i][j];
  }
}

(Do keep struct alignment/padding in mind however, since there's some special cases where that might trip union type punning over.)

answered Jan 11, 2023 at 10:46

Lundin

220k47 gold badges281 silver badges446 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Lundin Over a year ago

And just for completeness, the SEVERELY NOT RECOMMENDED version that does use macros: godbolt.org/z/rP5jf7W99. And this relies on a list on constants getting hardcoded.

Collectives™ on Stack Overflow

How to loop over variable names?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related