C pointer to 2d array in a struct

Question

I have a structure with a 2d array. I'm passing a pointer to this structure to a function, where I need to edit the 2d array. I am struggling to find the correct syntax to reference the 2d array from within my function.

#define TG_WIDTH 10
#define TG_HEIGHT 20

typedef enum {
   BLOCK_T = 0,
   BLOCK_LINE,
   BLOCK_SQUARE,
   BLOCK_L,
   BLOCK_L_REVERSE,
   BLOCK_S,
   BLOCK_S_REVERSE
} block_t;

typedef struct {
   char y_max[TG_WIDTH];
   block_t grid[TG_WIDTH][TG_HEIGHT];
} scratchpad_t;

bool
placeBlock(scratchpad_t *sp) {
   block_t (*g)[TG_WIDTH] = sp->grid;

   g[1][2] = BLOCK_T;
}

This gives me an "initialization from incompatible pointer type" warning though. What is the correct way to define/initialize "g" here?

Alok-- · Accepted Answer · 2013-09-05 01:40:13Z

3

Change TG_WIDTH to TG_HEIGHT:

block_t (*g)[TG_HEIGHT] = sp->grid;

answered Sep 5, 2013 at 1:40

Alok--

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nickie · Accepted Answer · 2013-09-05 01:37:03Z

2

What's wrong with the simplest possible?

sp->grid[1][2] = BLOCK_T;

If you have many array accesses in your function and you want to factor out sp->grid because you think this will be faster (fewer dereferences), I think that all optimizing C compilers take care of that pretty well.

answered Sep 5, 2013 at 1:37

nickie

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3 Comments

user1106281 Over a year ago

I'm porting a perl script to C and in the perl script I had used the g[1][2] syntax everywhere so that is what led me in this direction. Your solution works though...I should have thought of that :)

user1106281 Over a year ago

I'm still curious as to what the syntax should be for the way I was trying to do it originally

nickie Over a year ago

You already had it, just switched the dimensions... :-) It is block_t (*g)[TG_HEIGHT] = sp->grid; (a pointer, i.e. an array, of arrays of blocks, each of which having TG_HEIGHT elements). Somebody answered it but deleted his answer.

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