Thread: Calloc for an array of array with negative index in C

I have an array of array with negative index. It is an array which has real dimensions [dim_y + 40][dim_x + 40] but the user uses the array like it has dimensions [dim_y][dim_x].
So i see the array's rows lets say from -20 to dim_y + 20 but a user sees only from 0 to dim_y.
First i had global and already defined the dimensions dim_x, dim_y, so i had this:

Code:

int map_boundaries[dim_y + 40][dim_x + 40];
int (*map)[dim_x+40] = (int(*)[dim_x+40])&map_boundaries[20][20];

In fact, 'map' points to 'map_boundaries' , map[0][0] is map_boundaries[20][20].
I did what is posted in the second post here:
Negative array indexing - Everything2.com

I want 'map' to be global. Until now i had defined the dim_y and dim_x so that worked fine.
Now i just need to read from a user the dim_x and dim_y.
Until now i have global

Code:

int **map_boundaries;

and then in main i use calloc:

Code:

map_boundaries = (int **)calloc(dim_y + 40,sizeof(int*));
for(i = 0; i < dim_y + 40; i++){
    map_boundaries[i] = (int *)calloc(dim_x + 40,sizeof(int));
}

but i dont know how to declare this line now:

Code:

 int (*map)[dim_x+40] = (int(*)[dim_x+40])&map_boundaries[20][20];

If I understand
1. You have an (x, y) pair representing your map coordinates.
2. The "user" want to treat x and y as values that can be negative, i.e. x in [-20, 20] and y in [-20, 20]
3. C doesn't let you use negative array indices, so the program must map [-20, 20] to [0, 40].

Then you need to do some simple mapping -- this usually involves just some basic arithmetic -- that takes a map coordinate and turns it into the corresponding index array. Use the "index value" to access the array.

Perhaps make a list of each coordinate and it's corresponding array index. For example, for x

Code:

coord: -20 -19 -18 -17 -16 ...  -1   0   1 ...  18  19  20
index:   0   1   2   3   4 ...  19  20  21 ...  38  39  40

Now, what operation can you do to -20 that produces 0, that you can also to to -19 to get 1, ... and to 20 to get 40? Add, subtract, multiply or divide? By how much?

When you figure out the actual mapping process, I highly recommend you make a small function (or two functions if you want to map x and y separately) to do that for you. Call that everywhere. That way, if you want to change your map coordinates or dimensions, you only need to change one or two small functions instead of a dozen or more different places.