Question

Having the following code, how can I have “get” receive a map, and return the value at the specified position?

I am attempting to write a cellular automaton in C to try to wrap my head around pointers and memory allocation. Everything was fine until I decided to make “get” to obtain data instead of a direct map[x+world.w*y] as I used to.
I require this because in the future, I plan to have two maps of the same size, and use the same function to get data from them (so it’d be “get(&map2, x, y)” instead of “get(&map, x, y)”.

I do this because I was advised against using globals, so I will keep the two maps in main and send their addresses to functions to process.

This is C language, so no C++ solutions are valid.

I tried to search for this in google but all documentation is extremely technical and convoluted, and I am not sure of how this procedure is actually named… So, can anyone help me with this? How can I pass a malloc’ed array to a function and retrieve or alter data from it?

typedef struct Map {
int HP;
int type;
unsigned int flags;
} Map;

typedef struct World {
int w;
int h;
} World;

struct World world;

int tile (int x, int y) { return x + world.w * y; }

int get (/*unknown*/map , int x, int y){
int val = x + world.w * y;
return /*unknown ->?*/ type;
}


int main (){
Map* map;
world.w = 8;
world.h = 8;
int tiles = world.w * world.h;
map = (Map*)malloc(sizeof(Map) * tiles);
int i;
for(i = 0; i < tiles; i++){
    map[i].type = rand()%2;
}
int x,y;

while(1){
    put(0,0);
    for(y = 0; y < world.h; y++){
        printf("\n");
        for(x = 0; x < world.w; x++){
            printf("%i ", get(&map, x, y));
        }
    }
};
printf("\n");
return 0;

}

Answer 1

Instead of:

get(&map, x, y)

in which you pass the address of the address of the map pointer that malloc() returned, just pass the address itself:

get(map, x, y)

AFAICT from your code, malloc( ) returns exactly the thing that get( ) is looking for, i.e., a pointer to someplace in memory that has room for 64 tiles. So get( ) could look something like:

int
get( Map *map, int x, inty  ) {
  int val = x + map->w * y; // map is a pointer to struct, not the struct itself
  return val;               // get( ) returns an int, and it's in val
}

That might be closer to what you want.

— pete

There are a few other errors in your code, too, but this might let the compiler get off the ground.