Question

(Edit: put possible solution at end)

I’m a C/C++ programmer who is learning Objective C to develop iPhone apps. The programs that I will be writing will deal with large 2d arrays of objects. I’ve read about using NSArray’s of NSArray’s and have some working code, but I’m trying to understand how to use C style arrays to save overhead and to learn what you can and can’t do.

In this fragment MapClass only contains two properties, int x and int y. I have the following code fragment working with a statically defined array of 10×10.

MapClass *arr[10][10];

arr[2][3] = [[MapClass alloc] init];
arr[2][3].x = 2;
arr[2][3].y = 3;

NSLog(@"The location is %i %i", arr[2][3].x, arr[2][3].y);
// Output: "The location is 2 3"

This is an example of doing it with a one dimensional array and calculating where the cell is based on the X and Y:

MapClass **arr = (MapClass**) malloc(10 * 10 * sizeof(MapClass *));

arr[3 * 10 + 2] = [[MapClass alloc] init];
arr[3*10 + 2].x = 2;
arr[3*10 + 2].y = 3;

NSLog(@"The location is %i %i", arr[3*10 + 2].x, arr[3*10 + 2].y);
// Output: "The location is 2 3"

My question is this: How can I malloc my array as a two dimensional array so that I can use arr[2][3] style notation to access it?

Everything I’m trying is generating various errors such as “Subscript requires the size of [your class], which is not constant in non-fragile ABI”.

Can anyone give me a snippit on how to do this? I’ve been reading and experimenting and can’t figure it out. Does my one dimensional array example do anything wrong?

Answer?

After fooling around with xzgyb’s answer, I have the following block working. Anything wrong with it? Thanks!

int dimX = 20;
int dimY = 35;

MapClass ***arr = (MapClass***) malloc( dimX * sizeof(MapClass **));
for (int x = 0; x < dimX; ++x)
{
    arr[x] = (MapClass **) malloc( dimY * sizeof(MapClass*));

}

for (int x = 0; x < dimX; ++x)
{
    for (int y = 0; y < dimY; ++y)
    {
        arr[x][y] = [[MapClass alloc] init];
        arr[x][y].x = x;
        arr[x][y].y = y;

    }
}

for (int x = 0; x < dimX; ++x)
{
    for (int y = 0; y < dimY; ++y)
    {
        NSLog(@"%i %i is %i %i", x, y, arr[x][y].x, arr[x][y].y);

    }
}

// Cleanup
for (int x = 0; x < dimX; ++x) {
    for (int y = 0; y < dimY; ++y) {
        [arr[x][y] release];
    }
}

for (int x = 0; x < dimX; ++x)
{
    free(arr[x]);
}

free(arr);

Answer 1

Try the followed code:

MapClass ***arr = (MapClass***) malloc(10 * 10 * sizeof(MapClass *));

for ( int row = 0; row < 10; ++row ) {
    arr[ row ] = (MapClass **)&arr[ row * 10 ];
}

arr[0][1] = [[MapClass alloc] init];
arr[1][2] = [[MapClass alloc] init];