Question

I have the following code which works fine…

int testarr[3][3] = {
  {1,1,1},
  {1,0,1},
  {1,1,1}
};   
[self testCall: testarr];

Which calls this function:

- (void)testCall: (int[3][3]) arr {

    NSLog(@"cell value is %u",arr[1][1]);
}

I need the array to be of variable length – What is the best way to declare the function?

Using blanks doesn’t work:

- (void)testCall: (int[][]) arr { 

Thanks for your help.

Answer 1

I would write this as:

- (void) testCall: (int *) aMatrice;

Doing so allows you to avoid multiple mallocs and the math to calculate a single offset in a linear array based on x, y coordinates in a 2D array is trivial. It also avoids the multiple mallocs implied by int** and the limitations of 2D array syntax perpetuated by the language.

So, if you wanted a 4×5 array, you might do:

#define WIDTH 4
#define HEIGHT 5
#define INDEXOF(x,y) ((y*WIDTH) + x)

int *myArray = malloc(sizeof(int) * 5 * ELEMS_PER_ROW);

You could then initialize the array linearly or with a nested for loop:

for(int x=0; x<width; x++)
    for(int y=0; y<height; y++)
        myArray[INDEXOF(x,y)] = ... some value ...;

And you would pass it to the method like:

[foo testCall: myArray];

Though you might want to also carry along the width and the height or, better yet, create a IntMatrix subclass of NSObject that wraps all of the pointer arithmetic and storage beyond a nice clean API.

(all code typed into SO)