c中的非对称多维数组

Question

I'm porting a piece of code from PHP to c/objective-c, and I'm running into trouble handling this two dimensional array.

$PATTERN_LOOKUP = array(
    array(),
    array(6, 18),
    array(6, 22, 38),
    array(6, 26, 46, 66),
    array(6, 28, 50, 72, 94)
);

This is the closest I've gotten (it compiles), however I'm thinking I'll run into trouble later, because this is actually a 5x5 array, and there will be a bunch of undefined numbers. Is there a way to make an asymmetrical multidimensional array in c?

static int PATTERN_LOOKUP[][5] = {
    {0},
    {6, 18},
    {6, 22, 38},
    {6, 26, 46, 66},
    {6, 28, 50, 72, 94}
};

Answer 1

Well, you can always make a "symmetrical" array in C (ie a rectangular array) and only use an "asymmetrical" portion of it. This is a perfectly good solution for small arrays. And this is actually exactly what your declaration of PATTERN_LOOKUP does (the elements not explicitly initialized will be implicitly initialized with zeros).

However, for larger arrays it might get too expensive in therms of wasted memory. If it does get too expensive, you can always use a "simulated" 2D array in C (an array of pointers to subarrays)

static int array0[] = { 0 };
static int array1[] = { 6, 18 };
static int array2[] = { 6, 22, 38 };
static int array3[] = { 6, 26, 46, 66 };
static int array4[] = { 6, 28, 50, 72, 94 };

static int *const PATTERN_LOOKUP[] = {
  array0,
  array1,
  array2,
  array3,
  array4
};

You can even use a NULL entry in PATTERN_LOOKUP array to represent an empty array

static int *const PATTERN_LOOKUP[] = {
  NULL,
  array1,
  array2,
  array3,
  array4
};

In C99 you can use compound literals to achieve the same result without separate subarray declarations

static int *const PATTERN_LOOKUP[] = {
  (int []) { 0 },
  (int []) { 6, 18 },
  (int []) { 6, 22, 38 },
  (int []) { 6, 26, 46, 66 },
  (int []) { 6, 28, 50, 72, 94 }
};

Of course, it remains your responsibility to store/remember the actual size of each individual subarray.