Initialization of a three dimensional static const array

Question

I have a three dimensional static const array that is acting as a lookup table. Each cell of the array can return up to 8 separate numbers (each number is the corner of a cube - a voxel to be exact). So for example, MyStaticArray[0][1][1] might return 1,2,5,7. It can return a single number or a maximum of eight numbers, like so: 1,2,3,4,5,6,7,8.

I already have the lookup table prepared (on paper), my question is what the best way to store it? Initially I was going with a static const unsigned char[][][] but that is obviously not going to work in this case as the table can return more than one number (again, from 1-8).

My next solution was to return an unsigned char array. That is, static const unsigned char* [][][]. How do I initialize this (that is, how do I initialize the static const array to return to me unsigned char arrays that are also static so that I can access them without initializing a class)? Is there a better way?

I hope I was able to explain the question properly. If not, let me know and I will try to reword it and/or provide extra info.

Answer 1

You could pad each element up to the maximum length with dummy elements, and go with a 4D array:

static const unsigned char x[M][N][P][8] = { { 1, 2, 5, 7, -1, -1, -1, -1 },
                                             { 1, 2, 3, 4,  5,  6,  7,  8 },
                                             ...

or you could use individual bits as flags, e.g.:

static const unsigned char x[M][N][P] = { 0x53, // Bits 0, 1, 4, 6 are set
                                          0xFF,
                                          ...

If you find yourself needing more than 8 possibilities, upgrade to e.g. uint16_t or uint32_t.

Which method you go with depends on what you intend to do with the data.

Answer 2

As you only return 0-8 out of 8 possible numbers, I think you should stick with static const unsigned char[][][] and store the "numbers" as an one byte bitfield where bit n is set if n is one of the used numbers for this cell, so f.e. 1,2,5,7 might be encoded as 01010011 binary or 0x53 hexadecimal.

Answer 3

To use static initialization you'll need to know each dimension of your array. I presume you already know this but did not mention it in your question, so I'll use the dimensions [2][2][2] for my examples.

You'll also need a way to determine the number of elements at each position in the array. You can put the number of elements as the first value of the list, or create a sentinel value to mark the end of the list.

To pad each list to 8 elements, it would look like this. Any uninitialized values in the array will be set to 0.

static const unsigned char array[2][2][2][8] = {{{{1,2,3,4,5},{1,2,3,4,5,6,7,8}},{{4},{5}}},{{{...}}}};