In multidimensional arrays, can you use variable arrays or their names instead of the actual data?

Question

Referring to this thread How to declare and use "Variabled arrays" inside arrays

Example

byte array[2][4] = { {1,2,3,4}, {5,6,7,8} };
Serial.println(array[0][2]); is equal to '3'.

Instead of using {1, 2, 3, 4} and {5, 6, 7, 8} I want to be able to declare separate arrays for each of them and just put the name.

Another big issue is putting arrays of different sizes for each entry and it would be wrong to do so. Is this really not possible? Do I have to use some other concept instead if I want to have an array that contains entries of arrays that have different sizes? I heard about pointers but not familiar with that.

Example,

byte arrayX[3] = {1, 2, 3}
byte arrayY[4] = {5, 6, 7, 8}

byte array[2][4] = { arrayX , arrayY };

Take note that the code was not taken from the link I mentioned, I just used the same array names. Also I changed arrayX from the link to 3 entries because I want to emphasize the part of my question if it is possible if arrays have different sizes and you would just use the biggest size?

Now I think this is wrong and I haven't tested it currently but I want to ask if there is a solution that makes use of what I have mentioned? The site I linked to gave a solution which feels very redundant in terms of declaration and I would not want that. This is the solution mentioned there.

byte arrayZ[2][4] = {
  {arrayX[0],arrayX[1],arrayX[2],arrayX[3]},
  {arrayY[0],arrayY[1],arrayY[2],arrayY[3]}
};

The issue I had when searching the internet is the same as the link. "Surfed on the internet to find a bunch of examples but the only difference is that they don't use arrays that have been declared to have a name inside array."

Answer 1

Two-dimensional arrays come in two "flavours". Actually, multi-dimensional arrays are possible in any combination of these.

• All elements in a rectangular grid, at least from the abstract view of the programming language;
• A one-dimensional array of one-dimensional arrays.

If you declare:

byte array[n][m];

then you have reserved space for "n times m" bytes. From the programmer's view it is a rectangular grid, actually all bytes are in sequence in a one-dimensional array. This is because the memory of our computers are commonly organized as a big one-dimensional array of bytes. The compiler generates the correct calculation for the offset into this array, multiplying one index with the size of the width (or length, depending on your mental model) and adding the other index.

You can initialize such an array with the nested set of curly braces like this:

byte array[n][m] = {
    { /* ... */ },
    /* ... */
};

But you cannot initialize it as you show.

Otherwise, if you declare

byte* array[n];

then you have reseved space for "n" pointers to bytes, that is a one-dimensional array of pointers. Each of these pointers can point to a different or the same one-dimensional array of bytes, or to no array at all if it's nullptr.

Now you can set up the array as you like:

byte arrayX[m1] = { /* ... */ };
byte arrayY[m2] = { /* ... */ };

byte* array[n] = { arrayX, arrayY, /* ... */ };

The arrays arrayX and arrayY (and any further) don't need to have the same length. Each can have its own length. But beware, you need to trace the length somehow to avoid out-of-bounds accesses. And of course, you need to check or be sure that a pointer is not null before dereferencing it.

In both cases you can access a single element with:

byte element = array[i][j];
array[k][l] = 23;

This is because the syntax with the brackets is equivalent to the dereferencing operator:

x[y] == *(x + y)

Note 1: Since Arduinos are programmed in C++, there is another option. You can use references instead of pointers. Because it is really hard to create a null reference, some see it as a safety measure against null pointers.

Note 2: Again, since Arduinos are programmed in C++, you can implement an own class that provides the operator[]. With this option you can do any magical stuff, but it might break the user's expectation. Be careful if you go that route.