How to declare and use "Variabled arrays" inside arrays

Question

Good day folks. I'm new here and have been starting to be enthusiast in Arduino. I have a problem and I don't know what I should do, or whether I did something wrong regarding an array of arrays. 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.

Every array inside array I see that works is like this:

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

But mine is like this:

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

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

Serial.println(arrayX[2]); is equals to '3'.
Serial.println(arrayZ[0][2]); is equal to '0'.

Why?

Below is what in reality I am experiencing. I hope someone can try this out too.

softAddr = cycle * 2;

is an expression I used for looping where cycle is the iterated variable

for (byte cycle = 0; cycle < 7; cycle++) {}

I did a simple simulation and I guess the way I initialize the parent array, it initializes a new array in the name same from the declared variables but separate.

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

void setup() {
  Serial.begin(9600);
  byte glass[2][4] = {arrayX[4], arrayY[4]};
  Serial.print(F("OLD 1: ")); Serial.println(arrayX[0]);
  Serial.print(F("OLD 2: ")); Serial.println(arrayX[1]);
  Serial.print(F("NEW 1: ")); Serial.println(`glass[0][0]`);
  Serial.print(F("NEW 2: ")); Serial.println(`glass[0][1]`);
}

output:

OLD 1: 1

OLD 2: 2

NEW 1: 0

NEW 2: 0

//answered realized

I forgot that the array I put inside the array already had its data based on its index and the [4] resulted to nothing or 0 since there is no value at index [4] (knowing, indexes where 0, 1, 2, 3 on a 4 sized array). I realized this after I made a simple loop declaration not instantly like this:

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

But like this:

byte arrayX[4] = {10,11,12,13};
byte arrayY[4] = {20,21,22,23};
byte arrayZ[2][4];
void setup() {
  Serial.begin(9600);
  for (byte parent = 0; parent < 2; parent++) {
    for (byte child = 0; child < 4; child++) {
      if (parent == 0) {
        arrayZ[parent][child] = arrayX[child];
      } else {
        arrayZ[parent][child] = arrayY[child];
      }
    }
  }
  for (byte parent = 0; parent < 2; parent++) {
    for (byte child = 0; child < 4; child++) {
      Serial.print(F("value: ")); Serial.println(arrayZ[parent][child]);
    }
  }
}

Output:

value: 10

value: 11

value: 12

value: 13

value: 20

value: 21

value: 22

value: 23

Resulting in simply like this:

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

Answer 1

Something is wrong with your type description:

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

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

Serial.println(arrayX[2]); is equals to '3'.

This is normal ... arrayX[0] = 1, arrayX[1] = 2, arrayX[2] = 3

However, the next one is not good:

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

You are declaring a byte array, but you add arrayX[4] and arrayY[4] which, where arrayX[4] is the 5th elements of arrayX but it has only 4 elements, so it points to memory outside the array (probably into the next array.

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

byte* arrayZ[2] = { arrayX, arrayY };

Serial.print(arrayZ[0][2]);

This should print 3 (not tested).