Iterate through 1D array and 2D array with step - c++

Question

Iterating through 1D array (pseudo 2D) with step of 3:
arr = new int[height * width * 3];
for (int i = 0; i < height * width * 3; i+=3) {
arr[i] = 1;
}
I have tried this, but what I got is column of one third:
for (int y = 0; y < height * 3; y++) {
for (int x = 0; x < width; x+=3) {
arr[x + width * y] = 1;
}
}

Answer 1

Assuming your cells have a 'size' of 3 entries, you should use the * 3 on the inner loop. Otherwise you miss 2 thirds of your cells on each row.
You also need to multiply width by 3 to get the correct row.
for (int y = 0; y < height; y++) {
for (int x = 0; x < width * 3; x+=3) {
arr[x + width * 3 * y] = 1;
}
}
In general you need the following structure for such situations:
for (int y = 0; y < height; y++) {
for (int x = 0; x < width * cellWidth; x+= cellWidth) {
arr[x + width * cellWidth * y] = 1;
}
}
(Were cellWidth is 3 in your case)
To slightly simplify this, you could assume in the loops that your cells have a width of 1 (like a normal situation) and multiply by cellWidth when actually assigning the values:
for (int y = 0; y < height; y++) {
for (int x = 0; x < width; x++) {
int index = (x + width * y) * cellWidth;
arr[index + 0] = 1; // First 'cell entry'
arr[index + 1] = 1; // Second
...
arr[index + cellWidth - 1] = 1; // Last
}
}
Another solution is to create larger 'items' using a struct for example:
typedef struct { int r, int g, int b } t_rgb;
t_rgb* arr = new t_rgb[height * width];
for (int y = 0; y < height; y++) {
for (int x = 0; x < width; x++) {
arr[x + width * y].r = 1;
}
}
and you are able to use it as a regular array (the compiler does all calculations for you). This also makes it more clear what is happening in your code.

Answer 2

What are you trying to accomplish exactly? Setting a channel in a RGB image?
I usually do it like this:
for (int y = 0; y < height; y++)
for (int x = 0; x < width; x++)
arr[(x + width * y) * 3] = 1;
In general, to set RGB values, you can simply add an offset like this:
for (int y = 0; y < height; y++)
for (int x = 0; x < width; x++)
{
size_t base = (x + width * y) * 3;
arr[base + 0] = r;
arr[base + 1] = g;
arr[base + 2] = b;
}