How to increment two different values with for loops in C++

Question

At the moment I have this:

Shape* shapeArray[12];
for (int i = 0; i < vertices.size(); i += 3) {
    for (int j = 0; j < sizeof(shapeArray) / sizeof(shapeArray[0]); j++) {
        shapeArray[j] = new Triangle(vertices[i], vertices[i + 1], vertices[i + 2], vec3(0.8, 0.0, 1.0));
    }
}

I'm trying to create a cube out of triangles. I want to increment the j value by 1 each time and increment the i value by 3 each time resulting in something like this (if it were hard coded):

Shape* shapeArray[12];
shapeArray[0] = new Triangle(vertices[0], vertices[1], vertices[2], vec3(0.8, 0.0, 1.0));       
shapeArray[1] = new Triangle(vertices[3], vertices[4], vertices[5], vec3(0.8, 0.0, 1.0));       
shapeArray[2] = new Triangle(vertices[6], vertices[7], vertices[8], vec3(0.8, 0.0, 1.0));
shapeArray[3] = new Triangle(vertices[9], vertices[10], vertices[11], vec3(0.8, 0.0, 1.0));
shapeArray[4] = new Triangle(vertices[12], vertices[13], vertices[14], vec3(0.8, 0.0, 1.0));
shapeArray[5] = new Triangle(vertices[15], vertices[16], vertices[17], vec3(0.8, 0.0, 1.0));
shapeArray[6] = new Triangle(vertices[18], vertices[19], vertices[20], vec3(0.8, 0.0, 1.0));
shapeArray[7] = new Triangle(vertices[21], vertices[22], vertices[23], vec3(0.8, 0.0, 1.0));
shapeArray[8] = new Triangle(vertices[24], vertices[25], vertices[26], vec3(0.8, 0.0, 1.0));
shapeArray[9] = new Triangle(vertices[27], vertices[28], vertices[29], vec3(0.8, 0.0, 1.0));
shapeArray[10] = new Triangle(vertices[30], vertices[31], vertices[32], vec3(0.8, 0.0, 1.0));
shapeArray[11] = new Triangle(vertices[33], vertices[34], vertices[35], vec3(0.8, 0.0, 1.0));

However, it isn't giving me the desired result. I am getting 3 lots of the j value hence not drawing the cube properly.

How could I solve this issue with for loops? Or would I need to change from using an array to store the triangles. Thank you.

Answer 1

for (int i= 0, j= 0; i < vertices.size(); i+= 3, ++j) {
...
}

Something like this?

Answer 2

As pointed out in the comment , your inner loop is not necessary.

you need to iterate over the array and compute the vertice index acordingly when you create a new Triangle.

Shape* shapeArray[12];
for (int i = 0; i < vertices.size(); i += 3) {
        shapeArray[i] = new Triangle(vertices[i * 3], vertices[i * 3 + 1], vertices[i * 3 + 2], vec3(0.8, 0.0, 1.0));
}

Would be a simplier and working implementation.

Answer 3

You can change your for loop so that shapeArray indexes correspond accurately to vertices indexes. I deduced this on basis of your sample output.

Shape* shapeArray[12];
for (int i = 0; i < vertices.size(); i += 3)
{

    shapeArray[(int)i/3] = new Triangle(vertices[i * 3], vertices[i * 3 + 1], vertices[i * 3 + 2], vec3(0.8, 0.0, 1.0));

}

If you increment i by 3 then you will need to divide by 3 for shapeArray index. You do not need an inner loop in this scenario.