Question

In the top Form1 level I have:

List<float> cyclicSelectedIndex = new List<float>(2);
int currentCyclicIndex;

In the constructor I have:

currentCyclicIndex = 0;

And this is the mouse down event where I have the selectedIndex when clicking on a point:

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left) {
        label1.Text = e.X.ToString();
        label2.Text = e.Y.ToString();
        label1.Visible = true;
        label2.Visible = true;
        label3.Visible = true;
        label4.Visible = true;
        // find the index that is closest to the current mouse location
        MinDist = float.MaxValue;

        for (idx = 0; idx < Point_X.Count; ++idx) {
            float dx = Point_X[idx] - e.X;
            float dy = Point_Y[idx] - e.Y;
            float dist = (float)Math.Sqrt(dx * dx + dy * dy);

            if (dist < MinDist) {
                MinDist = dist;
                selectedIndex = idx;
            }
        }

        if (MinDist < 5) {
            mouseMove = true;
            OriginalX = Point_X[(int)selectedIndex];
            OriginalY = Point_Y[(int)selectedIndex];
        }
    }
} 

I want in the mouse down event in cyclic way logic to add the selectedIndex to the List according the INT variable pointing.

Once the int is 0 and once its 1. And each time it will contain or point at another selectedIndex number which meaning I clicked on two different points.

So if I click on one point it will add the point selectedIndex to the cyclic List at index 0 using the INT variable.

And then if I click on any other point it will add the other point selectedIndex to to index 1 in the cyclic List using the INT variable.

The idea is to make a cyclic List so when I click a two points it will not delete the last index but will puse the last one so once I click a point it will add the selectedIndex to index 0 clicked on another point it will add the selectedIndex to index 0 and move the other index to 1 and so on. Cyclic.

I need to make something in the mouse down event to add some code and make that when I click on a point like now and the I have the selectedIndex so if I click after it on another point I will have a new variable with the other clicked point index for example: selectedIndex1

And then I want in the mouse down event to use the int variable to add to the List<> each time the last selectedIndex or selectedIndex1 to the place in the List using the int.

Answer 1

A list is empty after creation, even if you specify a size in the constructor. This space is only the initially reserved space for the later addition of items. You have to add items with Add.

List<int> list = new List<int>();
list.Add(10);
list.Add(7);

If you have a fixed size, use an array instead

int[] cyclicArray = new int[2];
int index = 0;

Increment the index like this, using a modulo operation

index = (index + 1) % 2;

The index will cycle like this: 0, 1, 0, 1, …

cyclicArray[index] = someValue;

---

UPDATE

You can use the PointF structure to store x/y-coordinates as float values.

List<PointF> cyclicPoints = new List<PointF>(2);
int currentCyclicIndex;

After having found a OriginalX/Y, add a element to the list if its size is smaller than 2, or change a existing element at a cyclic position otherwise.

PointF point = new PointF(OriginalX, OriginalY);
if (cyclicPoints.Count < 2) {
    cyclicPoints.Add(point);
} else {
    cyclicPoints[currentCyclicIndex] = point;
    currentCyclicIndex = (currentCyclicIndex + 1) % 2;
}

You can access a point coordinate in the list like this:

float x = cyclicPoints[0].X;
float y = cyclicPoints[0].Y;

---

UPDATE #2

If you want to store the index, define your list as list of int, not of float or PointF

List<int> cyclicSelectedIndex = new List<int>(2); 
if (cyclicSelectedIndex.Count < 2) {
    cyclicSelectedIndex.Add((int)selectedIndex);
} else {
    cyclicSelectedIndex[currentCyclicIndex] = (int)selectedIndex;
    currentCyclicIndex = (currentCyclicIndex + 1) % 2;
}