C＃数学不准确？

Question

I have two variables: counter and frequency . I am using these two variables as such:

counter = counter >= frequency ? 0 : counter + 1;

As you can see, the counter counts up, until the point it matches the frequency, and then it resets to 0. At the moment, frequency is set to 100 . So the counter should reach 99, and then reset to 0.

I am using this counter to rotate an image at a rate of 100 times a second:

private int counter = 0;
private int frequency = 100;
private DispatcherTimer dispatch = new DispatcherTimer();

public Timer()
{
    this.dispatch.Interval = TimeSpan.FromMilliseconds(1000 / frequency);
    this.dispatch.Tick += new EventHandler(updateTimer);
    this.dispatch.Start();
}

private void updateTimer(object sender, EventArgs e)
{
    counter = counter >= frequency ? 0 : counter + 1;
    saveImage.RenderTransform = new RotateTransform(counter * 3.6);
}

The script will rotate the image around a 360 degree plane. Since frequency is set to 100, it will go the full 360 degrees, since 100 * 3.6 is 360. This works great if the frequency is set to 100. However, if I change the frequency, it may not go the full 360 degrees; or it may even go more than 360 degrees.

A simple fix to this would be to change the rotation math to the following:

    saveImage.RenderTransform = new RotateTransform(counter * (360 / frequency));

However, doing this in actuality produces an odd result. The image never reaches the full 360 degree turn; and instead only reaches 300 degrees, before it eventually resets to 0. What is wrong with my math here?

Answer 1

You may have rounding issues. Be aware that if you want floating point results you need to do floating point math. In your code, you're mixing int s and double s, which may result in (unexpected) integer divisions. For example 360 / frequency will not result in a floating point number, as both frequency and 360 are int s (for example 360 / 100 == 3!!).

Try changing your code to use floating point operations consistently as follows:

private double counter = 0.0;
private double frequency = 100.0;
private DispatcherTimer dispatch = new DispatcherTimer();

public Timer()
{
    // You may want integer division in this line to get full milliseconds
    this.dispatch.Interval = TimeSpan.FromMilliseconds((int)(1000.0 / frequency));
    this.dispatch.Tick += new EventHandler(updateTimer);
    this.dispatch.Start();
}

private void updateTimer(object sender, EventArgs e)
{
    counter = counter >= frequency ? 0.0 : counter + 1.0;
    saveImage.RenderTransform = new RotateTransform(counter * 3.6);
}

And the last example you gave should read:

saveImage.RenderTransform = new RotateTransform(counter * (360.0 / frequency));

with counter and frequency being double s.

---

EDIT: It would suffice to change the last line to

saveImage.RenderTransform = new RotateTransform(counter * (360.0 / frequency));

and leave the variables as int . Your last line in updateTimer doesn't work as 3.6 is hardcoded, but depending on frequency counter * 3.6 may not reach/be greater than 360°.

Example:

frequency = 50
counter = 49
counter * 3.6 = 176.4

frequency = 150
counter = 149
counter * 3.6 = 536.4

So you always need to calculate the degrees to rotate in every step according to frequency and not hardcode them as if frequency was 100.