Question

I have a simple C# function:

public static double Floor(double value, double step) {     return Math.Floor(value / step) * step; } 

That calculates the higher number, lower than or equal to ‘value’, that is multiple of ‘step’. But it lacks precision, as seen in the following tests:

[TestMethod()] public void FloorTest() {     int decimals = 6;     double value = 5F;     double step = 2F;     double expected = 4F;     double actual = Class.Floor(value, step);     Assert.AreEqual(expected, actual);     value = -11.5F;     step = 1.1F;     expected = -12.1F;     actual = Class.Floor(value, step);     Assert.AreEqual(Math.Round(expected, decimals),Math.Round(actual, decimals));     Assert.AreEqual(expected, actual); } 

The first and second asserts are ok, but the third fails, because the result is only equal until the 6th decimal place. Why is that? Is there any way to correct this?

Update If I debug the test I see that the values are equal until the 8th decimal place instead of the 6th, maybe because Math.Round introduces some imprecision.

Note In my test code I wrote the ‘F’ suffix (explicit float constant) where I meant ‘D’ (double), so if I change that I can have more precision.

Answer 1

If you omit all the F postfixes (ie -12.1 instead of -12.1F) you will get equality to a few digits more. Your constants (and especially the expected values) are now floats because of the F. If you are doing that on purpose then please explain.

But for the rest i concur with the other answers on comparing double or float values for equality, it’s just not reliable.