The code at the end demonstrates your problem. When casting from a float
number up to a double, you are getting some slightly skewed results. If you
divide 1/.1 in float or double you always get 10, however, casting a float
10 to a double results in some small amount of inaccuracy 9.99999985098839.
Generally you can work this off by using an error quotient or some small
decimal that gives a degree of freedom. This could be demonstrated by doing
(float fixedError = .0001f; Math.Floor((min/increment)+fixedError);) You
could also make use of the Round function instead of the floor method, or
you could roll your own floor method for floats.
using System;
public class Locality {
private static void Main(string[] args) {
float min=1.0f, increment=0.1f;
double mind=1.0D, incrementd=0.1D;
Console.WriteLine(min/increment);
Console.WriteLine(mind/incrementd);
Console.WriteLine((double) (min/increment));
Console.WriteLine(Math.Floor(min/increment));
Console.WriteLine(Math.Floor(mind/incrementd));
double result = Math.Floor(min/increment)*increment;
Console.WriteLine(result);
}
}
--
Justin Rogers
DigiTec Web Consultants, LLC.
"James Thurley" <ne********@jamesthurley.NO__SPAM.com> wrote in message
news:bi**********@titan.btinternet.com...
Michael Mayer wrote:
I guess I'm curious why you can't cast back to floats? Do you need the
precision or not? I have a feeling you're running into this problem:
http://www.pobox.com/~skeet/csharp/floatingpoint.html
(thanks to Jon Skeet for the article)
I don't need double precision, but the inaccuracy caused by casting can
make, for example, the Floor function give the incorrect answer:
float min=1.0f, increment=0.1f;
double result = Math.Floor(min/increment)*increment;
I'm leaving the result as a double in this example - its casting from
float to double that's causing the problem here. In theory,
min/increment will be 10.0 which is also the Floor. Multiply it by
increment and you get 1.0 again. But result comes out as
0.90000001341104507. (Try it - I swear it's true!)
Now I get that number if I put "Math.Floor(1.0f/(double)0.1f)*0.1f" into
the watch window. Even wierder, If I copy and paste
"Math.Floor(min/increment)*increment" into the watch window it gives me
the correct answer of 1.0. So I guess the compiler must be making the
conversion to double earlier than I would have expected, but either way
it's causing a problem.
I'm starting to think I should just use doubles... processors of the
future will be 64bit anyway I guess.
