In C#: double d = 0.4 - 1.2; What is d?

Ivor S Sargoytchev

Well? What is d equal to? Ivor S. Sargoytchev Dundas Software

Daniel Turini

It will return the same as any IEEE floating point software will: rounding errors. http://www.statsdirect.com/discussion/_disc1/0000000d.htm[^]

// Quantum sort algorithm implementation
while (!sorted)
;

leppie

Thru a microscope, everything looks big. Believe me that is -0.8. Try Console.Write(). leppie::AllocCPArticle(Generic DFA State Machine for .NET);

Tim Smith

Integer Math != IEEE Math Tim Smith I'm going to patent thought. I have yet to see any prior art.

Ivor S Sargoytchev

It is not since "if (d == -0.8)" will fail. Ivor S. Sargoytchev Dundas Software

Nemanja Trifunovic

Hehehe, I ran into this problem 15 years ago while writing a Fortran program.

Ivor S Sargoytchev

I guess I should use decimals. Ivor S. Sargoytchev Dundas Software

Daniel Turini

Ivor S. Sargoytchev wrote: I guess I should use decimals. Decimals are very, very slow. Depending on what you're doing, there are techniques to workaround most IEEE rounding problems.

// Quantum sort algorithm implementation
while (!sorted)
;

Daniel Turini

Yes, but you should compare it like:

if (Math.Abs(d - 0.8) < Double.Epsilon)

And everything looks fine. See Double.Epsilon docs on MSDN.

// Quantum sort algorithm implementation
while (!sorted)
;

Ivor S Sargoytchev

Daniel, This is not true. Double.Epsilon is a very very very small number: Double.Epsilon = 4.94065645841247E-324 But in C# I get 0.4 - 1.2 = -0.79999999999999993. As you can see the deviation from -0.8 is hundreds of times bugger than 4.94065645841247E-324. Ivor S. Sargoytchev Dundas Software

Chris Maunder

Ivor S. Sargoytchev wrote: if (d == -0.8) You're comparing floating point values for equality?? :omg: cheers, Chris Maunder

Ivor S Sargoytchev

Chris, With respect, some constructive criticism please. Ivor S. Sargoytchev Dundas Software

peterchen

ok, you asked for it... It is *well known* (;P) that floating point numbers have a limited resolution (around 7 digits for single, 15 for double*), and comparing the result of an arithmetic operation using "==" is prone to err. if you know the order of magnitude of one comparand, or at least you know that it's "far enough" from zero, use the abs(a-b) < 1e-6*(abs of known comparand) comparison. Otherwise it gets a little bit more tricky. *) don't quote me on the numbers, look up the IEEE definitions for the format

"Der Geist des Kriegers ist erwacht / Ich hab die Macht" StS
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Chris Maunder

The value represented in a floating point variable is only an approximation to a value, not an exact representation. Because of this you cannot assume that something like 2.0 == 4.0 * 0.5 will return true. When comparing floating point values you need to estimate how close is close enough then do a fuzzy comparision (something like ABS(A-B) < Epsilon) cheers, Chris Maunder

Lost User

Chris Maunder wrote: The value represented in a floating point variable is only an approximation to a value, not an exact representation. Because of this you cannot assume that something like 2.0 == 4.0 * 0.5 will return true. So this is how you count your beer consumption isn't? Especially when Kiwi and Aussie mates are involved. ;P Michael Martin Australia "I personally love it because I can get as down and dirty as I want on the backend, while also being able to dabble with fun scripting and presentation games on the front end." - Chris Maunder 15/07/2002

Brit

If it's anything like C++, then d = -0.79999999999999993 ------------------------------------------ The ousted but stubbornly non-dead leader reportedly released an audiotape this weekend, ending by calling on Iraqis to, quote, "resist the occupation in any way you can, from writing on walls, to boycotting, to demonstrating and taking up arms." adding, "you know, pretty much anything I used to kill you for."

Michael Dunn

Chris Maunder wrote: 2.0 == 4.0 * 0.5 Won't that actually be true because all those numbers are powers of 2? :cool: --Mike-- Ericahist | Homepage | RightClick-Encrypt | 1ClickPicGrabber Latest art~!@#2rDFA#@(#*%$Rfa39f3fqwf--= NO CARRIER

Michael Dunn

That reminds me of the old C= BASIC, there was some number (I'll just use 42 as an example) where you could type:

PRINT 42

and the response would be:

42.0001

:wtf: --Mike-- Ericahist | Homepage | RightClick-Encrypt | 1ClickPicGrabber Latest art~!@#2rDFA#@(#*%$Rfa39f3fqwf--= NO CARRIER

John M Drescher

Yes it should... John

Navin

Ivor S. Sargoytchev wrote: Dundas Software THe only way you're going to get an answer in this crowd, when posting in the Lounge, is if you offer to hook us up with those women from the Summer Sale ads. :-D "When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it's longer than any hour. That's relativity." - Albert Einstein