Bug 20017 - Rounding doubles differs between calculated and hard coded values
Summary: Rounding doubles differs between calculated and hard coded values
Alias: None
Product: iOS
Classification: Xamarin
Component: XI runtime ()
Version: 7.2.3
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
Depends on:
Reported: 2014-05-23 14:59 UTC by Josh Hogan
Modified: 2014-05-25 10:52 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

RoundingTest project (4.14 KB, application/ms-download)
2014-05-23 14:59 UTC, Josh Hogan

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Description Josh Hogan 2014-05-23 14:59:22 UTC
Created attachment 6870 [details]
RoundingTest project

I've encountered a strange problem with rounding using Math.Round's double signatures.

Hard coded doubles seem to round correctly, while "calculated" doubles do not.

I had a little trouble illustrating the problem clearly, but I've attached a test project.

I'd be happy to provide additional information if necessary.
Comment 1 Sebastien Pouliot 2014-05-23 16:15:18 UTC
The joy of IEEE floating point values…

Your calculated result is not what you think it is. You can use the"R" (roundtrip) to see the exact value (that the CPU computed), e.g.

	double discounted = price - (price * 0.10);
	Console.WriteLine("10% off   = {0}", discounted.ToString ("R"));

and you'll get:

	10% off   = 13.454999999999998

and that (real value) will round down to 13.45.

To avoid this you might want to use the Decimal type for any financial calculations.
Comment 2 Josh Hogan 2014-05-23 17:06:03 UTC

So obvious in hindsight.  Decimal it is!

This probably isn't the right forum for discussion, but why doesn't the debugger show me 13.454999999999998?

Comment 3 Sebastien Pouliot 2014-05-25 10:52:58 UTC
By default debuggers (both XS and VS) use `ToString` to show the content of the value.

.NET set this format value for float/double to a fixed point format (not the the "R" roundtrip format) likely because it's easier to read that way. It works wel (most of the time) but you can get weird results some time.