Floating point accuracy in VBA 1

[phudgens]

I am getting slightly different results from code used by C++ to do Eigen vector calculations compared to what the same code in VBA gives me using the same input data. I am wondering if my conversion of the hypot and atan2 methods (available in C++, Matlab, Perl, etc) are not capturing the same floating point accuracy as is standard in C++. For hypot I am using: Sqr((A1 * A1) + (A2 * A2)). And for atan2 I am using: Application.WorksheetFunction.Atan2(HypotA1A2, A3)). Does anyone have experience with using advanced methods like these in Excel VBA, in particular as they relate to floating point accuracy?

 

[SkipVought]

Hi,

Lots of hypothetical info, but nothing specific.

Are you refereing to accuracy or precision?

Can you give some specific examples?

Please answer BOTH questions clearly, concisely and completely.

Skip,
[sub]
Just traded in my old subtlety...
for a NUANCE![/sub]

 

[SkipVought]

You might also find a discussion like this interesting...

http://www.cprogramming.com/tutorial/floating_point/understanding_floating_point_representation.html

Skip,
[sub]
Just traded in my old subtlety...
for a NUANCE![/sub]

 

[DougAJ4]

How much difference are you getting? There shouldn't be any difference up to the 14th significant figure.

Was "Application.WorksheetFunction.Atan2(HypotA1A2, A3) a typo? It should be Atan2(adjacent, opposite)

If you are using multiple copies of this function you will find a significant performance hit from using the Worksheetfunction call.

You can download a worksheet file with VBA trig functions (including ATan2) from:

http://newtonexcelbach.wordpress.com/2008/02/23/trignometric-functions-in-vba/

Or VBA functions calling a Fortran Dll from:

http://newtonexcelbach.wordpress.com/2008/12/11/trigonometric-functions-dll/

Doug Jenkins

http://newtonexcelbach.wordpress.com/

 

[phudgens]

DougAJ4,

Your mention of the ATan2 function prompted me to do a web search and I found some good information on Wikipedia. It turns out that some computer programs reverse the order of the parameters. Reversing the paramenters in my VBA code solved the problem - we are now getting exact answers.

Thanks very much,
Paul Hudgens