[Rd] long double, C, fortran

[Matt Shotwell]

In working with small numbers, I usually look for a solution involving a
log transformation. For instance, when a or b are very small, we can
compute the ratio

a / ( a + b )

more safely by

1 / ( 1 + exp( log(b) - log(a) ) )

Alternative data types are sure to cause issues with portability. It's
not worth the headache if you can find an analytical solution like that
above. Some supposedly standard data types (including long double) are
simply not standard in regard to their size in memory. This short
Wikipedia article ought to scare anyone who cares about portability:
<http://en.wikipedia.org/wiki/Long_double>. 

If you are doing some specialty computations involving very large or
small numbers (e.g. computing the Bell number), you might consider the
CRAN package gmp, which wraps the GNU multiple precision arithmetic
library. Or consider using a language like Ruby, where multiple
precision is automatic
<http://en.wikipedia.org/wiki/Arbitrary-precision_arithmetic>.

-Matt

On Thu, 2010-10-07 at 14:28 -0400, Richard D. Morey wrote:
> I'm using .Call() to call C code from R under Windows (on an Intel 
> Core 2 duo). The C code involves some very small numbers, and I think 
> I'm losing precision using doubles. I thought I might use long doubles 
> to see if I can get that precision back. I have a few questions:
> 
> 1. Does this affect the portability to other OSs or processors?
> 2. I'm returning the results in a matrix. Will a matrix of REALs be 
> sufficient for holding long doubles, or will it be cast back to doubles?
> 3. Will calls to FORTRAN BLAS (like dsymv, dpotrf, dpotri) still work 
> with long doubles?
> 
> Thanks for any help you can provide.
> 

-- 
Matthew S. Shotwell
Graduate Student 
Division of Biostatistics and Epidemiology
Medical University of South Carolina