(I didn't see anyone else answer this, so ...)
You can probably find the R code in src/main/ but I'm not sure. You are
talking about a very simple calculation, so it seems unlike that the
algorithm is the cause of the difference. I have done much more
complicated things and usually get machine precision comparisons. There
are four possibilities I can think of that could cause (small) differences.
0/ Your code is wrong, but that seems unlikely on such a simple
calculations.
1/ You are summing a very large number of numbers, in which case the sum
can become very large compared to numbers being added, then things can
get a bit funny.
2/ You are using single precision in fortran rather than double. Double
is needed for all floating point numbers you use!
3/ You have not zeroed the double precision numbers in fortran. (Some
compilers do not do this automatically and you have to specify it.) Then
if you accidentally put singles, like a constant 0.0 rather than a
constant 0.0D+0, into a double you will have small junk in the lower
precision part.
(I am assuming you are talking about a sum of reals, not integer or
complex.)
HTH,
Paul Gilbert
On 2/14/19 2:08 PM, Rampal Etienne wrote:
Hello,
I am trying to write FORTRAN code to do the same as some R code I have.
I get (small) differences when using the sum function in R. I know there
are numerical routines to improve precision, but I have not been able to
figure out what algorithm R is using. Does anyone know this? Or where
can I find the code for the sum function?
Regards,
Rampal Etienne
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