[Rd] code for sum function

[Paul Gilbert]

(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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