[R] prod and F90 product

[Jim Holtman]

sounds like FAQ 7.31

Sent from my iPad

On Nov 7, 2013, at 18:39, Filippo <ingfimo at gmail.com> wrote:

> Hi,
> I'm having strange differences between the R function prod ad the F90 function product.
> Processing the same vector C (see attachment). I get 2 different results:
> prod(C) = 1.069678e-307
> testProduct(C) = 0
> 
> where testProd is the following wrapping function:
> 
> testProd <- function(x) {
>    return(.Fortran('testProd', as.double(x), as.double(0), as.double(0), as.integer(length(x))))
> }
> 
> subroutine testProd(x, p, q,  n)
>    implicit none
>    integer, intent (in) :: n
>    double precision, intent (in) :: x(n)
>    double precision, intent (out) :: p
>    double precision, intent (out) :: q
>    integer :: i
> 
>    p = product(x)
>    q=1
>    do i = 1, n
>        q = q*x(i)
>    end do
> end subroutine testProd
> 
> I check the lowest possible number and seems to be the same for both R and F90.
> Can anyone help me understanding this behaviour?
> Thank you in advance
> Regards,
> Filippo
> 
> <C>
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