Re: [R] prod and F90 product

[Duncan Murdoch]

On 13-11-07 6:39 PM, Filippo 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?

Some intermediate results may be stored with 80 bit precision in R, 64 
bit precision in Fortran.

Duncan Murdoch

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