R help - Apr 2008 - behaviour of .Fortran in terms of "double precision" and "real" datatype

Hello R-world!

This is my first post to the R list, so I feel that I need to say thanks to
the community (I'm using R now for 5 months)!

I'd greatly appreciate if somebody could explain some odd behavior of
".Fortran" to me. Let's assume a primitive Fortran subroutine

barfoo.f
----------------------------
      subroutine bar(y, v)
      double precision y
      real v

      y = 1.455523
      v = 3.2

      end
----------------------------

after $ R CMD SHLIB barfoo.f (uses gfortran & gcc), I switch to R-2.6.2
R> dynload("barfoo.so")
R> .Fortran("bar", y=as.double(3), v=as.real(2.1))
$y
[1] 1.455523

$v
[1] 2.099999
--------------
the double value of y changes according to the programme, but the v value is
not being updated (besides an error is made).

The reason why I"m interested is because I want to run an elaborate Fortran
programme within a MCMC in R. All arguments for the programme are
'real'.
Could somebody explain to me what's going on? Also, feel free to try that on
your machine.

thanks very much in advance,
Sebastian

	[[alternative HTML version deleted]]

On 29/04/2008 6:52 PM, Sebastian Eiser wrote:
> Hello R-world!
> 
> This is my first post to the R list, so I feel that I need to say thanks to
> the community (I'm using R now for 5 months)!
> 
> I'd greatly appreciate if somebody could explain some odd behavior of
> ".Fortran" to me. Let's assume a primitive Fortran subroutine
> 
> barfoo.f
> ----------------------------
>       subroutine bar(y, v)
>       double precision y
>       real v
> 
>       y = 1.455523
>       v = 3.2
> 
>       end
> ----------------------------
> 
> after $ R CMD SHLIB barfoo.f (uses gfortran & gcc), I switch to R-2.6.2
> R> dynload("barfoo.so")
> R> .Fortran("bar", y=as.double(3), v=as.real(2.1))
> $y
> [1] 1.455523
> 
> $v
> [1] 2.099999
> --------------
> the double value of y changes according to the programme, but the v value
is
> not being updated (besides an error is made).
as.real() is the same as as.double().  You want as.single(), or to 
convert the Fortran code to use double.  R doesn't use single precision 
internally; it will convert everything back and forth from double just 
for the call.

Duncan Murdoch
> 
> The reason why I"m interested is because I want to run an elaborate
Fortran
> programme within a MCMC in R. All arguments for the programme are
'real'.
> Could somebody explain to me what's going on? Also, feel free to try
that on
> your machine.
> 
> thanks very much in advance,
> Sebastian
> 
> 	[[alternative HTML version deleted]]
> 
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