Re: [R] Missing values in argument of .Fortran.

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Tue 07 Jun 2005 - 01:26:41 EST

On 6/6/2005 9:52 AM, Rolf Turner wrote:
> I wish to pass a vector ``y'', some of whose entries are NAs to a
> fortran subroutine which I am dynamically loading and calling by
> means of .Fortran(). The subroutine runs through the vector entry by
> entry; obviously I want to have it do one thing if y[i] is present
> and a different thing if it is missing.
>
> The way I am thinking of proceeding is along the xlines of:
>
> ymiss <- is.na(y)
> rslt <- .Fortran(
> "foo",
> NAOK=TRUE,
> as.double(y),
> as.logical(ymiss),
> etc,
> etc
> )
>
> and inside ``foo'' have a logical branch based on the value of
> xmiss(i).
>
> Questions:
>
> (1) Is there a sexier way to proceed? E.g. is it possible
> within (g77) fortran to detect the fact that y(i) is/was an
> NA (or not) and make the nature of y(i) the basis of an
> if-statement?

In C you can use the macros

ISNA(x) True for Rís NA only
ISNAN(x) True for Rís NA and IEEE NaN
R_FINITE(x) False for Inf, -Inf, NA, NaN

where the R function is.na() is closest to ISNAN(), I think. There's no supplied way to do these things in Fortran, but presumably you could call a C function which did one of these tests.

> (2) Are there any lurking pitfalls in the use of the NAOK=TRUE
> argument?

I think the way you did it looks perfectly safe. Following my advice above will be a little trickier, because some other user of your code might use a different Fortran compiler, and it might handle C functions differently.

> (3) Is there an entirely different and better way to proceed?

I'd do it your way if I was using Fortran.

Duncan Murdoch



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