On Thu, 18 Jun 2009, Christophe Genolini wrote:
Hi the list,
I am writing a R function that call a C function. The C function needs
integers but I do not manage to give a NA integer as argument :
--- C code ---
void essai(int *t){
Rprintf("\nT0=%i T1=%i T2=%i T3=%i",t[0],t[1],t[2],t[3]);
}
--- R ---
boub <- c(1,2,3,4)
.C("pour",as.integer(boub),NAOK=TRUE)
# T0=1 T1=2 T2=3 T3=4[[1]]
# [1] 1 2 3 4
boub <- c(1,2,NA,4)
.C("essai",as.integer(boub),NAOK=TRUE)
# T0=1 T1=2 T2=-2147483648 T3=4[[1]]
# [1] 1 2 NA 4
--- ---
In the second example, T2=-2147483648 and not NA.
I check the "writing R extension", there is a part that explain that the test
of NA is not the same between double and integer (NA_INTEGER, ISNA), but I
did not find explanation on passing NA argument as integer.
Any idea of what is wrong in my code?
Simple: Rprintf does not know about NAs (and nor does printf). From
the manual:
The most useful function for printing from a C routine compiled into
R is Rprintf. This is used in exactly the same way as printf, but
is guaranteed to write to R's output (which might be a GUI console
rather than a file).
The value of NA is stored as NA_INTEGER = -2^32, and if you want your
C code to be aware of it, *you* need to program so that value is
treated specially. (Since double NAs are stored as a particular NaN,
the default C handling of doubles will probably do something sensible
but careful code will also need to take the difference between NaNs
into account.)
--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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