[Rd] Argument as.integer(NA) to a function C

[Prof Brian Ripley]

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,                  ripley at 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