[R] Optimizing C code

[Romain Francois]

Bonjour Christophe,

NA and NaN are different things... Actually this is tricky because NA is 
implemented as a special kind of NaN :

See this extract of R_ext/Arith.h :

int R_IsNA(double);		/* True for R's NA only */
int R_IsNaN(double);		/* True for special NaN, *not* for NA */
int R_finite(double);		/* True if none of NA, NaN, +/-Inf */
#define ISNA(x)	       R_IsNA(x)

/* ISNAN(): True for *both* NA and NaN.
    NOTE: some systems do not return 1 for TRUE.
    Also note that C++ math headers specifically undefine
    isnan if it is a macro (it is on OS X and in C99),
    hence the workaround.  This code also appears in Rmath.h
*/
#ifdef __cplusplus
   int R_isnancpp(double); /* in arithmetic.c */
#  define ISNAN(x)     R_isnancpp(x)
#else
#  define ISNAN(x)     (isnan(x)!=0)
#endif

Romain

PS: the question would be more appropriate in R-devel.


On 01/22/2010 11:14 AM, Christophe Genolini wrote:
>
> Hi the list,
>
> I need to write some efficient distances function, so I read the code
> for the Euclidean distance.
> I do not understand the purpose of the line 11 : if x[i] and y[i] are
> not NA (line 9), can dev be NA ?
>
> Christophe
>
>
> #define both_FINITE(a,b) (R_FINITE(a) && R_FINITE(b))
> #define both_non_NA(a,b) (!ISNAN(a) && !ISNAN(b))
>
> 1. static double R_euclidean2(double *x, double *y, int taille)
> 2. {
> 3. double dev, dist;
> 4. int count, i;
> 5.
> 6. count= 0;
> 7. dist = 0;
> 8. for(i = 0 ; i < taille ; i++) {
> 9. if(both_non_NA(x[i], y[i])) {
> 10. dev = (x[i] - y[i]);
> 11. if(!ISNAN(dev)) {
> 12. dist += dev * dev;
> 13. count++;
> 14. }
> 15. }
> 16. }
> 17. if(count == 0)return NA_REAL;
> 18. if(count != taille) dist /= ((double)count/taille);
> 19. return sqrt(dist);
> 20.}

-- 
Romain Francois
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http://romainfrancois.blog.free.fr
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