[R] Optimizing C code

[Duncan Murdoch]

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 ?
>   

As Romain said, the test is for NaN as well as NA.  One way it could 
happen is if both x[i] and y[i] were infinite:  then the difference is NaN:

 > Inf - Inf
[1] NaN

Duncan Murdoch
> 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.}
>
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