make correlation distance robust

src/distance_T.cpp

@@ -2,7 +2,7 @@
  * \brief all functions requiered for R dist function and C hcluster function.
  *
  *  \date Created: probably in 1995
- *  \date Last modified: Time-stamp: <2011-11-08 21:06:41 antoine>
+ *  \date Last modified: Time-stamp: <2012-01-08 12:16:41 antoine>
  *
  *  \author R core members, and lately: Antoine Lucas 
  *
@@ -395,7 +395,7 @@ template<class T> T  distance_T<T>::R_correlation(double * x, double * y , int n
 						  int i1, int i2,
 						  int * flag, T_tri & opt)
 {
-  T num,denum,sumx,sumy,sumxx,sumyy,sumxy;
+  T num,denum2,sumx,sumy,sumxx,sumyy,sumxy;
   int count,j;
 
   count= 0;
@@ -424,8 +424,12 @@ template<class T> T  distance_T<T>::R_correlation(double * x, double * y , int n
       return NA_REAL;
     }
   num = sumxy - ( sumx*sumy /count );
-  denum = sqrt( (sumxx - (sumx*sumx /count ) )* (sumyy - (sumy*sumy /count ) ) );
-  return 1 - (num / denum);
+  denum2 =  (sumxx - (sumx*sumx /count ) )* (sumyy - (sumy*sumy /count ) ) ;
+  if(denum2 <=0)
+    {
+      return 0;
+    }
+  return 1 - (num / sqrt(denum2));
 }
 
 /** \brief Absolute Distance correlation (Uncentered Pearson)