Update var.h

Variance: Change from the unstable one-pass formula back to the stable two-pass formula.
Details: #223

Author: MattPD

inst/include/Rcpp/sugar/functions/var.h

@@ -34,10 +34,10 @@ class Var : public Lazy< double , Var<RTYPE,NA,T> > {
     Var( const VEC_TYPE& object_ ) : object(object_){}
 
     double get() const{
-        double
-            m = mean(object).get(),
-            ssq = sum( pow(object, 2.0) ).get();
-        return (ssq - m * m * object.size()) / (object.size() - 1);
+        const double
+            average = mean(object).get(),
+            sum_squared_deviations = sum( pow(object - average, 2.0) ).get();
+        return sum_squared_deviations / (object.size() - 1);
     }
 
 private: