This is the same as the R function sum when used on a vector.

double sumv(Rcpp::NumericVector A)
   {    int na= A.size();
         double b=0;
         for (int i = 0; i < na; i++)
         { b=b+A(i);
         }
         return b;
   }

Multiply two vectors together using a dot product.

double dot(Rcpp::NumericVector c,Rcpp::NumericVector d)
   {     int nc= c.size();
         double e=0;
         for (int i = 0; i < nc; i++)
         {  e=e+c(i)*d(i); }
         return e;
   }