Re-type ComputeY as a List.

The number of unique elements in different columns in x1 may be
different, which leads to attempting to add vectors of different length
into res. Previously, `push_back` on a DataFrame changed the DataFrame
to a list, which is being addressed in RcppCore/Rcpp#1099. This change,
combined with attempting to add vectors of different lengths, leads to
an error.

src/testxvalid.cpp

@@ -24,15 +24,15 @@ using namespace Rcpp;
 //' @param tau2 the residual heterogeneity
 //' @keywords internal
 // [[Rcpp::export(".ComputeY")]]
-DataFrame ComputeY(DataFrame x1, NumericVector y,
+List ComputeY(DataFrame x1, NumericVector y,
                 NumericVector vi, NumericVector tau2) {
   int nsplit;
   int i;
   int j;
-  DataFrame res;
+  List res;
   for (nsplit = 0; nsplit < x1.ncol(); nsplit++) {
     IntegerVector Nodes = x1[nsplit];
-    IntegerVector uniNodes = Rcpp::sort_unique(Nodes); 
+    IntegerVector uniNodes = Rcpp::sort_unique(Nodes);
     NumericVector swyNodes;
     for (i = 0; i < uniNodes.length(); i++) {
       // compute the weighted sum for each node
@@ -48,7 +48,6 @@ DataFrame ComputeY(DataFrame x1, NumericVector y,
      swyNodes.push_back(sumWY/sumW, to_string(uniNodes[i]));
 
     }
-
     res.push_back(swyNodes);
   }