In rcpp - how to return a vector with names

Question

Suppose I have the following matrix:

testM <- as.matrix(read.table(textConnection(" 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 5 4 1 3 2 2 1 "))) 

This matrix has column names V1 to V6

Suppose there is another matrix where I delete the column name:

 > testM2<-testM > colnames(testM2)<-NULL 

Then, if I try colMeans on testM and testM2 R returns the numeric class in both cases, except for the first response of the colnames answer.

 > colMeans(testM) V1 V2 V3 V4 V5 V6 2.166667 2.333333 2.833333 2.500000 2.666667 2.666667 > colMeans(testM2) [1] 2.166667 2.333333 2.833333 2.500000 2.666667 2.666667 

Now suppose I have the same function written in RCpp as follows:

 double do_mean(NumericVector x) { return mean(na_omit(x)); } //[[Rcpp::export]] NumericVector colMeansCppMt(NumericMatrix& x) { int nCols=x.ncol(); NumericVector out=no_init(nCols); for (int i=0;i<nCols;i++) { NumericMatrix::Column tmp=x(_,i); out[i]=do_mean(tmp); } return out; } 

The output for colMeansCppMt for testM and testM2 returns numeric vectors, but for testM does not contain colnames since it was not set.

Now suppose I change the colMeansCppMt function to include attributes like this:

 //[[Rcpp::export]] NumericVector colMeansCppMt(NumericMatrix& x) { int nCols=x.ncol(); NumericVector out=no_init(nCols); for (int i=0;i<nCols;i++) { NumericMatrix::Column tmp=x(_,i); out[i]=do_mean(tmp); } out.attr("names")=x.attr("names"); return out; } 

The output for testM is still a vector that does not contain column names.

I also tried out.attr("names")=x.attr("colnames") and out.attr("colnames")=x.attr("colnames") .

a). How can I check RCpp where the colnames matrix (for example, x in the example function above) were set or not?

b) How can I return a numeric vector in R with names in Rcpp?

Answer 1

To set the landscape:

• Regular R vectors have the (optional) attribute names ,
• data.frame have data.frame attribute for rows and names attribute for columns, and
• matrix have the (optional) attribute dimnames ; this attribute is a list containing 2 character vectors (rows, then columns).

So you want the column names x be bound to the 'names' out attribute, so something like:

 out.attr("names") = VECTOR_ELT(x.attr("dimnames"), 1); 

will work.

(I can't remember if Rcpp good API for getting / setting dimension names in arrays ...)