Most effecient way of converting DataFrame to Matrix and vice versa

Question

I have been trying to implement some of the basic R functions like split in Rcpp for Data frames and matrices. For that I need to know the most efficient method of converting Data Frame and Matrix back and forth. so far I use the DataFrame constructor to convert the matrix to DataFrame. How can I convert DataFrame Back to Matrix.Lets assume that all the data are of type double.

The current approach is

matrix to DataFrame

NumericMatrix x;
DataFrame y= DataFrame(x);
y.attr("names")=x.attr("names");

DataFrame to Matrix

DataFrame x;
int xsize=x.size();
NumericVector col=x(0); 
NumericMatrix y(col.size(),xsize);
for(int i=0;i<xsize;i++){
    y(_,i)=col;
    if(i<xsize-1){
       col=x(i+1);
    }
}
y.attr("names")=x.attr("names");

Is there a more efficient way of doing this conversion?

Also I am a newbie to Rcpp. Can Anybody explain how to find the source code of a particular class implementation, for example NumericMatrix?

Also the last line

y.attr("names")=x.attr("names")

does not set the column names of x as column names of y. Can anybody explain how to set the column names as the column names of the data frame.

Answer 1

You are missing the nrow method for DataFrame. This could simplify your code. You don't need the special casing for the first column, etc ...

int xsize=x.size();
NumericMatrix y(x.nrows(),xsize);
for(int i=0;i<xsize;i++){
    y(_,i) = NumericVector(x[i]);
}

As for setting the column names, you can go through the dimnames attributes:

y.attr("dimnames") = List::create( R_NilValue, x.attr("names") ) ;

The source code for Matrix is here but I'm not sure this is going to help you.