[Rcpp-devel] Mimicing of X[ X[, col]==val, ] with RcppArmadillo

[Dirk Eddelbuettel]

Hi,
On 31 May 2012 at 13:07, Søren Højsgaard wrote:
| Dear List
| 
| I've tried to make an Rcpp version of X[ X[,col]==val, ] and came up with the following:
| 
| sqbr <- cxxfunction(signature(r_X='float', r_col='int', r_val='double'), body =
| '
|   using namespace arma;
|   using namespace Rcpp;
|   using namespace std;
|   
|   mat XX     = as<mat>(r_X);
|   double val = as_scalar(as<fvec>(r_val));
| 
|   colvec col = XX.cols(as<uvec>(r_col)-1);
|   vec valvec = val * ones<vec>(XX.n_rows,1);
| 
|   NumericVector valvec2 = wrap(valvec), col2 = wrap(col);
|   LogicalVector rowbool = valvec2 == col2;
| 
|   uvec rowidx = find(as<uvec>(rowbool)>0);
| 
|   Rcout << col;  Rcout << valvec;
|   Rcout << "rowbool" << endl << as<uvec>(rowbool); Rcout << "rowidx" << endl << rowidx;
| 
|   mat ans = XX.rows(rowidx);
|   return(wrap(ans));
| '
| , plugin='RcppArmadillo')
| 
| X <- matrix(c(1,2,3,4,4,5,6,7,7),nr=3)
| cc <- 2
| vv <- 4
| sqbr(X,cc,vv)
| 
| I would expect that there is a way of doing this without having to jump "into" armadillo "back" to Rcpp and "into" Armadillo again. I just can't spot an easier way. If a simpler solution exists I would be interested to know...

I don't see wju valvec2 and rowbool cannot be Armadillo types. That way you'd
avoid a 'jump back'. 
 
| A related question. Suppose I want to do X[ X[,col1]==val & X[,col2]>val2, ]. Is there any elegant way of doing this. I am thinking whether the operators "==" and ">" can be passed on as arguments so that one can do something similar to 
| > `>`(5,2)
| [1] TRUE
| > `==`(5,2)
| [1] FALSE

Not that I can think of right now.  

Cheers, Dirk
 
| Best regards
| Søren
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com