[Rcpp-devel] Splitting a matrix by its rows into a list

Dirk Eddelbuettel edd at debian.org
Tue Feb 19 02:37:06 CET 2013


On 18 February 2013 at 17:02, Kevin Ushey wrote:
| One solution: X.row(ii) is actually generating a CharacterMatrix::Row ; ie, a
| view of that row in the CharacterMatrix, and I think you need to explicitly
| copy it to a CharacterVector and then assign that CharacterVector to the list.
| 
| You could write something like:
| 
| src <- '
|   CharacterMatrix X(XX_);
|   int Nx = X.nrow();
|   List ans;
|   for (int ii=0; ii<Nx; ii++) {
|     CharacterMatrix::Row view = X.row(ii);
|     CharacterVector tmp = no_init( X.ncol() );
|     for( int j=0; j < X.ncol(); j++ ) {
|       tmp[j] = view[j];
|     }
|     ans.push_back( tmp );
|   }
|   return(wrap(ans));
|| 
| although it is a bit clunkier. I'm not sure if there's a more idiomatic method
| available.

I think all approaches have to be clunky as all our data structures store
column-wise, because that is what R does.  

So I would probably start by element-wise copying, see if that works / if
that is fast enough and leave it at that.

Dirk
| 
| FWIW, trying to copy and assign directly, e.g. 'CharacterVector tmp = X.row
| (ii);', doesn't seem to work, even though the same code works (and is in the
| quick-start vignette) for NumericVector.
| 
| -Kevin
| 
| On Mon, Feb 18, 2013 at 3:37 PM, Søren Højsgaard <sorenh at math.aau.dk> wrote:
| 
|     I want to split a matrix by its rows into a list as:
|     > a<- matrix(letters[1:6],ncol=2)
|     > split(a,row(a))
|     $`1`
|     [1] "a" "d"
| 
|     $`2`
|     [1] "b" "e"
| 
|     $`3`
|     [1] "c" "f"
| 
|     I do as follows and get a strange result. Any suggestions?
| 
|     Thanks in advance!
|     Søren
| 
|     ---------------
| 
|     src <- '
|       CharacterMatrix X(XX_);
|       int Nx = X.nrow();
|       List ans;
|       for (int ii=0; ii<Nx; ii++){
|         ans.push_back(X.row(ii));
|       }
|       return(wrap(ans));
|     '
|     split_ <- cxxfunction(signature(XX_="matrix"), plugin="Rcpp", body=src)
| 
|     split_(a)
| 
|     [[1]]
|     [[1]][[1]]
|     <CHARSXP: "a">
| 
|     [[1]][[2]]
|     <CHARSXP: "d">
| 
| 
|     [[2]]
|     [[2]][[1]]
|     <CHARSXP: "b">
| 
|     [[2]][[2]]
|     <CHARSXP: "e">
| 
| 
|     [[3]]
|     [[3]][[1]]
|     <CHARSXP: "c">
| 
|     [[3]][[2]]
|     <CHARSXP: "f">
| 
| 
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| 
| 
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com  


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