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

[John Merrill]

There is a sleazy hack which works, but wastes a fair amount of memory:
take the transpose of the matrix in R, and use the fact that the columns of
that transpose are the rows of the original.


On Mon, Feb 18, 2013 at 5:37 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

>
> 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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