[Rcpp-devel] binding, combining vectors

[Romain Francois]

Le 13/12/12 17:29, Dirk Eddelbuettel a écrit :
>
> On 13 December 2012 at 10:01, Romain Francois wrote:
> | Hello,
> |
> | I'd like to add a functionality to "bind" vectors of the same type (well
> | sugar expressions really, but let's say it is vectors for the sake of
> | this email).
> |
> | So essentially I'd like something similar to what "c" does in R :
> |
> |  > c( letters, letters )
> |   [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p"
> | "q" "r" "s"
> | [20] "t" "u" "v" "w" "x" "y" "z" "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"
> | "k" "l"
> | [39] "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"
> |
> |
> | I don't want to call it "c", and I'd like suggestions : combine, bind ?
> |
> | The idea is to avoid writing code like this:
> |
> | CharacterVector x, y ;
> | CharacterVector z( x.size() + y.size() ) ;
> | int i=0;
> | for( ; i<x.size(); i++) z[i] = x[i] ;
> | for( int j=0; j<y.size(); i++, j++) z[i] = y[j] ;
> |
> | I know it is not a big deal for people to write this code, but for
> | example this does not handle the names of the elements, ... and
> | internally I can use more efficient code
>
> The STL more or less has that already, just combine two calls to copy():
>
> -----------------------------------------------------------------------------
> #include <Rcpp.h>
>
> // [[Rcpp::export]]
> std::vector<std::string> concat(std::vector<std::string> x,
> std::vector<std::string> y) {
>
>    std::vector<std::string> z(x.size() + y.size());
>
>    std::copy(x.begin(), x.end(), z.begin());
>    std::copy(y.begin(), y.end(), z.begin() + x.size());
>
>    return(z);
> }
>
>
> /*** R
> concat(letters[1:5], letters[1:4])
> ***/
> -----------------------------------------------------------------------------
>
>
> R> sourceCpp("/tmp/concat.cpp")
>
> R> concat(letters[1:5], letters[1:4])
> [1] "a" "b" "c" "d" "e" "a" "b" "c" "d"
> R>
>
>
> Dirk

I'm not asking how to implement it, I have a fairly good understanding 
of std::copy.

I was just asking how to name this.

I'd like to have, e.g. the possibility to do:

CharacterVector x, y ; // from somewhere

CharacterVector z1 = concat(x, y ) ;
CharacterVector z1 = concat(x, "foo", y ) ;

Or perhaps dealing with sugar exprs;

CharacterVector z1 = concat( rep(x, 2), rep_each(y, 3) ) ;


... anyway. I think I like "concat"


Romain

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Romain Francois
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