Skip to content

[Rcpp-devel] binding, combining vectors

3 messages · Dirk Eddelbuettel, Romain Francois

Original
Romain Francois
# 
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

Romain
Dirk Eddelbuettel
#
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
Romain Francois
# 
Le 13/12/12 17:29, Dirk Eddelbuettel a ?crit :
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