[Rcpp-devel] data.frame from R to C++?

[Dirk Eddelbuettel]

On 8 April 2011 at 07:20, Dirk Eddelbuettel wrote:
| 
| On 8 April 2011 at 00:27, deqiang sun wrote:
| | Hi all,
| | 
| | Suppose I want to read a csv file including TEXT and NUMBER fields in R by function read.table. 
| | Is there a way to use the result in C++? Like the way I use in R, say, 2nd column is x[,2], 3rd row is x[3,].
| | I have gone through the examples in RInside and did not find anything helpful.
| 
| You will want to look at _Rcpp_ not RInside for these things.  
| 
| It is also a good idea to check the mailing list archive. Here I quickly
| search Google for "gmane rcpp-devel Rcpp::DataFrame" and found e.g.
| 
| http://article.gmane.org/gmane.comp.lang.r.rcpp/1205 
|      -- returning a data.frame from R to C++
| 
| and a bunch more. 
| 
| But I think you can't pass an entire dataframe from R to C++ _as a single

Actually, you can as a DataFrame is really just a list.   

Here is a complete example of passing a data.frame in, accessing each column
and returning.  Notice, though, how both the original data gets modified for
int and char (but not date).  That was discussed here at length too; we pass
pointers which is why the change propagates.  Notice also how I effectively
split the data.frame back up into its columns -- by name.

Dirk

edd at max:/tmp$ cat deqiang.R 

library(inline)

D <- data.frame(a=1:3, b=LETTERS[1:3], c=Sys.Date()+0:2, stringsAsFactors=FALSE)
print(D)

src <- '
  Rcpp::DataFrame DL = Rcpp::DataFrame(x);
  Rcpp::IntegerVector a = DL["a"];
  Rcpp::CharacterVector b = DL["b"];
  Rcpp::DateVector c = DL["c"];

  // do something, et
  a[2] = 42;
  b[1] = "foo";
  c[0] = c[0] + 7;                      // move up a week

  return(Rcpp::List::create(Rcpp::Named("DataFrame")=DL,
                            Rcpp::Named("IntVec")=a,
                            Rcpp::Named("CharVec")=b,
                            Rcpp::Named("DateVec")=c));
'

fun <- cxxfunction(signature(x="misc"), body=src, plugin="Rcpp")
print(fun(D))
edd at max:/tmp$ r deqiang.R 
Loading required package: methods
  a b          c
1 1 A 2011-04-08
2 2 B 2011-04-09
3 3 C 2011-04-10
$DataFrame
   a   b          c
1  1   A 2011-04-08
2  2 foo 2011-04-09
3 42   C 2011-04-10

$IntVec
[1]  1  2 42

$CharVec
[1] "A"   "foo" "C"  

$DateVec
[1] "2011-04-15" "2011-04-09" "2011-04-10"

edd at max:/tmp$ 

-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com