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

[Dirk Eddelbuettel]

On 12 April 2011 at 05:38, deqiang sun wrote:
| How should I read the txt in the following context?
| c3	3a
| c1	1b
| c2	2c
| 
| The code I have is 
| 		RInside R(argc, argv);
| 		SEXP ans;
| 
| 		std::string txt = "a=read.csv('xxx.xls',sep='\t',head=F)";
| 		ans = R.parseEval(txt);
| 		Rcpp::DataFrame DF(ans);
| 
| 		Rcpp::StringVector x1 = DF["V1"];
| 		Rcpp::StringVector x2 = DF["V2"];
| 
| But the print of x1 and x2 is 
| 3	3	
| 1	1	
| 2	2	
| 
| What's wrong with my code?

The read.csv() function and its cousins default to stringsAsFactors=TRUE. 

So the '3 1 2' are the factor levels. You want either 

    std::string txt = "a=read.csv('xxx.xls',sep='\t',"
                      "head=FALSE,stringsAsFactors=FALSE)";

or set it as a global option.  Google for stringsAsFactors, this is
documented and are ways around it.  

Dirk

PS  I personally would also set column names in the data file or R code.

 
| Thanks,
| 
| Dsun
| On Apr 12, 2011, at 4:14 AM, Romain Francois wrote:
| 
| > Le 12/04/11 10:57, deqiang sun a écrit :
| >> Hi Dirk,
| >> 
| >> Thanks very much for your example. From this example I learned how to passing data back and forth between R and C++.
| >> The example is R code and uses piece of C++ code in side R program.
| >> Well, it's better if you put this example(by only making slight changes to make R embeded in C++) inside package RInside.
| >> 
| >> Accessing data by column name is the feature I ( and other people) usually want.
| >> However, is there a way to access the data Frame by row number?
| > 
| > No. You access the column, and then do whatever with each element, as in 
| > Dirk's example.
| > 
| >> Regards.
| >> 
| >> Dsun
| >> On Apr 8, 2011, at 9:56 AM, Dirk Eddelbuettel wrote:
| >> 
| >>> 
| >>> Ok, I now committed a slightly nice example to RcppExamples, a package I
| >>> should revamp to contain more examples.  In there, we now receive a
| >>> data.frame, operate on each column, create a new data and return old and new.
| >>> 
| >>> The core of the code (inside the try/catch protection) is
| >>> 
| >>>      // construct the data.frame object
| >>>      Rcpp::DataFrame DF = Rcpp::DataFrame(Dsexp);
| >>> 
| >>>      // and access each column by name
| >>>      Rcpp::IntegerVector a = DF["a"];
| >>>      Rcpp::CharacterVector b = DF["b"];
| >>>      Rcpp::DateVector c = DF["c"];
| >>> 
| >>>      // do something
| >>>      a[2] = 42;
| >>>      b[1] = "foo";
| >>>      c[0] = c[0] + 7;                      // move up a week
| >>> 
| >>>      // create a new data frame
| >>>      Rcpp::DataFrame NDF =
| >>> 	  Rcpp::DataFrame::create(Rcpp::Named("a")=a,
| >>> 				  Rcpp::Named("b")=b,
| >>> 				  Rcpp::Named("c")=c);
| >>> 
| >>>      // and return old and new in list
| >>>      return(Rcpp::List::create(Rcpp::Named("origDataFrame")=DF,
| >>> 				Rcpp::Named("newDataFrame")=NDF));
| >>> 
| >>> 
| >>> Dirk
| >>> --
| >>> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
| > 
| > -- 
| > Romain Francois
| > Professional R Enthusiast
| > +33(0) 6 28 91 30 30
| > http://romainfrancois.blog.free.fr
| > http://romain-francois.com
| > |- http://bit.ly/fhqbRC : Rcpp workshop in Chicago on April 28th
| > |- http://bit.ly/dFyZGB : Hydraulique au Montpellier Comedie Club
| > `- http://bit.ly/eVXit9 : Eponyme : 40 minutes stand up
| > 
| > 
| 

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