从 C++ 写一个 Rdata 文件

Question

Suppose I have a C++ program that has a vector of objects that I want to write out to an Rdata data.frame file, one observation per element of the vector. How can I do that? Here is an example. Suppose I have

vector<Student> myStudents;

And Student is a class which has two data members, name which is of type std::string and grade which is of type int .

Is my only option to write a csv file?

Note that Rdata is a binary format so I guess I would need to use a library.

A search for Rdata [r] [C++] came up empty.

Answer 1

I think nobody has bothered to extract a binary file writer from the R sources to be used independently from R.

Almost twenty years ago I did the same for Octave files as their format is simply: two integers for 'n' and 'k', followed by 'n * k' of data -- so you could read / write with two function calls each.

I fear that for R you would have to cover too many of R's headers -- so the easiest (?) route may be to give the data to R, maybe via Rserve ('loose' connection over tcp/ip) and RInside (tighter connection via embedding), and have R write it.

Edit: In the years since the original answer was written, one such library has been created: librdata .

Answer 2

I don't know if this will fit everyone needs (of those who is googling this question), but this way you can save individual or multiple variables:

using namespace std;
using namespace Rcpp;
using Eigen::Map; 
using Eigen::MatrixXi;
using Eigen::MatrixXd;

Environment base("package:base");

Function save = base["save"];
Function saveRDS = base["saveRDS"];

MatrixXd M = MatrixXd::Identity(3,3);

NumericMatrix xx(wrap(M));
NumericMatrix xx1(wrap(M));
NumericMatrix xx2(wrap(M));

base["xx"] = xx;
base["xx1"] = xx1;
base["xx2"] = xx2;

vector<string> lst;
lst.push_back("xx");
lst.push_back("xx1");
lst.push_back("xx2");
CharacterVector all = wrap(lst);

save(Named("list", all), Named("envir", base) , Named("file","Identities.RData"));
saveRDS(xx,Named("file","Identity.RDs"));
return wrap(M);

library(inline)
library(Rcpp)
library(RcppEigen)

src <- '
#put here cpp code shown above 
'

saveworkspace <- cxxfunction(signature(), src, plugin = "RcppEigen")
saveworkspace()
list.files(pattern="*.RD*")


[1] "Identity.RDs"
[2] "Identities.RData"

I'm not 100% sure if this C++ code will work in standalone library/executable.

NB : Initially I missed the comment that the solution should be independent of R , but for those who is searching for exactly the same question, but they are ok with dependency on R , this could be helpful.

Answer 3

Here is an example of a function that saves a list in a RData. This example is based on the previous answer :

void save_List_RData(const List &list_Data, const CharacterVector &file_Name)
{
  Environment base("package:base");
  Environment env = new_env();
  env["list_Data"] = list_Data;
  Function save = base["save"];
  CharacterVector all(1);
  all[0] = "list_Data";
  save(Named("list", all), Named("envir", env), Named("file", file_Name));
  Rcout << "File " << file_Name << " has been saved! \\n";
}