Using Rcout for output synchronised with R

Dirk Eddelbuettel — Jan 8, 2013 | source

The Writing R Extensions manual, which provides the gold standard of documentation as far as extending R goes, strongly suggests to use Rprintf and REprintf for output (from C/C++ code). The key reason is that these are matched to the usual output and error streams maintained by R itself.

In fact, use of std::cout and std::cerr (as common in standard C++ code) is flagged when running R CMD check and no longer permitted when uploading to CRAN.

Thanks to an initial patch by Jelmer Ypma, which has since been reworked and extended, we have devices Rcout (for standard output) and Rcerr (for standard error) which intercept output and redirect it to R.

To illustrate, we create a simple function which prints a value:

#include <RcppArmadillo.h>   // as we use RcppArmadillo below
                             // this first example use only Rcpp 

using namespace Rcpp;

// [[Rcpp::export]]
void showValue(double x) {
    Rcout << "The value is " << x << std::endl;
}

We can use this from R, and output will be properly synchronised:

cat("Before\n")
Before
showValue(1.23)
The value is 1.23
cat("After\n")
After

As of the 0.10.* abd 0.11.* releases, Rcpp itself still lacks the converter code to “pretty-print simple non-scalar data structures. But there are alternatives. First, RcppArmadillo can do so via its operator<<() as the (Rcpp)Armadillo output is automatically redirected to R output stream. See below for a recent alternative from Rcpp itself.

#include <RcppArmadillo.h>

// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
void showMatrix(arma::mat X) {
    Rcout << "Armadillo matrix is" << std::endl << X << std::endl;
}
M <- matrix(1:9,3,3)
print(M)
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
showMatrix(M)
Armadillo matrix is
   1.0000   4.0000   7.0000
   2.0000   5.0000   8.0000
   3.0000   6.0000   9.0000

Having output from R and C++ mix effortlessly is a very useful feature. We hope to over time add more features to output more of Rcpp basic objects.

Alternatively, starting with R version 0.11.5, we now have function print() which can print any SEXP object – by calling the internal R function Rf_PrintValue().

A simple illustration follow. We first define helper function.

// [[Rcpp::export]]
void callPrint(RObject x) { 
    Rcpp::print(x);             // will work on any SEXP object
}

A few examples calls follow below.

callPrint(1:3)             # print a simple vector
[1] 1 2 3
callPrint(LETTERS[1:3])    # or characters
[1] "A" "B" "C"
callPrint(matrix(1:9,3))   # or a matrix
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
callPrint(globalenv())    # or an environment object
<environment: R_GlobalEnv>

tags: basics 

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