Chapter 6 Printing messages

You can print messages and values of objects on the R console screen by using Rprintf() and Rcout.

REprintf() and Rcerr can be used for printing error messages.

6.1 Rcout, Rcerr

The way of using Rcout and Rcerr is the same as std::cout and std::cerr. Connecting messages or variables with << in the order you want. When you give a vector object to <<, it will print all the elements of the vector.

// [[Rcpp::export]]
void rcpp_rcout(NumericVector v){
  // printing value of vector
  Rcout << "The value of v : " << v << "\n";

  // printing error message
  Rcerr << "Error message\n";
}

6.2 Rprintf(), REprintf()

The way of using Rprintf() and REprintf() is the same as std::printf(), it prints a message by specifying format.

Rprintf( format, variables)

In the format string, you can use following format specifiers for printing the values of variables. When you want to print multiple variables, you have to pass these variables in the order that its corresponding specifier appears in the format string.

Only a part of the format specifier is presented below, please refer to other documentation for detail (For example, cplusplus.com).

specifier	explanation
%i	printing signed integer (int)
%u	printing unsigned integer (unsigned int)
%f	printing floating point number (double)
%e	printing floating point number (double) in exponential style
%s	printing C string (char*)

Additionally, Rprintf() and REprintf() can only print data types that exist in standard C language, thus you cannot pass data types defined by Rcpp package (such as NumericVector) to Rprintf() directly. If you want to print the values of elements of an Rcpp vector using Rprintf(), you have to pass each element separately to it (see below).

// [[Rcpp::export]]
void rcpp_rprintf(NumericVector v){
    // printing values of all the elements of Rcpp vector  
    for(int i=0; i<v.length(); ++i){
        Rprintf("the value of v[%i] : %f \n", i, v[i]);
    }
}