ropenmprcpp

Rcpp Compiler error in r tools for Visual Studio 2015

I am a new user of Rcpp and OpenMP.
I have written code to multiply two arrays.
I want use omp as well just to check its performance with big matrices.

I use R tools in Visual Studio 2015 to work and run this code:

This code

When I compile it in r tools for Visual Studio 2015, I get this error:

c:/Rtools/mingw_64/bin/g++ -m64 -I"C:/PROGRA~1/MIE74D~1/ROPEN~1/R-35~1.1/include" -DNDEBUG   -I"C:/Users/amaa11/Documents/R/win-library/3.5/Rcpp/include" -I"C:/Users/amaa11/DOCUME~1/VISUAL~1/Projects/RPROJE~1/RPROJE~1"   -I"C:/swarm/workspace/External-R-3.5.1/vendor/extsoft/include"     -O2 -Wall  -mtune=core2 -c mul_mat.cpp -o mul_mat.o
mul_mat.cpp:10:4: error: stray '#' in program
/ /#pragma omp parallel for
^
mul_mat.cpp:4:1: error: expected unqualified-id before '/' token
/ / [[Rcpp::export(mul_mat)]]<br>
^
make: *** [mul_mat.o] Error 1

How can I fix it?

Solution

  • The quick and easy answer is: Remove the space between comment forward slashes. For example, change

    / /#pragma omp parallel for

    to

    //#pragma omp parallel for

    However, I also have other comments/tips:

    Update

    So, for completeness, this is the fixed code that compiled fine for me:

    #include <Rcpp.h>
#include <omp.h>

using namespace Rcpp;

// [[Rcpp::export]]
NumericMatrix mul_mat(int n, NumericMatrix A, NumericMatrix B) {
    int i, j, k;
    NumericMatrix mult(n, n);
    for ( i = 0; i < n; ++i ) {
        //#pragma omp parallel for
        for ( j = 0; j < n; ++j ) {
            mult(i, j) = 0;
        }
    }
    for ( i = 0; i < n; ++i ) {
        for ( j = 0; j < n; ++j ) {
            for ( k = 0; k < n; ++k ) {
                mult(i, j) += A(i, k) * B(k, j);
            }
        }
    }
    return mult;
}

    And we can see it exported the function to R (this part added in response to now deleted comments regarding the proper way to export the function to R):

    > Rcpp::sourceCpp("so-answer.cpp")
> ls()
[1] "mul_mat"

    You initially had a(n incorrectly typed) comment delimiter before the #pragma omp parallel for; if you really want to use it, remove the comment delimiter as well as add // [[Rcpp::plugins(openmp)]]; this is discussed (among other places) in an Rcpp Gallery post:

    Additionally, we can make use of the OpenMP library to use multiple cores. For the OpenMP implementation, we need to enable OpenMP support. One way of doing so is by adding the required compiler and linker flags as follows:

    Sys.setenv("PKG_CXXFLAGS"="-fopenmp")
    Sys.setenv("PKG_LIBS"="-fopenmp")

    Rcpp version 0.10.5 and later will also provide a plugin to set these variables for us:

    // [[Rcpp::plugins(openmp)]]

    So, try:

    #include <Rcpp.h>
#include <omp.h>

// [[Rcpp::plugins(openmp)]]

using namespace Rcpp;

// [[Rcpp::export]]
NumericMatrix mul_mat(int n, NumericMatrix A, NumericMatrix B) {
    int i, j, k;
    NumericMatrix mult(n, n);
    for ( i = 0; i < n; ++i ) {
        omp_set_num_threads(2); // For example
        #pragma omp parallel for
        for ( j = 0; j < n; ++j ) {
            mult(i, j) = 0;
        }
    }
    for ( i = 0; i < n; ++i ) {
        for ( j = 0; j < n; ++j ) {
            for ( k = 0; k < n; ++k ) {
                mult(i, j) += A(i, k) * B(k, j);
            }
        }
    }
    return mult;
}

    which also compiled just fine on my machine.