[Rcpp-devel] help with Rcpp::List of Rcpp::List

[THIOULOUSE JEAN]

Dear Rcpp-devel list

I need your help again to solve a problem I have when submitting a new release of the ade4 package (https://github.com/sdray/ade4) to CRAN. I wrote a C++ function where I need to use a list of lists (see code below). It works perfectly on all platforms, but the Windows and Linux compilers (not the Mac one) raise a warning saying :

Flavor: r-devel-windows-x86_64
   testsCpp.cpp:478:28: warning: ISO C++ forbids variable length array 'spl1' [-Wvla]

Flavor: r-devel-linux-x86_64-debian-gcc
    testsCpp.cpp:478:19: warning: variable length arrays are a C99 feature [-Wvla-extension]

This warning happens here:

testsCpp.cpp: In function 'arma::vec RVintrarandtestCpp(const arma::mat&, const arma::mat&, Rcpp::IntegerVector, int)':
testsCpp.cpp:478:28: warning: ISO C++ forbids variable length array 'spl1' [-Wvla]
  478 |                 Rcpp::List spl1[i];
      |                            ^~~~

Here is the C++ code, as you can see I declare a Rcpp::List of Rcpp::List with varying lengths (these lists are used to store the row numbers of a data table that belong to the same levels of a factor). So my question is: what should I do about this warning ? Do I need to find a workaround (which one ?), or should I ignore it ?

Sorry for the long message, and thanks in advance for any help...

Jean


// [[Rcpp::export]]
arma::vec RVintrarandtestCpp(const arma::mat & X, const arma::mat & Y, Rcpp::IntegerVector fac, const int nrepet)
{
/*--------------------------------------------------
Get the number of levels of the factor
--------------------------------------------------*/
	Rcpp::CharacterVector faclevs = fac.attr("levels");
	int nlev = faclevs.length();
/*--------------------------------------------------
 List of lists to store the row numbers belonging to each factor level
--------------------------------------------------*/
	Rcpp::List spl1(nlev);
	for (i=0; i<nlev; i++) {
		Rcpp::List spl1[i];
	}
/*--------------------------------------------------
Fill the list with the row numbers that belong to each factor level
--------------------------------------------------*/
	for (j=0;  j<nlev; j++) {
		Rcpp::List listej;
		for (i=0; i<l1; i++) {
			if (fac(i) == j+1) {
				listej.push_back(i+1);
			}
		}
		spl1[j] = listej;
	}

—-
Jean THIOULOUSE - https://urlz.fr/jmA8 (Lab home page)
https://orcid.org/0000-0001-7664-0598 (ORCID page)
https://www.springer.com/fr/book/9781493988488 (ade4 Book)
https://jthome.thioulouse.fr/ref/