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

[Jean Thioulouse]

Thanks for the (private) answers that helped me to solve the problem, by just removing these stupid useless lines.

Jean

> Le 27 janv. 2023 à 19:30, THIOULOUSE JEAN <Jean.Thioulouse at univ-lyon1.fr> a écrit :
> 
> 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/
> 
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