[Rcpp-devel] "transposing" a list

[Antonio Piccolboni]

That worked, thanks


Antonio


On Tue, Nov 26, 2013 at 2:48 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

>
> On 26 November 2013 at 14:26, Antonio Piccolboni wrote:
> | Hi all,
> | I have a list of lists, the inner lists all of the same lengths. I would
> like
> | to create a new list  of lists such that ll[[i]][[j]] == tll[[j]][[i]]`,
> |  where the old one is ll and the new one is tll. In R this can be
> achieved by
> | the function
> |
> | f = splat(mapply.list)
> |
> | mapply.list =
> |   function(...) mapply(FUN=list, ..., SIMPLIFY = FALSE)
> |
> | In my application these lists can get pretty long (both inner and outer)
> so
> | here's my attempt with Rcpp. The empty case is checked in R code, so the
> outer
> | list is non-empty
> |
> | SEXP t_list(SEXP _ll) {
> |   Rcpp::List ll(_ll);
> |   Rcpp::List first_col(Rcpp::wrap(ll[0]));
> |   std::vector<std::vector<SEXP> > tll(first_col.size());
> |   for(int i = 0; i < ll.size(); i++) {
> |     Rcpp::List l(Rcpp::wrap(ll[i]));
> |     for(int j = 0; j < l.size(); j++)
> |       tll[j].push_back(Rcpp::clone(Rcpp::wrap(l[j])));}
> |   std::vector<SEXP> results ;
> |   for(int i = 0; i < tll.size(); i++) {
> |      results.push_back(Rcpp::clone(Rcpp::wrap(tll[i])));}
> |   return(Rcpp::wrap(results));}
>
> What is in the inner lists?  Do you really need STL types, or is this for
> convenience?
>
> "Our" types are thin proxies.  Generally no copies.
>
> You are mixing our types, as<>(), wrap(), push_back() as well as clone()
> quite extensively and be almost guaranteed that you will create copies
> here.
> There may be a better way involving few copies, but this is somewhat
> dependent on your data and problem.
>
> | It works just fine for a lists of three lists of length 10 or 100, but
> at 1000
> | it starts mixing up or skipping elements and it can also crash the
> interpreter.
> | One set of test lists was created with ll = list(as.list(1:10^n), as.list
> | (rnorm(10^n)), as.list(as.factor(1:10^n))) where n was set to 2, 3  or 4
> in
> | different tests. I suppose this code is somehow reusing memory that
> should
> |  not be reused and when garbage collection happens the errors start, but
> I
> | don't see how that could happen. Suggestions? Thanks
>
> Avoid clone.
>
> Avoid wrap if you don't need it.
>
> Dirk
>
>
> | Antonio
> |
> |
> | ----------------------------------------------------------------------
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> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
>
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