[Rcpp-devel] add new components to list without specifying list size initially

[Dirk Eddelbuettel]

On 11 August 2011 at 03:06, Walrus Foolhill wrote:
| Hello,
| I need to create a list and then fill it sequentially by adding components in a
| for loop. Here is an example that works:
| 
| library(inline)
| src <- '
| Rcpp::List mylist(2);
| for(int i=0; i<2; ++i)
|   mylist[i] = i;
| mylist.names() = CharacterVector::create("a","b");
| return mylist;
| '
| fun <- cxxfunction(body=src, plugin="Rcpp")
| print(fun())
| 
| But what I really want is to create an empty list and then fill it, that is
| without specifying its number of components before hand... This is because I
| don't know in advance at which step of the for loop I will need to create a new
| component. Here is an example, that obviously doesn't work, but that should
| show what I am looking for:
| 
| Rcpp::List mylist;
| CharacterVector names = CharacterVector::create("a", "b");

If you know how long names is, you know how long mylist going to be ....

| for(int i=0; i<2; ++i){
|   mylist.add(names[i], IntegerVector::create());
|   mylist[names[i]].push_back(i);

I don't understand what that is trying to do.

| }
| return mylist;
| 
| Do you know how I could achieve this? Thanks.

Rcpp::List is an alias for Rcpp::GenericVector, and derives from Vector. You
can look at the public member functions -- there are things like

    push_back()
    push_front()
    insert()
    
etc that behave like STL functions __but are inefficient as we (almost
always) need to copy the whole object__ so they are not recommended.

When I had to deal with 'unknown quantities of data' returning I was mostly
able to either turn it into a 'fixed or known columns, unknow rows' problem
(easy, just grow row-wise) or I 'cached' in a C++ data structure first before
returning to R via Rcpp structures -- and then I knew the dimensions for the
to-be-created object too.

Dirk


-- 
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http://dirk.eddelbuettel.com/blog/2011/08/04#rcpp_classes_2011-09_and_2011-10