Dynamically increase size of list in Rcpp

Question

I am trying to implement a "coupling to the past" algorithm in Rcpp. For this I need to store a matrix of random numbers, and if the algorithm did not converge create a new matrix of random numbers and store that as well. This might have to be done 10+ times or something until convergence.

I was hoping I could use a List and dynamically update it, similar as I would in R. I was actually very surprised it worked a bit but I got errors whenever the list size becomes large. This seems to make sense as I did not allocate the needed memory for the additional list elements, although I am not that familiar with C++ and not sure if that is the problem.

Here is an example of what I tried. however be aware that this will probably crash your R session:

library("Rcpp")

cppFunction(
includes = ' 
NumericMatrix RandMat(int nrow, int ncol)
 {
  int N = nrow * ncol;
  NumericMatrix Res(nrow,ncol);
  NumericVector Rands  = runif(N);
   for (int i = 0; i < N; i++) 
  {
    Res[i] = Rands[i];
  }
  return(Res);
 }',

code = '
void foo()
{
  // This is the relevant part, I create a list then update it and print the results:
  List x;
  for (int i=0; i<10; i++)  
  {
   x[i] = RandMat(100,10);
   Rf_PrintValue(wrap(x[i]));
  }
}
')


foo()

Does anyone know a way to do this without crashing R? I guess I could initiate the list at a fixed amount of elements here, but in my application the amount of elements is random.

Answer 1

You have to "allocate" enough space for your list. Maybe you can use something like a resizefunction:

List resize( const List& x, int n ){
    int oldsize = x.size() ;
    List y(n) ;
    for( int i=0; i<oldsize; i++) y[i] = x[i] ;
    return y ;
}

and whenever you want your list to be bigger than it is now, you can do:

x = resize( x, n ) ;

Your initial list is of size 0, so it expected that you get unpredictable behavior at the first iteration of your loop.