Index element from list in Rcpp - r

Question

Suppose I have a List in Rcpp, here called x containing matrices. I can extract one of the elements using x[0] or something. However, how do I extract a specific element of that matrix? My first thought was x[0](0,0) but that does not seem to work. I tried using * signs but also doesn't work.
Here is some example code that prints the matrix (shows matrix can easily be extracted):
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()
{
List x;
x[0] = RandMat(3,3);
Rf_PrintValue(wrap( x[0] )); // Prints first matrix in list.
}
')
foo()
How could I change the line Rf_PrintValue(wrap( x[0] )); here to print the the element in the first row and column? In the code I want to use it for I need to extract this element to do computations.

Answer 1

Quick ones:
Compound expression in C++ can bite at times; the template magic gets in the way. So just assign from the List object to a whatever the element is, eg a NumericMatrix.
Then pick from the NumericMatrix as you see fit. We have row, col, element, ... access.
Printing can be easier using Rcpp::Rcout << anElement but note that we currently cannot print entire matrices or vectors -- but the int or double types are fine.
Edit:
Here is a sample implementation.
#include <Rcpp.h>
// [[Rcpp::export]]
double sacha(Rcpp::List L) {
double sum = 0;
for (int i=0; i<L.size(); i++) {
Rcpp::NumericMatrix M = L[i];
double topleft = M(0,0);
sum += topleft;
Rcpp::Rcout << "Element is " << topleft << std::endl;
}
return sum;
}
/*** R
set.seed(42)
L <- list(matrix(rnorm(9),3), matrix(1:9,3), matrix(sqrt(1:4),2))
sacha(L) # fix typo
*/
And its result:
R> Rcpp::sourceCpp('/tmp/sacha.cpp')
R> set.seed(42)
R> L <- list(matrix(rnorm(9),3), matrix(1:9,3), matrix(sqrt(1:4),2))
R> sacha(L)
Element is 1.37096
Element is 1
Element is 1
[1] 3.37096
R>

Answer 2

You have to be explicit at some point. The List class has no idea about the types of elements it contains, it does not know it is a list of matrices.
Dirk has shown you what we usually do, fetch the element as a NumericMatrix and process the matrix.
Here is an alternative that assumes that all elements of your list have the same structure, using a new class template: ListOf with enough glue to make the user code seamless. This just moves to a different place the explicitness.
#include <Rcpp.h>
using namespace Rcpp ;
template <typename WHAT>
class ListOf : public List {
public:
template <typename T>
ListOf( const T& x) : List(x){}
WHAT operator[](int i){ return as<WHAT>( ( (List*)this)->operator[]( i) ) ; }
} ;
// [[Rcpp::export]]
double sacha( ListOf<NumericMatrix> x){
double sum = 0.0 ;
for( int i=0; i<x.size(); i++){
sum += x[i](0,0) ;
}
return sum ;
}
/*** R
L <- list(matrix(rnorm(9),3), matrix(1:9,3), matrix(sqrt(1:4),2))
sacha( L )
*/
When I sourceCpp this file, I get:
> L <- list(matrix(rnorm(9), 3), matrix(1:9, 3), matrix(sqrt(1:4), 2))
> sacha(L)
[1] 1.087057