Index element from list in Rcpp

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:

1. 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.

2. Then pick from the NumericMatrix as you see fit. We have row, col, element, ... access.

3. 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>