Implementing trees in R

Hi all,

   I am rather new to R. Recently I have been trying to implement some
tree algorithms in R. I used lists to model tree nodes. I thought
something like this would work:

   parent <- list();
   child <- list();
   parent$child1 <- child;
   child$parent <- parent;

   When I tried to check whether a node is its parent's first child
using "if (node$parent$child1 == node)", it always returned false. Then
I realized that it does not really work because "parent$child1 <- child"
actually makes a copy of child instead of referencing it. I think one
possible fix is to keep a list of node objects, and make references
using the positions in the list. For example, I think the following
would work:

   parent <- list();
   child <- list();
   nodes <- list(parent, child);
   parent$child1 <- 2;
   child$parent <- 1;

   Then the "first child" test can be rewritten as "if
(nodes[[nodes[[nodeId]]$parent]]$child1 == nodeId)". However, I would
prefer not to implement trees in this way, as it requires the
inconvenient and error-prone manipulations of node IDs.

   May I know if there is a way to make object references to lists? Or
are there other ways to implement tree data structures in R?

   BTW, I checked how hclust was implemented, and noticed that it calls
an external Fortran program. I would want a solution not involving any
external programs.

   Thanks.

--


       God bless.

       Kevin