[Rcpp-devel] How to modifying the elements of a list in the global environment?

[Giovanni Petris]

Thanks Dirk!

By the way, I just figured out a working solution for what I wanted to achieve - 'code_f' below.

Best,
Giovanni
=====

### Small reproducible example
K <- 2; N <- 5; M <- 3
x <- matrix(rnorm(N * M), N, M)
mu <- matrix(rnorm(M * K, mean = 1 : K), M, K, TRUE)

### What I would do in R
f_R <- function() {
    ## center observations in 'x' using columns of 'mu'
    w <- lapply(1L:K, function(i) x - rep(mu[, i], each = N))
    ## eventually more stuff here...
    w
}

### For the Rcpp part, set aside working space in the global environment
workspace <- lapply(1:K, function(k) matrix(0.0, N, M))

code_f <- '
    Rcpp::NumericMatrix cpp_x(x);
    Rcpp::NumericMatrix cpp_mu(mu); 
    int K = cpp_mu.ncol();
    int M = cpp_mu.nrow();
    int N = cpp_x.nrow();
    Environment glob = Environment::global_env(); 
    List cpp_work(glob.get("workspace"));
    NumericMatrix tmpMatrix;

    for (int k=0; k < K; k++) {
        tmpMatrix = as<SEXP>(cpp_work[k]);
        for (int i=0; i < M; i++) 
            tmpMatrix(_, i) = cpp_x(_, i) - cpp_mu(i, k);
    }

    return R_NilValue;
'
## library(Rcpp); library(inline)
f_cpp <- cxxfunction(signature(x = "numeric", mu = "numeric"), code_f, plugin = "Rcpp")

## try it
f_cpp(x, mu)
all.equal(workspace, f_R())


________________________________________
From: Dirk Eddelbuettel [edd at debian.org]
Sent: Thursday, October 18, 2012 9:15 PM
To: Giovanni Petris
Cc: Dirk Eddelbuettel; rcpp-devel at lists.r-forge.r-project.org
Subject: RE: [Rcpp-devel] How to modifying the elements of a list in the        global  environment?

On 19 October 2012 at 01:32, Giovanni Petris wrote:
| Hi Dirk,
|
| Thank you for the quick reply. I will look for more examples on the net.
|
| About your suggestion of allocating scrap space inside the C++ routine, am I wrong to think that when the matrices are large and the function is called repeatedly within a Gibbs sampler loop, this is not a very efficient approach?

Yes, if you want to use it several times you can pass it around for reuse.
As I mentioned, a reference is just a pointer and hence unrelated to the size
of the object.

Dirk

| Thanks again!
|
| Best,
| Giovanni
|
| ________________________________________
| From: Dirk Eddelbuettel [edd at debian.org]
| Sent: Thursday, October 18, 2012 8:06 PM
| To: Dirk Eddelbuettel
| Cc: Giovanni Petris; rcpp-devel at lists.r-forge.r-project.org
| Subject: Re: [Rcpp-devel] How to modifying the elements of a list in the        global  environment?
|
| Giovanni,
|
| On 18 October 2012 at 17:46, Dirk Eddelbuettel wrote:
| | | > code_f <- '
| | | +     Rcpp::NumericMatrix cpp_x(x);
| | | +     Rcpp::NumericMatrix cpp_mu(mu);
| | | +     int K = cpp_mu.ncol();
| | | +     int M = cpp_mu.nrow();
| | | +     int N = cpp_x.nrow();
| | | +     Environment glob = Environment::global_env();
| | | +     List cpp_work(glob.get("workspace"));
| | | +
| | | +     for (int k=0; k < K; k++) {
| | | +         cpp_work[k] = clone(cpp_x);
| | | +         for (int i=0; i < M; i++)
| | | +             cpp_work(_, i) = cpp_work(_, i) - cpp_mu(i, k);
|
| That can't work.  You have the types confused.
|
| You could just pass the list object down, and then pick from it.  Working
| examples for that are in the list archives, in the RcppExamples package, and
| in other places on the net.
|
| Or, as I suggested, just allocate the scrap space inside the C++ routine.
|
| Hth,  Dirk
|
| --
| Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com

--
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com