[Rcpp-devel] '[' operator of NumericMatrix

[Dirk Eddelbuettel]

On 22 August 2012 at 17:11, Peng Yu wrote:
| Hi,
| 
| I'm trying to use '[' operator to extract elements from NumericMatrix.
| But it seems that it is not what I expected. Is it a bug or a feature?

Neither but mostly your misunderstanding of C syntax carrying forward in C++
as never ever worked in a matrix class.

In C (and C++), the expression   i,j  leaved just j.

You want    x(i, j)   here.

Dirk

| I can use Column to extract columns and then extract elements in each
| column, which is a tedious solution. Any better solution? Thanks!
| 
| BTW, where is '[' is documented for NumericMatrix?
| 
| > suppressPackageStartupMessages(library(inline))
| > src='
| + Rcpp::NumericMatrix x(mx);
| + for(unsigned i = 0; i < x.nrow(); ++ i) {
| +   for(unsigned j = 0; j < x.ncol(); ++ j) {
| +     std::cout << i << " " << j << " " << x[i, j] << std::endl;
| +   }
| + }
| + return Rcpp::wrap(0);
| + '
| > fun=cxxfunction(signature(mx="numeric"), src,
| +   plugin="Rcpp")
| > orig=matrix(1:12, 3, 4)
| > orig
|      [,1] [,2] [,3] [,4]
| [1,]    1    4    7   10
| [2,]    2    5    8   11
| [3,]    3    6    9   12
| > fun(orig)
| 0 0 1
| 0 1 2
| 0 2 3
| 0 3 4
| 1 0 1
| 1 1 2
| 1 2 3
| 1 3 4
| 2 0 1
| 2 1 2
| 2 2 3
| 2 3 4
| [1] 0
| >
| 
| 
| -- 
| Regards,
| Peng
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com