Getting different results for Sample

[shokoohi]

Hello
I ran the following code but I get different results

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadilloExtensions/sample.h>
using namespace Rcpp ;
// [[Rcpp::export]]
CharacterVector csample_char( CharacterVector x, 
                              int size,
                              bool replace, 
                              NumericVector prob = NumericVector::create()
) {
    CharacterVector ret = RcppArmadillo::sample(x, size, replace, prob) ;
    return ret ;
}


/*** R
N <- 10
set.seed(7)
sample.r <- sample(letters, N, replace=T)
print(sample.r)
set.seed(7)
sample.c <- csample_char(letters, N, replace=T)
print(sample.c)
print(identical(sample.r, sample.c))
*/

Here is what I get:

> N <- 10
> set.seed(7)
> sample.r <- sample(letters, N, replace=T)
> print(sample.r)
 [1] "j" "s" "g" "b" "o" "z" "v" "h" "x" "c"
> set.seed(7)
> sample.c <- csample_char(letters, N, replace=T)
> print(sample.c)
 [1] "z" "k" "d" "b" "g" "u" "i" "z" "e" "l"
> print(identical(sample.r, sample.c))
[1] FALSE

[eddelbuettel]

Wow. That is an excellent issue ticket. The only thought I had -- and it turns out it was the right one -- is that the original article on this sample() approach via RcppArmadillo is so old ... that it was affected by the one change made in R 3.6.0. See under 'Details' in help(set.seed):

 ‘sample.kind’ can be ‘"Rounding"’ or ‘"Rejection"’, or partial
 matches to these.  The former was the default in versions prior to
 3.6.0: it made ‘sample’ noticeably non-uniform on large
 populations, and should only be used for reproduction of old
 results.  See PR#17494 for a discussion.

When I use that with your code I get same sample as sample.c gets:

> RNGkind(sample.kind="Rounding")
Warning message:
In RNGkind(sample.kind = "Rounding") : non-uniform 'Rounding' sampler used
> set.seed(7)
> (sample.r <- sample(letters, N, replace=T))
 [1] "z" "k" "d" "b" "g" "u" "i" "z" "e" "l"
> print(identical(sample.r, sample.c))
[1] TRUE
> 

I will add a note to the article.

[eddelbuettel]

I just added short note to this piece. Thanks again for pointing this out.