From: Henrik Bengtsson <hb_at_biostat.ucsf.edu>

Date: Wed, 03 Nov 2010 11:07:51 -0700

R-devel_at_r-project.org mailing list

https://stat.ethz.ch/mailman/listinfo/r-devel Received on Wed 03 Nov 2010 - 18:10:21 GMT

Date: Wed, 03 Nov 2010 11:07:51 -0700

On Wed, Nov 3, 2010 at 11:02 AM, Henrique Dallazuanna <wwwhsd_at_gmail.com> wrote:

> The resample function in the example section from sample help page does it

*> or not?
*

Yes, I just noticed that one [at the very end of the example in help("sample")]. So, maybe resample() should be a function available in R?

/Henrik

*>
*

> On Wed, Nov 3, 2010 at 3:54 PM, Henrik Bengtsson <hb@biostat.ucsf.edu>wrote:

*>
**>> Hi, consider this one as an FYI, or a seed for further discussion.
**>>
**>> I am aware that many traps on sample() have been reported over the
**>> years. I know that these are also documents in help("sample"). Still
**>> I got bitten by this while writing
**>>
**>> sample(units, size=length(units));
**>>
**>> where 'units' is an index (positive integer) vector. It works in all
**>> cases as expected (=I expect) expect for length(units) == 1. I know,
**>> it is well known. However, it got to make me wonder if it is possible
**>> to use sample() to draw a single value from a set containing only one
**>> value. I don't think so, unless you draw from a value that is <= 1.
**>>
**>> For instance, you can sample from c(10,10) by doing:
**>>
**>> > sample(rep(10, times=2), size=2);
**>> [1] 10 10
**>>
**>> but you cannot sample from c(10) by doing:
**>>
**>> > sample(rep(10, times=1), size=1);
**>> [1] 9
**>>
**>> unless you sample from a value <= 1, e.g.
**>>
**>> sample(rep(0.31, times=1), size=1);
**>> [1] 0.31
**>>
**>> sample(rep(-10, times=1), size=1);
**>> [1] -10
**>>
**>> Note also the related issue of sampling from a double vector of length 1,
**>> e.g.
**>>
**>> > sample(rep(1.2, times=2), size=2);
**>> [1] 1.2 1.2
**>> > sample(rep(1.2, times=1), size=1);
**>> [1] 1
**>>
**>> I the latter case 1.2 is coerced to an integer.
**>>
**>> All of the above makes sense when one study the code of sample(), but
**>> sample() is indeed dangerous, e.g. imagine how many bootstrap
**>> estimates out there quietly gets incorrect.
**>>
**>>
**>> In order to cover all cases of length(units), including one, a solution is:
**>>
**>> sampleFrom <- function(x, size=length(x), ...) {
**>> n <- length(x);
**>> if (n == 1L) {
**>> res <- x;
**>> } else {
**>> res <- sample(x, size=size, ...);
**>> }
**>> res;
**>> } # sampleFrom()
**>>
**>> > sampleFrom(rep(10, times=2), size=2);
**>> [1] 10 10
**>>
**>> > sampleFrom(rep(10, times=1), size=1);
**>> [1] 10
**>>
**>> > sampleFrom(rep(0.31, times=1), size=1);
**>> [1] 0.31
**>>
**>> > sampleFrom(rep(-10, times=1), size=1);
**>> [1] -10
**>>
**>> > sampleFrom(rep(1.2, times=2), size=2);
**>> [1] 1.2 1.2
**>>
**>> > sampleFrom(rep(1.2, times=1), size=1);
**>> [1] 1.2
**>>
**>>
**>> I want to add sampleFrom() to the wishlist of functions to be
**>> available in default R. Alternatively, one can add an argument
**>> 'sampleFrom=FALSE' to the existing sample() function. Eventually such
**>> an argument can be made TRUE by default.
**>>
**>> /Henrik
**>>
**>> ______________________________________________
**>> R-devel_at_r-project.org mailing list
**>> **https://stat.ethz.ch/mailman/listinfo/r-devel
**>>
**>
**>
**>
**> --
**> Henrique Dallazuanna
**> Curitiba-Paranį-Brasil
**> 25° 25' 40" S 49° 16' 22" O
**>
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**>
**>
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**>
**>
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