A different error in sample()

On 20 Sep 2018, at 01:53 , lmo via R-devel <r-devel at r-project.org> wrote:

Although it seems to be pretty weird to enter a numeric vector of length one that is not an integer as the first argument to sample(), the results do not seem to match what is documented in the manual. In addition, the results below do not support the use of round rather than truncate in the documentation. Consider the code below.
The first sentence in the details section says: "If x has length 1, is numeric (in the sense of is.numeric) and x >= 1, sampling via sample takes place from 1:x."
In the console:> 1:2.001
[1] 1 2

1:2.9

[1] 1 2

truncation:

trunc(2.9)

[1] 2

So, this seems to support the quote from in previous emails: "Non-integer positive numerical values of n or x will be truncated to the next smallest integer, which has to be no larger than .Machine$integer.max."
However, again in the console:> set.seed(123)

table(sample(2.001, 10000, replace=TRUE))

   1    2    3 
5052 4941    7

So, neither rounding nor truncation is occurring. Next, define a sequence.

x <- seq(2.001, 2.51, length.out=20)

Now, grab all of the threes from sample()-ing this sequence.

set.seed(123)
threes <- sapply(x, function(y) table(sample(y, 10000, replace=TRUE))[3])

Check for NAs (I cheated here and found a nice seed).> any(is.na(threes))
[1] FALSE
Now, the (to me) disturbing result.

is.unsorted(threes)

[1] FALSE

or equivalently

all(diff(threes) > 0)

[1] TRUE

So the number of threes grows monotonically as 2.001 moves to 2.5. As I hinted above, the monotonic growth is not assured. My guess is that the growth is stochastic and relates to some "probability weighting" based on how close the element of x is to 3. Perhaps this has been brought up before, but it seems relevant to the current discussion.
A potential aid to this issue would be something like
if(length(x) == 1 && !all.equal(x, as.integer(x))) warning("It is a bad idea to use vectors of length 1 in the x argument that are not integers.")
Hope that helps,luke

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