R devel - May 2009 - Bug in base function sample ( ) (PR#13727)

Full_Name: Michael Chajewski
Version: 2.9.0
OS: Windows XP
Submission from: (NULL) (150.108.71.185)


I was programming a routine which kept reducing the array from which a random
sample was taken, resulting in a single number. I discovered that when R
attempts to sample from an object with only one number it does not
reproduce/report the number but instead chooses a random number between 1 and
that number. 

Example 1:

# I am assigning a single number
gg <- 7

# Creating an array to store sampled values
ggtrack <- 0

# I am sampling 10,000 observations from my single value
# object and storing them
for (i in 1:10000) {
	g0 <- sample(gg, (i/i))
	ggtrack <- c(ggtrack,g0)
}

# Deleting the initial value in the array
ggtrack <- ggtrack[-1]

# The array ought to be 10,000 samples long (and it is)
length(ggtrack)

# The array should contain 10,000 "7", but it does not
# See the histogram of sampled values
hist(ggtrack)

Example 2:

# Here is the same example, but now with
# two number. Note that now the function performs
# as expected and only samples between the two.

gg <- c(7,2)
ggtrack <- 0
for (i in 1:10000) {
	g0 <- sample(gg, (i/i))
	ggtrack <- c(ggtrack,g0)
}

ggtrack <- ggtrack[-1]
length(ggtrack)
hist(ggtrack)


Highest Regards,
Michael Chajewski

On Thu, 2009-05-28 at 09:30 +0200, chajewski at fordham.edu
wrote:
> Full_Name: Michael Chajewski
> Version: 2.9.0
> OS: Windows XP
> Submission from: (NULL) (150.108.71.185)
> 
> 
> I was programming a routine which kept reducing the array from which a
random
> sample was taken, resulting in a single number. I discovered that when R
> attempts to sample from an object with only one number it does not
> reproduce/report the number but instead chooses a random number between 1
and
> that number. 
This is working as documented/intended in ?sample. 'x' is of length 1,
so it is interpreted as 1:x (if x >=1), resulting in the behaviour you
have encountered.

That help page even goes so far as to warn you that this "convenience
feature may lead to undesired behaviour..." and gives an example
function (in Examples) that handles the sort of use case you have. See
the Examples section and the resample() function created there.

HTH

G
> 
> Example 1:
> 
> # I am assigning a single number
> gg <- 7
> # Creating an array to store sampled values
> ggtrack <- 0
> 
> # I am sampling 10,000 observations from my single value
> # object and storing them
> for (i in 1:10000) {
> 	g0 <- sample(gg, (i/i))
> 	ggtrack <- c(ggtrack,g0)
> }
> 
> # Deleting the initial value in the array
> ggtrack <- ggtrack[-1]
> 
> # The array ought to be 10,000 samples long (and it is)
> length(ggtrack)
> 
> # The array should contain 10,000 "7", but it does not
> # See the histogram of sampled values
> hist(ggtrack)
> 
> Example 2:
> 
> # Here is the same example, but now with
> # two number. Note that now the function performs
> # as expected and only samples between the two.
> 
> gg <- c(7,2)
> ggtrack <- 0
> for (i in 1:10000) {
> 	g0 <- sample(gg, (i/i))
> 	ggtrack <- c(ggtrack,g0)
> }
> 
> ggtrack <- ggtrack[-1]
> length(ggtrack)
> hist(ggtrack)
> 
> 
> Highest Regards,
> Michael Chajewski
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
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>
> ...I discovered that when R attempts to sample from an object with only one
> number it does not
> reproduce/report the number but instead chooses a random number between 1
> and that number.
>
This is the documented behavior.

In my opinion, it is a design error, but changing it would no doubt break
lots of code.

As a general rule, the designers of R seem to have preferred convenience to
consistency, which often makes things easier or more concise, but sometimes
causes unfortunate surprises like this.

           -s

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