R devel - Jun 2010 - proposed change to 'sample'

There is a weakness in the 'sample'
function that is highlighted in the
help file.  The 'x' argument can be
either the vector from which to sample,
or the maximum value of the sequence
from which to sample.

This can be ambiguous if the length of
'x' is one.

I propose adding an argument that allows
the user (programmer) to avoid that
ambiguity:

function (x, size, replace = FALSE, prob = NULL,
     max = length(x) == 1L && is.numeric(x) && x >= 1)
{
     if (max) {
         if (missing(size))
             size <- x
         .Internal(sample(x, size, replace, prob))
     }
     else {
         if (missing(size))
             size <- length(x)
         x[.Internal(sample(length(x), size, replace, prob))]
     }
}
<environment: namespace:base>


This just takes the condition of the first
'if' to be the default value of the new 'max'
argument.

So in the "surprise" section of the examples
in the 'sample' help file

sample(x[x > 9])

and

sample(x[x > 9], max=FALSE)

have different behaviours.

By the way, I'm certainly not convinced that
'max' is the best name for the argument.

-- 
Patrick Burns
pburns at pburns.seanet.com
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')

> -----Original Message-----
> From: r-devel-bounces at r-project.org 
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Patrick Burns
> Sent: Sunday, June 20, 2010 3:08 AM
> To: r-devel at r-project.org
> Subject: [Rd] proposed change to 'sample'
> 
> There is a weakness in the 'sample'
> function that is highlighted in the
> help file.  The 'x' argument can be
> either the vector from which to sample,
> or the maximum value of the sequence
> from which to sample.
> 
> This can be ambiguous if the length of
> 'x' is one.
> 
> I propose adding an argument that allows
> the user (programmer) to avoid that
> ambiguity:
> 
> function (x, size, replace = FALSE, prob = NULL,
>      max = length(x) == 1L && is.numeric(x) && x >= 1)
S+'s sample() has an argument 'n' to achieve
the same result.  It has been there since at
least 2005 (S+ 7.0.6).  sample(n=n) means to
return a sample from seq_along(n), where n must
be a scalar nonnegative integer.  sample(x=x)
retains it old ambiguous meaning.
  sample(x, size = n, replace = F, prob = NULL, n = NULL, ...)

S+ also has an rsample function where n (with
the same meaning) is the only way to specify the
population.  It also has an order=TRUE/FALSE argument
where order=TRUE means to randomly order the output.
order=FALSE means that the ordering of the output is
unspecified, but it allows the person writing rsample
methods to use the quickest way to get a random sample
(for big data it can be fastest to return the sample
from 1:n in increasing order). 
  rsample(n, size = n, replace = F, prob = NULL,
        bigdata = F, minimal = NULL, ..., order = T)
I like the idea of separating the concepts of sampling
and permuting data.  Many statistics are invariant to
ordering of the data and it can be a waste of time
to randomly order a sample to feed to such functions.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> {
>      if (max) {
>          if (missing(size))
>              size <- x
>          .Internal(sample(x, size, replace, prob))
>      }
>      else {
>          if (missing(size))
>              size <- length(x)
>          x[.Internal(sample(length(x), size, replace, prob))]
>      }
> }
> <environment: namespace:base>
> 
> 
> This just takes the condition of the first
> 'if' to be the default value of the new 'max'
> argument.
> 
> So in the "surprise" section of the examples
> in the 'sample' help file
> 
> sample(x[x > 9])
> 
> and
> 
> sample(x[x > 9], max=FALSE)
> 
> have different behaviours.
> 
> By the way, I'm certainly not convinced that
> 'max' is the best name for the argument.
> 
> -- 
> Patrick Burns
> pburns at pburns.seanet.com
> http://www.burns-stat.com
> (home of 'Some hints for the R beginner'
> and 'The R Inferno')
> 
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>