[R] unexpected subset select results?

[ivo welch]

thanks, everyone.  I did not even know about transform() and with(),
but they look quite useful.

actually, what I had intended to state was that Boolean variables in
select parts of subset statements, especially when mixed with other
variables that are just part of the data frame, leads to unexpected
results.  (my d statement was intended to show how easy it is to
forget that a variable is not part of the data frame, but just part of
the global environment.)  unless this mixed-treatment covers an
important functional aspect, this might be better to cause a warning
(or an error) than a silent recycling of variables.

(Related: The recycling rules are generally convenient, but can also
be rather problematic in catching errors.  It would be nice to be able
to turn them off.)

regards,

/iaw


On Tue, Aug 24, 2010 at 3:55 AM, Gavin Simpson <gavin.simpson at ucl.ac.uk> wrote:
> On Mon, 2010-08-23 at 17:51 -0400, ivo welch wrote:
>> quizz---what does this produce?
>
> Henrique has provided an answer to the question, but...
>
>>    d=data.frame( a=1:1000, b=2001:3000, z= 5001:6000 )
>>    attach(d); c <- (a+b)>25; detach(d)
>
> ...this is ugly and will potentially catch you out one day if you forget
> to detach. These three calls can be achieved using a single with() :
>
> c <- with(d, (a + b) > 25)
>
> And the version you wanted:
>
> attach(d); d$c <- (a+b)>25; detach(d)
>
> can be done using within():
>
> d <- within(d, c <- (a + b) > 25)
>
> and with the latter, the intention is pretty clear.
>
> HTH
>
> G
>
>>    d= subset(d, TRUE, select=c( a, b, c ))
>>
>> yes, I know I have made a mistake, in that the code does not do what I
>> presumably would have wanted.  it does seem like unexpected behavior,
>> though, without an error.  there probably is some reason why this does
>> not ring an alarm bell...
>>
>> /iaw
>> ----
>> Ivo Welch (ivo.welch at brown.edu, ivo.welch at gmail.com)
>>
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>
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