On Aug 24, 2010, at 7:36 AM, ivo welch wrote:
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.
I think you will find intense resistance to the request for a warning
when arguments to functions like table() and subset() are given
vectors of the correct length but are not part of a data.frame or in
hte data argument. I suppose the subset situation might be arguably
different than the table situation, but it is rather common practice
to construct utility index or flag vectors that are never incorporated
into part of the main data,frame that is being analyzed.
(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.)
You do get warnings and it is possible to raise the level of action
taken by the system to that of an error.
?options
... and take note of hte various options beginning with "warn...
... or take note of the error option and write your own.
regards,
/iaw
On Tue, Aug 24, 2010 at 3:55 AM, Gavin Simpson <gavin.simp...@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.we...@brown.edu, ivo.we...@gmail.com)
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