On 23/10/2016 3:43 PM, Bert Gunter wrote:
Yes, variables in the formula should be handled by nse with the data
argument. Got it -- thanks. But still ... can with() be used to handle
those and/or any other variables in foo that appear as arguments. I see no
problems in doing so, but ... ?
One possible problem:
Formulas aren't just the bits you can see, they have environments
attached. The NSE part of evaluating them makes sure that the
environment contains the variables in the data argument early in the chain.
This means that your function can pass the evaluated formula to another
function, and it will carry the appropriate data with it.
If you evaluate a formula within with(), things will get complicated.
The environment attached to the formula in
with(foo, f( y ~ x1 + x2 + x3 + ... , data = foo, ...))
will likely contain two different copies of the variables in foo. The
first will be the usual one described above. But since formulas can
refer to things in the environment that called f(), it is added to the
chain of environments that are the parent of the first one.
Environments are reference objects, so f() could decide to modify some
of the variables. It would likely get very confused if there were two
copies of them, one in one environment, one in another.
So I'd advise to use one form or the other, i.e. don't use with(), or if
you do, don't use data=.
Duncan Murdoch
Bert
(But see inline below)
On Oct 23, 2016 7:24 PM, "Jeff Newmiller" <jdnew...@dcn.davis.ca.us> wrote:
No. And I don't know why you are conflating the treatment of variables in
the formula with treatment of variables passed as other arguments. It is
sort of like thinking the x symbols in foo$x[ x < 0 ] refer to the same
data.
In my query they explicitly do, though. Nevertheless your response was
apropos.
foo$y ~ foo$x1 + foo$x2 + foo$x3 is not preferable, and given the
availability of a data argument such redundancy is unnecessary. NSE is
already in use for the formula. It is not (necessarily) in use for the
other arguments, so you just have to learn which arguments are being
handled with NSE by any particular function and which are not... good docs
would be the preferred avenue but recognizing the error message that arises
when you fail to specify foo$ for the non-formula arguments gets me by if
the docs are unclear.
However, it is dangerous to apply NSE tricks recursively, so piling
"with" on top of the existing formula eval-with-data is only likely to
confuse the evaluation context even more.
This is what I'm not sure of. Can you give an example of when such
confusion would occur?
--
Sent from my phone. Please excuse my brevity.
On October 23, 2016 9:18:17 AM PDT, Bert Gunter <bgunter.4...@gmail.com>
wrote:
As has been noted oftimes on this list
f( y ~ x1 + x2 + x3 + ... , data = foo, ...)
is much preferable to
f( foo$y ~ foo$x1 + foo$x2 + foo$x3 + ..., ...)
(with no data argument), using nse = non-standard evaluation to set the
environment for formula evaluation. However, as queries here recently
demonstrate, the formula variables (y, x1, x2, x3, ...) or other
variables
in foo are also sometimes needed as further arguments of f, and these
have
to be explicitly and tediously given as foo$whatever or equivalent
indexing.
So my question is, can/should with() be used instead in the form
with(foo, f( y ~ x1 + x2 + x3 + ... , data = foo, ...)) with no
explicit
$ or indexing in ... variables?
or even
with(foo, f( y ~ x1 + x2 + x3 + ... , ...))
with no data argument for nse or indexing, though this seems to me
questionable in that it may affect the formula's environment
differently.(??)
Please correct any misstatements of fact in the above as well as
clarifying
anything else I seem confused about.
Many thanks.
Bert
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