Am 14.12.21 um 21:57 schrieb Prof Brian Ripley:
On 14/12/2021 20:26, Blackwell, Matthew wrote:
Hello all,
In attempting to create a one-sided formula from a two-sided formula,
I discovered that the following syntax will successfully complete this
operation:
f <- y ~ x + z
f[2] <- NULL
f
~x + z
str(f)
Class 'formula' language ~x + z
..- attr(*, ".Environment")=<environment: R_GlobalEnv>
In searching through the formula documentation, I couldn't find this
technique as documented and wondered whether or not it is expected and
if it makes sense to develop a package against the behavior. I'm using
R 4.1.0, but I see the same on R-devel (r81303). I asked on Twitter,
but someone thought this list might be a better venue.
Apologies if I missed some documentation and thanks in advance.
See ?"~", which says
A formula has mode ‘call’. It can be subsetted by ‘[[’: the
components are ‘~’, the left-hand side (if present) and the
right-hand side _in that order_.
That would suggest that
f <- y ~ x + z
f[[2]] <- NULL
was the documented way (and the one I would have used).
I'd also mention delete.response() here. It takes a "terms" object (a
formula with attributes) and uses the same technique internally to
remove the response -- if there is one... I.e., be sure that
length(f)==3 before dropping the second element.
Best regards,
Sebastian Meyer
However, ?"["
says
‘[’ and ‘[[’ are sometimes applied to other recursive objects such
as calls and expressions. Pairlists are coerced to lists for
extraction by ‘[’, but all three operators can be used for
replacement.
Cheers,
Matt
~~~~~~~~~~
Matthew Blackwell
Associate Professor of Government
Harvard University
https://www.mattblackwell.org
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