Re: [Rd] Dropping RHS of a formula using NULL assignment
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")= 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 __ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
Re: [Rd] Dropping RHS of a formula using NULL assignment
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")= 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). 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 -- Brian D. Ripley, rip...@stats.ox.ac.uk Emeritus Professor of Applied Statistics, University of Oxford __ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
Re: [Rd] Dropping RHS of a formula using NULL assignment
On 14/12/2021 3:26 p.m., 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")= 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. The source "y ~ x + z" parses to a call to the `~` function with arguments y and x + z. Calls have the function as the first element, and arguments follow: so f[1] would be ~, f[2] would be y, f[3] would be x + z. You can see this if you pass f through as.list(): > as.list(f) [[1]] `~` [[2]] y [[3]] x + z Setting element 2 to NULL removes it, so you see > f[2] <- NULL > as.list(f) [[1]] `~` [[2]] x + z I think it's safe to make use of this even if it's undocumented. It's a pretty basic aspect of formulas. I'd guess there are lots of packages already using it, but I can't point to any particular examples. (I've ignored the difference between an unevaluated formula and an evaluated one, but they're almost the same, the only important difference in the attributes: evaluating it gives it a class and an environment.) Duncan Murdoch __ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
[Rd] Dropping RHS of a formula using NULL assignment
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")= 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. Cheers, Matt ~~ Matthew Blackwell Associate Professor of Government Harvard University https://www.mattblackwell.org __ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
