Hi listers,
I have a package asremlPlus on CRAN that manipulates formulae. It uses the
keep.order argument, as in the stats::terms function, to allow control over the
order of terms in a model.
However, when stats::update.formula is used to update a formula, there is no
keep.order argument for update.formula. The result is that the updated formula
is always simplified to a sum of single terms and these terms are always
re-ordered. I have written a function that is able to update a formula and keep
the order. The following example illustrates the problem and my solution.
#Functions to keep.order when a formula is updated
update_keep_order <- function(object, ...) {
UseMethod("update_keep_order")
}
update_keep_order.formula <- function(old, new, keep.order = TRUE) {
tmp <- .Call(stats:::C_updateform, as.formula(old), as.formula(new))
formula(terms.formula(tmp, keep.order = keep.order, simplify = TRUE))
}
#Generate some factors and a formula
facs <- expand.grid(A=1:2, B=1:2, C=1:2, D=1:2)
form <- with(facs, formula(~ A*B + C*D))
#Update with update.formula
(upd <- update(form, ~ . - C, keep.order = TRUE))
#Update with update_keep_order.formula
(upd_keep <- update_keep_order(form, ~ . - C, keep.order = TRUE))
However, the function calls the undocumented function stats::C_updateform and
so cannot be added to my package.
Is there another solution to achieve this outcome that does not require this
undocumented function and so could be incorporated in an R package on CRAN?
Thanks in advance for any help in solving this issue.
Cheers,
Chris
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