[R-pkg-devel] update.formula does not keep.order

[Duncan Murdoch]

On 27/10/2021 7:16 p.m., Chris Brien wrote:
> 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.

I can't think of a short term solution.  The long term solution is to 
prepare and submit a patch to R to add the keep_order argument to 
update.formula.

As a workaround, perhaps you could modify the result of update() to 
restore the original order.  This sounds messy, but if you already have 
code to manipulate formulae, maybe it's mostly in place.  The steps 
would be:

  Decompose the formula into a vector of terms, e.g. ~A + B + A:B + C + 
D + C:D becomes

old <- expression(A, B, A:B, C, D, C:D)

  Update the formula, and decompose the new formula in the same way.

So A + B + D + A:B + C:D becomes

new <- expression(A, B, D, A:B, C:D)


  Match each term in the new vector to its location in the original 
vector, and sort it.

index <- match(new, old)
sorted <- order(index)
new[sorted]

In your example, this gives expression(A, B, A:B, D, C:D) .


  (You'll need to decide what to do with terms that weren't present in 
the original.  They'll have NA values in index.  Presumably they go to 
the end.)

  Rebuild the formula after sorting.

Duncan Murdoch