[R] Using update() within a function with a changed formula

[Wolfgang Viechtbauer]

Perfectly clear. Thanks!

Best,
Wolfgang

On Sat, 13 Nov 2021, Duncan Murdoch wrote:
> On 13/11/2021 2:24 p.m., Viechtbauer, Wolfgang (SP) wrote:
>> Hello all,
>> 
>> Say I would like to change the outcome in a formula to a variable not part 
>> of the original dataset. This works just fine:
>> 
>> res <- lm(mpg ~ wt + cyl, data=mtcars)
>> res
>> y <- rnorm(nobs(x))
>> update(x, formula = y ~ .)
>> 
>> But not when doing so within a function:
>> 
>> rm(y)
>> 
>> f <- function(x) {
>>     y <- rnorm(nobs(x))
>>     update(x, formula = y ~ .)
>> }
>> 
>> f(res)
>> 
>> Is there a way to make this work? Using y <<- ... inside the function 
>> works, but I would like to avoid such a heavy-handed approach.
>
> Formulas have associated environments, and that's where functions using them 
> look for variables.  update() leaves the formula environment unchanged, as 
> documented in ?update.formula. Since yours started out as globalenv(), you 
> would need to put y there, and that's what you want to avoid.
>
> However, the default update() method (the one you call on res, that calls the 
> formula method) allows you to change other things.
>
> So you could do it this way:
>
> res <- lm(mpg ~ wt + cyl, data=mtcars)
>
> f <- function(x) {
>  y <- rnorm(nobs(x))
>  newdata <- cbind(model.frame(x), y)
>  update(x, formula = y ~ ., data = newdata)
> }
> f(res)
>
> Duncan Murdoch