[R] trouble automating formula edits when log or * are present; update trouble

[R. Michael Weylandt]

Hi Paul,

I haven't quite thought through this yet, but might it not be easier
to convert your formula to a character and then use gsub et al on it
directly?

Something like this

# Using m2 as you set up below
m2 <- lm(y ~ log(x1) + x2*x3, data=dat)

f2 <- formula(m2)

as.formula(paste(f2[2], f2[1],gsub("x1", "x1c", as.character(f2[3]))))

It's admittedly unwieldy, but it seems pretty robust.

Something like:

changeFormula <- function(form, xIn, xOut){
    as.formula(paste(form[2], form[1], gsub(xIn, xOut, as.character(form[3]))))
}

changeForm(formula(m2), "x1", "x1c")

I'm not sure if this will play nice with environments and what not so
you might need to change those manually.

Hope this gets you started,
Michael

On Tue, May 29, 2012 at 11:43 AM, Paul Johnson <pauljohn32 at gmail.com> wrote:
> Greetings
>
> I want to take a fitted regression and replace all uses of a variable
> in a formula. For example, I'd like to take
>
> m1 <- lm(y ~ x1, data=dat)
>
> and replace x1 with something else, say x1c, so the formula would become
>
> m1 <- lm(y ~ x1c, data=dat)
>
> I have working code to finish that part of the problem, but it fails
> when the formula is more complicated. If the formula has log(x1) or
> x1:x2, the update code I'm testing doesn't get right.
>
> Here's the test code:
>
> ##PJ
> ## 2012-05-29
> dat <- data.frame(x1=rnorm(100,m=50), x2=rnorm(100,m=50),
> x3=rnorm(100,m=50), y=rnorm(100))
>
> m1 <- lm(y ~ log(x1) + x1 + sin(x2) + x2 + exp(x3), data=dat)
> m2 <- lm(y ~ log(x1) + x2*x3, data=dat)
>
> suffixX <- function(fmla, x, s){
>    upform <- as.formula(paste0(". ~ .", "-", x, "+", paste0(x, s)))
>    update.formula(fmla, upform)
> }
>
> newFmla <- formula(m2)
> newFmla
> suffixX(newFmla, "x2", "c")
> suffixX(newFmla, "x1", "c")
>
> The last few lines of the output. See how the update misses x1 inside
> log(x1) or in the interaction?
>
>
>> newFmla <- formula(m2)
>> newFmla
> y ~ log(x1) + x2 * x3
>> suffixX(newFmla, "x2", "c")
> y ~ log(x1) + x3 + x2c + x2:x3
>> suffixX(newFmla, "x1", "c")
> y ~ log(x1) + x2 + x3 + x1c + x2:x3
>
> It gets the target if the target is all by itself, but not otherwise.
>
> After messing with this for quite a while, I conclude that update was
> the wrong way to go because it is geared to replacement of individual
> bits, not editing all instances of a thing.
>
> So I started studying the structure of formula objects.  I noticed
> this really interesting thing. the newFmla object can be probed
> recursively to eventually reveal all of the individual pieces:
>
>
>> newFmla
> y ~ log(x1) + x2 * x3
>> newFmla[[3]]
> log(x1) + x2 * x3
>> newFmla[[3]][[2]]
> log(x1)
>> newFmla[[3]][[2]][[2]]
> x1
>
> So, if you could tell me of a general way to "walk" though a formula
> object, couldn't I use "gsub" or something like that to recognize each
> instance of "x1" and replace with "x1c"??
>
> I just can't figure how to automate the checking of each possible
> element in a formula, to get the right combination of [[]][[]][[]].
> See what I mean? I need to avoid this:
>
>> newFmla[[3]][[2]][[3]]
> Error in newFmla[[3]][[2]][[3]] : subscript out of bounds
>
> pj
>
> --
> Paul E. Johnson
> Professor, Political Science    Assoc. Director
> 1541 Lilac Lane, Room 504     Center for Research Methods
> University of Kansas               University of Kansas
> http://pj.freefaculty.org            http://quant.ku.edu
>
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