I have a formula that contains some terms and a data frame (the output of an earlier model.frame() call) that contains all of those terms and some more. I want the subset of the model frame that contains only the variables that appear in the formula.
ff <- log(Reaction) ~ log(1+Days) + x + y
fr <- data.frame(`log(Reaction)`=1:4,
`log(1+Days)`=1:4,
x=1:4,
y=1:4,
z=1:4,
check.names=FALSE)
The desired result is fr minus the z column (fr[,1:4] is cheating -- I need a programmatic solution ...)
Some strategies that don't work:
fr[all.vars(ff)]
## Error in `[.data.frame`(fr, all.vars(ff)) : undefined columns selected
(because all.vars() gets "Reaction", not log("Reaction"))
stripwhite <- function(x) gsub("(^ +| +$)","",x)
vars <- stripwhite(unlist(strsplit(as.character(ff)[-1],"\\+")))
fr[vars]
## Error in `[.data.frame`(fr, vars) : undefined columns selected
(because splitting on + spuriously splits the log(1+Days) term).
I've been thinking about walking down the parse tree of the formula:
ff[[3]] ## log(1 + Days) + x + y
ff[[3]][[1]] ## `+`
ff[[3]][[2]] ## log(1 + Days) + x
but I haven't got a solution put together, and it seems like I'm going down a rabbit hole. Ideas?
This should work:
> fr[gsub(" ","",rownames(attr(terms.formula(ff), "factors")))]
log(Reaction) log(1+Days) x y
1 1 1 1 1
2 2 2 2 2
3 3 3 3 3
4 4 4 4 4
And props to Roman Luštrik for pointing me in the right direction.
Edit: Looks like you could pull it out off the "variables" attribute as well:
fr[gsub(" ","",attr(terms(ff),"variables")[-1])]
Edit 2: Found first problem case, involving I() or offset():
ff <- I(log(Reaction)) ~ I(log(1+Days)) + x + y
fr[gsub(" ","",attr(terms(ff),"variables")[-1])]
Those would be pretty easy to correct with regex, though. BUT, if you had situations like in the question where a variable is called, e.g., log(x) and is used in a formula alongside something like I(log(y)) for variable y, this will get really messy.