I really appreciate the excellent ideas from Bill Dunlap and Greg Snow.
Both suggestions almost work perfectly. Greg's recognizes expressions
such as sex=='female' but not ones such as age > 21, age < 21, a - b >
0, and possibly other legal R expressions. Bill's idea is similar to
what Duncan Murdoch suggested to me. Bill's doesn't catch the case when
a variable appears both in an expression and as a regular variable (sex
in the example below):
f <- function(formula) {
trms <- terms(formula)
variables <- as.list(attr(trms, "variables"))[-1]
## the 'variables' attribute is stored as a call to list(),
## so we changed the call to a list and removed the first element
## to get the variables themselves.
if (attr(trms, "response") == 1) {
## terms does not pull apart right hand side of formula,
## so we assume each non-function is to be renamed.
responseVars <- lapply(all.vars(variables[[1]]), as.name)
variables <- variables[-1]
} else {
responseVars <- list()
}
## omit non-name variables from list of ones to change.
## This is where you could expand calls to certain functions.
variables <- variables[vapply(variables, is.name, TRUE)]
variables <- c(responseVars, variables) # all are names now
names(variables) <- vapply(variables, as.character, "")
newVars <- lapply(variables, function(v) as.name(paste0(toupper(v),
"z")))
formula(do.call("substitute", list(formula, newVars)),
env=environment(formula))
}
a <- cat + (age + Heading("Females") * (sex == "Female") * sbp) *
Heading() * g + (age + sbp) * Heading() * trio ~ Heading() *
country * Heading() * sex
f(a)
Output:
CATz + (AGEz + Heading("Females") * (SEXz == "Female") * SBPz) *
Heading() * Gz + (AGEz + SBPz) * Heading() * TRIOz ~ Heading() *
COUNTRYz * Heading() * SEXz
The method also doesn't work if I replace sex == 'Female' with x3 > 4,
converting to X3z > 4. I'm not clear on how to code what kind of
expressions to ignore.
Thanks!
Frank
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