maybe something like:
f <- y ~ x1 + x2 attr(terms(f), "term.labels")
but this wan't work if you have a more complex formula (e.g., f <- y ~ x1*x2 + I(x1^2)) and you want only c("x1", "x2").
I hope it helps.
Best, Dimitris
---- Dimitris Rizopoulos Ph.D. Student Biostatistical Centre School of Public Health Catholic University of Leuven
Address: Kapucijnenvoer 35, Leuven, Belgium Tel: +32/16/336899 Fax: +32/16/337015 Web: http://www.med.kuleuven.ac.be/biostat http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm
----- Original Message ----- From: "Daniel Almirall" <[EMAIL PROTECTED]>
To: <r-help@stat.math.ethz.ch>
Sent: Tuesday, January 11, 2005 9:55 PM
Subject: [R] getting variable names from formula
R-list,
1. Given a formula (f) w variables referencing some data set (dat), is
there any easier/faster way than this to get the names (in character form)
of the variables on the RHS of '~' ?
dat <- data.frame(x1 = x1 <- rnorm(100,0,1), x2 = x2 <- rnorm(100,0,1), y = x1 + x2 + rnorm(100,0,1))
f <- y ~ x1 + x2
mf <- model.frame(f, data=dat)
mt <- attr(mf, "terms")
predvarnames <- attr(mt, "term.labels")
predvarnames[1] "x1" "x2"
-----
2. Also, is there an easy/fast way to do it, without having the data set
(dat) available? That is, not using 'model.frame' which requires 'data'?
I understand that one approach for this is to use the way formulas are
stored as 'list's. For example, this works
predvarnames <- character()
for (i in 2:length(f[[3]]) ){
predvarnames <- c(predvarnames, as.character(f[[3]][[i]]))
}
predvarnames[1] "x1" "x2"
but is there a better way?
Thanks, Danny
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