R help - Jan 2005 - 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

Daniel Almirall wrote:
> 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
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
That's exactly what the all.vars function does.  If you apply it to the 
formula you get all the names of variables referenced in the formula. 
If you only want the right hand side then apply it to the third 
component of the formula

 > f <- y ~ x1 + x2
 > all.vars(f)
[1] "y"  "x1" "x2"
 > all.vars(f[[3]])
[1] "x1" "x2"

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" <dalmiral at umich.edu>
To: <r-help at 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
>