[R] getting variable names from formula

[Douglas Bates]

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
> 
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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"