[R] quickly extract response from formula

[William Dunlap]

You can bullet-proof it a bit by making sure that length(formula)==3
before assuming that formula[[2]] is the response.   If length(formula)==2
then there is no response term, only predictor terms.  E.g., replace
   resp <- frm[[2]]
with
   resp <- if (length(frm)==3) frm[[2]] else NULL
(or call stop(), or warning(), ...)

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Andreas Leha
> Sent: Friday, November 01, 2013 2:50 PM
> To: r-help at stat.math.ethz.ch
> Subject: Re: [R] quickly extract response from formula
> 
> Hi David,
> 
> thanks for your quick answer!
> 
> David Winsemius <dwinsemius at comcast.net> writes:
> 
> > On Oct 31, 2013, at 1:27 PM, Andreas Leha wrote:
> >
> >> Hi all,
> >>
> >> what is the recommended way to quickly (and without much burden on the
> >> memory) extract the response from a formula?
> >
> > If you want its expression value its just form[[2]]
> >
> > If you wnat it evaluated in the environment of a dataframe then this should be fairly
> efficient:
> >
> > x <- stats::runif(20)
> > y <- stats::runif(20)
> > dfrm <- data.frame(x=x,y=y)
> > extractResponse <- function(frm, dat) { resp <- frm[[2]]; print(resp) # that's optional
> >                                          fdat <- eval(resp,
> >                                          envir=dat); return(fdat) }
> 
> This is what I'll be using.  Thanks again!
> 
> [...]
> 
> Regards,
> Andreas
> 
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