[R] building formula objects

[Bill.Venables@cmis.csiro.au]

In response to a query from Russel Senior, Jon Baron writes:

>  -----Original Message-----
> From: 	Jonathan Baron [mailto:baron at cattell.psych.upenn.edu] 
> Sent:	Tuesday, July 09, 2002 9:38 PM
> To:	Russell Senior
> Cc:	r-help at stat.math.ethz.ch
> Subject:	Re: [R] building formula objects
> 
> On 07/09/02 03:48, Russell Senior wrote:
> >
> >I want to write a function to take an argument as the response
> >variable of a linear model, e.g. to do anova's across a list of
> >variables, something like the following (except, of course, this
> >doesn't work):
> >
> >  function(x) { anova(lm(x ~ my.factor,data=my.data)) }
> >
> >The x in lm() above is getting evaluated at the wrong level.  How
> >can I make this work? 
> 
> I don't know.  But what I do instead is this:
> 
> my.coeffs <- matrix(NA,mynumber,3) # set up empty matrix
> for (i in 1:my.number) {
>  mycoeffs[i] <- as.numeric(try(coef(lm(Y[i]~X1[i]+X2[i]+X3[i]))[2:4]))}
> 
> The "try()" is in case it doesn't work for a given value of i
> (e.g., too many missing data), and that makes "as.numeric()"
> necessary. The coef() extracts the coefficients, excluding the
> constant,, which is all I was interested in, but you could store
> the whole thing if you use a list instead of my.coeffs.
> 
> I would like to know how to do it with a function.

[WNV]  It's not too bad, but a little trickier than most people would
expect.  See the Programmer's Niche article in the latest edition of R news
for a general strategy for handling this kind of problem (at the risk of
touting my own stuff).

Bill Venables

> Jon
> -- 
> Jonathan Baron, Professor of Psychology, University of Pennsylvania
> R page:               http://finzi.psych.upenn.edu/
>
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