[R] Variable names in model formula

[Duncan Murdoch]

On Tue, 02 May 2000 17:11:52 +1000, Bill Venables wrote in message
<3.0.32.20000502171151.0126c440 at pophost.nsw.cmis.csiro.au>:

>At 10:37 PM 5/1/00 -0400, E. S. Venkatraman wrote:
>>I have the following problem.  I have survival data (time, status) along
>>with several covariates (X1, X2,..., Xn).  I want to fit a Cox model for
>>each of the covariate (univariately) and obtain the fitted probability of
>>survival at a fixed time point t0 and covariate value Xi0.  
>
>You could try
>
>covnames <- names(...)
>form <- Surv(time, status) ~ dummy
>for(nam in covnames) {
>	form[[3]] <- as.name(nam)
>	survout <- coxph(form, data = dataname)
>	print(summary(survfit(survout, newdata), t0))
>}
>
>...  A slightly more direct way would be to use some
>column of the data frame on the right hand side:
>
>for (i in 1:5) {
>	survout <- coxph(Surv(time, status) ~ dataname[, i], dataname)
>.....
>}
>

I just discovered a nice way to do this in S-PLUS the other day; it
doesn't work in R 0.99, but I don't have version 1 installed on this
machine to check there.

The terms() function in S-PLUS returns an expression listing the terms
in a model formula.  It is nice to manipulate:

 formula(terms(somefit)[1]) gives the model formula using just the
first term,

 formula(terms(somefit)[-1]) gives the model formula leaving out the
first term, 

etc.

I don't know if copying this functionality would break anything (or
maybe it's already there in 1.x), but it seems to me like a nice
addition.

Duncan Murdoch
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