[R] Variable names in model formula

[Bill Venables]

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.  I tried to do
>this in a for loop where the index is the variable name as follows
>
>covnames <- names(...)
>for (covi in covnames) {
>  survout <- coxph(Surv(time,status) ~ covi, data = dataname)
>  print(summary(survfit(survout,newdata),t0))
>}

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))
}

>
>Unfortunately "covi" is being treated as the variable name instead of 
>being evaluated and the coxph call gives an error.  None of as.character,
>eval, substitute (at least the way I called them) worked.  How does one
>pass on variable names to model formula?

The above way is one.  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)
.....
}

There is a distinction between character strings and names.
>
>Thanks,
>Venkat
>-----------------------------------------------------------------------
>E. S. Venkatraman, Ph.D.     Phone: (212) 639-8520  Fax: (212) 717-3137
>Assistant Attending Member     Memorial Sloan-Kettering Cancer Center
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