[R] Variable names in model formula

[E. S. Venkatraman]

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))
}

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?

Thanks,
Venkat
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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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