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I have not needed to use c(f1, f2) but it seems to me that Peter's example points out more a deficiency in c or the need for another binding function for this case (which can be done with factor(c(as.character(f1),as.character(f2))) depending on how NAs are handled.
More often than I'd like, I have to combine several related data frames together. The following showed that the rbind method for data frames does do a reasonable job: # Suppose dat1 and dat2 are two data frames with same variables.
dat1$ind <- factor(rep("dat1", nrow(dat1)))
dat2$ind <- factor(rep("dat2", nrow(dat2)))
dat <- rbind(dat1, dat2)
levels(dat$ind)
[1] "dat1" "dat2"
Frank Harrell
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