rbind data.frames with character vectors?

That is not how you are intended to put character strings in data frames 
in S.  Rather, there is

A <- data.frame(a=1, b=I("A"))
B <- data.frame(a=2, b=I("B"))
AB <- rbind(A,B)

etc works (at least in R-devel)

Using $ on data frame is underhand, and avoids some of the consistency 
checks.

We are planning to use I("foo") to put the column in as a character 
column and use it consistently, but for 1.8.0 not 1.7.x


On Mon, 31 Mar 2003, Spencer Graves wrote:

"rbind(A, B)" converts character columns of A and B to factors.  This 
means that "A <- rbind(A, B)" generates NAs unless the character strings 
in B are already levels of the corresponding columns of A.

I've got a work-around, but I'm not happy with it.  What do you suggest?

Example:

 > A <- data.frame(a=1)
 > A$b <- "A"
 > B <- data.frame(a=2)
 > B$b <- "B"
 > sapply(A, data.class)

           a           b
   "numeric" "character"

 > AB <- rbind(A,B)
 > sapply(AB, data.class)

         a         b
"numeric"  "factor"

 > C. <- data.frame(a=3)
 > C.$b <- "C"
 > rbind(AB, C.)

     a    b
1   1    A
11  2    B
111 3 <NA>
Warning message:
invalid factor level, NAs generated in: "[<-.factor"(*tmp*, ri, value = 
"C")

 > sapply(rbind(AB, C.), data.class)

         a         b
"numeric"  "factor"
Warning message:
invalid factor level, NAs generated in: "[<-.factor"(*tmp*, ri, value = 
"C")

Thanks,
Spencer Graves
p.s.  This example produces the desired result in S-Plus 2000 and 6.1 
Professional for Windows 2000.

I am not at clear sure that is intentional, though.