[R] retain class after merge

[Heinz Tuechler]

Dear All,

is there a simple way to retain the class attribute of a column, if 
merging two data.frames?
When merging the example data.frames form help(merge) I am unable to 
keep the class attribute as set before merging (see below).
Two columns are assigned new classes before merge (myclass1, 
myclass2), but after merge the resulting column has class "character".

best regards,

Heinz


## use character columns of names to get sensible sort order
authors <- data.frame(
     surname = I(c("Tukey", "Venables", "Tierney", "Ripley", "McNeil")),
     nationality = c("US", "Australia", "US", "UK", "Australia"),
     deceased = c("yes", rep("no", 4)))
books <- data.frame(
     name = I(c("Tukey", "Venables", "Tierney",
              "Ripley", "Ripley", "McNeil", "R Core")),
     title = c("Exploratory Data Analysis",
               "Modern Applied Statistics ...",
               "LISP-STAT",
               "Spatial Statistics", "Stochastic Simulation",
               "Interactive Data Analysis",
               "An Introduction to R"),
     other.author = c(NA, "Ripley", NA, NA, NA, NA,
                      "Venables & Smith"))

class(authors$surname) <- 'myclass1'
class(books$name) <- 'myclass2'
(m1 <- merge(authors, books, by.x = "surname", by.y = "name"))
class(m1$surname)

[1] "character"
 >
 > sessionInfo()
R version 2.13.1 Patched (2011-08-08 r56671)
Platform: i386-pc-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=German_Switzerland.1252  LC_CTYPE=German_Switzerland.1252
[3] LC_MONETARY=German_Switzerland.1252 LC_NUMERIC=C
[5] LC_TIME=German_Switzerland.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base