[R] problem with lapply(x, subset, ...) and variable select argument

[joerg van den hoff]

I need to extract identically named columns from several data frames in 
a list. the column name is a variable (i.e. not known in advance). the 
whole thing occurs within a function body. I'd like to use lapply with a
variable 'select' argument.


example:

tt <- function (n) {
    x <- list(data.frame(a=1,b=2), data.frame(a=3,b=4))
    for (xx in x) print(subset(xx, select = n))   ### works
    print (lapply(x, subset, select = a))   ### works
    print (lapply(x, subset, select = "a"))  ### works
    print (lapply(x, subset, select = n))  ### does not work as intended
}
n = "b"
tt("a")  #works (but selects not the intended column)
rm(n)
tt("a")   #no longer works in the lapply call including variable 'n'


question: how  can I enforce evaluation of the variable n such that
the lapply call works? I suspect it has something to do with eval and
specifying the correct evaluation frame, but how? ....


many thanks

joerg