R help - Oct 2005 - problem with lapply(x, subset, ...) and variable select argument

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

On Mon, 10 Oct 2005, joerg van den hoff wrote:
> 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.
You would probably be better off using "[" rather than subset().

tt <- function (n) {
     x <- list(data.frame(a=1,b=2), data.frame(a=3,b=4))
     print(lapply(x,"[",n))
}

seems to do what you want.

 	-thomas
> 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>
Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle

The problem is that subset looks into its parent frame but in this
case the parent frame is not the environment in tt but the environment
in lapply since tt does not call subset directly but rather lapply does.

Try this which is similar except we have added the line beginning
with environment before the print statement.

tt <- function (n) {
   x <- list(data.frame(a=1,b=2), data.frame(a=3,b=4))
   environment(lapply) <- environment()
   print(lapply(x, subset, select = n))
}

n <- "b"
tt("a")

What this does is create a new version of lapply whose
parent is the environment in tt.


On 10/10/05, joerg van den hoff <j.van_den_hoff at fz-rossendorf.de>
wrote:
> 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>