[R] Substituted arguments surviving multiple function calls

[Sundar Dorai-Raj]

Michael Hoffman wrote:
> I am using R 2.1.1 and have written a function that will retrieve a 
> named column from a data frame:
> 
> d = data.frame(a1=c(4, 2), a2=c(6, 7))
> f1 = function(x)
> {
>    do.call("$", list(d, substitute(x)))
> }
> 
> So this works:
> 
>  > f1(a1)
> [1] 4 2
> 
> However, I want to make another function, f2, which also accepts a 
> column name as an argument and then calls the first function with it:
> 
> f2 = function(x)
> {
>    f1(substitute(x))
> }
> 
> However, this does not work:
> 
>  > f2(a1)
> Error in list(a1 = c(4, 2), a2 = c(6, 7))$substitute(x) :
>          invalid subscript type
> 
> It works if I take the substitute() out of f1(), but then I can only 
> call f1() through f2() or something that does the substitution for it.
> Is there a better way to do this?
> 
> Thanks for any help you can offer.

Hi, Michael,

How about the following:

d <- data.frame(a1 = c(4, 2), a2 = c(6, 7))

f1 <- function(x, d) {
   eval(substitute(x), d)
}

f2 <- function(x, d) {
   eval(substitute(f1(x, d)), list(x = substitute(x), d = d))
}

f1(a1, d)
f2(a1, d)

Note that I redefined "f1" to do away with the "do.call". Also, it's 
always safest to pass "d" as an argument rather than rely on it being in 
  .GlobalEnv.

There may be others who suggest a different approach.

HTH,

--sundar