Methods for recursive concatenations on user defined classes in R? - r

Question

All the concatenation functions in R can be rewritten as recursive functions. For instance, I could use c as a binary operator and I could define a new concat function as
concat <- function(...) {
Reduce(c, ...)
}
and concat would function as c actually functions in R base.
R syntactically uses many such functions, for instance c for vectors and lists, cbind for arrays, data.frames and matrices. When defining new object classes, it makes sense to create a method for combining them using a function which takes ... as an argument.
I know R can match methods to objects when they are the first object in the argument list, but what if I define a method like
concat <- function(...) {
UseMethod('concat')
}
concat.numeric <- function(...) {
c(...)
}
concat.character <- function(...) {
c(...)
}
myCon <- function(charPart, numPart) {
out <- list(charPart=charPart, numPart=numPart)
class(out) <- "myClass"
out
}
concat.myClass <- function(...) {
myCon(sapply(..., `[[`, 'charPart'), sapply(..., `[[`, 'numPart'))
}
> concat(4, 6, 'a')
"4" "6" "a"
> myObj1 <- myCon('a', 1)
> myObj2 <- myCon('b', 2)
> concat(myObj1, myObj2)
Error in get(as.character(FUN), mode = "function", envir = envir) :
object 'p' of mode 'function' was not found
At what point does R identify the types of arguments supplied to concat? How can I convince R to attempt to cast arguments to concat up to my specific object class?

Answer 1

You're not quite passing what you think to sapply you need to put it in list(...) so sapply can iterate through the elements rather than parsing them as extra arguments in the wrong place.
concat.myClass <- function(...)
{
myCon(sapply(list(...), `[[`, 'charPart'), sapply(list(...), `[[`, 'numPart'))
}
> myObj1
$charPart
[1] "a"
$numPart
[1] 1
attr(,"class")
[1] "myClass"
> myObj2
$charPart
[1] "b"
$numPart
[1] 2
attr(,"class")
[1] "myClass"
this then gives:
> concat(myObj1, myObj2)
$charPart
[1] "a" "b"
$numPart
[1] 1 2
attr(,"class")
[1] "myClass"
.. which I presume is what you want???