overriding a method in R, using NextMethod

Question

how dows this work in R...

I am using a package (zoo 1.6-4) that defines a S3 class for time series sets. I am writing a derived class where I want to override a few methods and can't get past this one:[.zoo!

in my derived class rows are indexed by timestamp, like in zoo, but differently from zoo, I allow only POSIXct values in the index. my users will be selecting columns all of the time, while slicing series only occasionally so I want to offer obj[name] instead of obj[, name].

my objects have class c("delftfews", "zoo").

but...

how do I override a method?

I tried this:

"[.delftfews" <- function(x, i, j, drop=TRUE, ...) {
  if (missing(i)) return(NextMethod())
  if (all(class(i) == "character") && missing(j)) {
    return(NextMethod('[', x=x, i=1:NROW(x), j=i, drop=drop, ...))
  }
  NextMethod()
}

but I get this error: Error in rval[i, j, drop = drop., ...] : incorrect number of dimensions.

I have solved by editing the source from zoo: I removed those ..., but I don't get why that works. anybody can explain what is going on here?

Answer 1

A point of clarification: allowing only POSIXct index values does not allow indexing columns by name only. I'm not sure how you arrived at that conclusion.

You're overriding zoo correctly, but I think you misunderstand NextMethod. The error is caused by if (missing(i)) return(NextMethod()), which calls [.zoo if i is missing, but [.zoo requires i because zoo's internal data structure is a matrix. Something like this should work:

if (missing(i)) i <- 1:NROW(x)

though I'm not sure if you have to explicitly pass this new i to NextMethod...

You may be interested in the xts package, if you haven't already taken a look at it.

Answer 2

The problem is that with the above definition of [.delftfews this code:

library(zoo)
z <- structure(zoo(cbind(a = 1:3, b = 4:6)), class = c("delftfews", "zoo"))
z["a"]
# generates this call: `[.zoo`(x = 1:6, i = 1:3, j = "a", drop = TRUE, z, "a")

Your code does work as is if you write the call like this:

z[j = "a"]
# generates this call: `[.zoo`(x = z, j = "a")

I think what you want is to change the relevant line in [.delftfews to this:

return(NextMethod(.Generic, object = x, i = 1:NROW(x), drop = drop))
# z["a"] now generates this call: `[.zoo`(x = z, i = 1:3, j = "a", drop = TRUE)