R help - Jan 2011 - Generic Functions and Dates

I'm trying to write a generic function that calls different methods
depending on the structure of the argument, but not the exact type of its
contents. For example, the function 'nan2last' below works for a numeric
vector but not for a vector of Date objects. Is there any way to make it work on
any vector?


setGeneric("nan2last", function(x) {
standardGeneric("nan2last") })

setMethod(nan2last, "vector",
          function(x) {

            naLocs <- (1:length(x))[is.na(x)]

            if (length(naLocs) == 0)
              return (x)

            naLocs <- naLocs[naLocs>1]

            for (i in 1:length(naLocs)) {
              x[naLocs[i]] <- x[naLocs[i]-1]
            }

            return(x)
          })

## Works
x <- 1:10;
x[sample(10,3)] <- NA
print(cbind(x, nan2last(x)))

## Doesn't work
x <- seq(as.Date("2011-01-01"), as.Date("2011-01-31"),
"days")
x[sample(length(x), 5)] <- NA
nan2last(x) 

Thanks.

- Elliot

On 01/31/2011 12:15 PM, Elliot Joel Bernstein wrote:
> I'm trying to write a generic function that calls different methods
> depending on the structure of the argument, but not the exact type of
> its contents. For example, the function 'nan2last' below works for
a
> numeric vector but not for a vector of Date objects. Is there any way
> to make it work on any vector?
Hi Elliot --

In principle one could write a method that advertises that it operates
on signature "ANY"

setMethod(nan2last, "ANY", function(x) <...>)

but in practice it's unlikely that one could do something useful on
_any_ object.

If I
> getClass("Date")
Virtual Class "Date" [package "methods"]

Slots:

Name:   .S3Class
Class: character

Extends: "oldClass"

I see that Date does not extend "vector", so I'd write a method
that
delegates as appropriate, along the lines of

.nan2last <- function(x) { <general implementation> }

setMethod(foo, "vector", .na2last)
setMethod(foo, "Date", .na2last)
> setGeneric("nan2last", function(x) {
standardGeneric("nan2last") })
>
> setMethod(nan2last, "vector",
>           function(x) {
>
>             naLocs <- (1:length(x))[is.na(x)]      
probably naLocs <- which(is.na(x))

>
>             if (length(naLocs) == 0)
>               return (x)
>
>             naLocs <- naLocs[naLocs>1]
>
>             for (i in 1:length(naLocs)) {    
seq_along(naLocs), otherwise you'll surprise yourself with
1:length(integer(0))

>               x[naLocs[i]] <- x[naLocs[i]-1]
>             }     
just x[naLocs] <- x[naLocs - 1], outside the loop (so no seq_along)

Martin


>
>             return(x)
>           })
>
> ## Works
> x <- 1:10;
> x[sample(10,3)] <- NA
> print(cbind(x, nan2last(x)))
>
> ## Doesn't work
> x <- seq(as.Date("2011-01-01"),
as.Date("2011-01-31"), "days")
> x[sample(length(x), 5)] <- NA
> nan2last(x) 
> 
> Thanks.
> 
> - Elliot
> 
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