[Rd] S4 methods for "+"

[Robin Hankin]

On 25 Aug 2006, at 12:56, Prof Brian Ripley wrote:

> Just like any other S4 method:
>
> setMethod("+", c("track", "track"),
>           function(e1, e2) new("track", x=c(e1 at x, e2 at x), y=c 
> (e1 at y,e2 at y)))
>
> If you want to write a group generic for the S4 Ops group, you do it
> very like S3.  There are worked examples in 'S Programming' (that  
> at least
> at one point worked in R).
>


I had a long train journey over the weekend and took S Programming
with me.  Most of my  questions about S4 methods can be answered
by reading chapters 4 and 5 over and over and over again, with  
occasional
reference to the Green Book.

?SetMethod does not reference Venables & Ripley.  Could this be added?





>
> On Fri, 25 Aug 2006, Robin Hankin wrote:
>
>> Hi
>>
>> I'm trying to implement S4 methods in a package, and I am having
>> difficulty defining "+" to do what I want.
>>
>> In the Green Book, there is a discussion of a "track" object,
>>
>> setClass("track", representation(x="numeric", y="numeric"))
>>
>> OK.
>>
>> track1 <- new("track",x=c(1,4,6),y=c(10,11,12))
>> track2 <- new("track",x=c(2,5),y=c(100,101))
>>
>>
>> What I want to do is to define "+" for track object so that if
>>
>> track3 <-  track1 + track2
>>
>> has track3 at x  == c(1,2,4,5,6)
>> and
>> track3 at y = c(10,100,11,101,12)
>>
>> maybe adding a track object to a scalar would shift the values of the
>> x slot.
>>
>> The algorithm itself is no problem...but what is the S4 equivalent to
>> the S3 technique of writing an Ops.track() function that tells
>> R what "+" means?
>>
>>
>>

--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743