# [Rd] vector S4 classes

Martin Maechler maechler at stat.math.ethz.ch
Tue Aug 29 12:11:23 CEST 2006

```>>>>> "Robin" == Robin Hankin <r.hankin at noc.soton.ac.uk>
>>>>>     on Tue, 29 Aug 2006 10:42:21 +0100 writes:

Robin> In the Green Book, section 7.5 discusses new vector classes and uses
Robin> quaternions
Robin> as an example of a vector class that needs more than one number per
Robin> element.

Robin> I would like to define a new class that has a numeric vector and a
Robin> logical
Robin> vector of the same length that specifies whether the measurement was
Robin> accurate.

Robin> The following code does not behave as desired:

>> setClass("thing",representation("vector",accurate="logical"))
Robin> [1] "thing"
>> dput(x <- new("thing",1:10,accurate=rep(T,10)))
Robin> structure(c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), accurate = c(TRUE,
Robin> TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE), class =
Robin> structure("thing", package = ".GlobalEnv"))
>> x[1:3]
Robin> [1] 1 2 3
>> dput(x[1:3])
Robin> c(1, 2, 3)
>>

Robin> because, although the "accurate" slot is filled as desired in "x",
Robin> when extracting the first
Robin> three elements, it seems to be lost.

and you would really expect that ``R'' magically knows to it
also must subset the accurate slote ?

Robin> What is the appropriate setClass() call to do what I want?  Or indeed
Robin> is making "thing"
Robin> a vector class as sensible idea here?

I think you need to define at least a  subset and subassign
method for your class as well.

Defining it as "vector" will automatically inherit all the
method definitions for "vector" --- none of which will ``know anything''
Therefore, I tend to think you rather define a class with "all slots"

setClass("Thing", representation(x = "numeric", accurate = "logical"))

and then you probably have to define many methods for that
class, notably for "[" and also "[<-" where the latter should
happen via setReplaceMethod("Thing",

Also, I'd define a validity method, where you have to decide if
'accurate' must have the same length as 'x' -- or what it should
mean if not.

Martin

```