[R] is.na for S4 object

[cgenolin]

Hi the list,

I have a variable y that is either NA or some S4 object. I would like to
know in which case I am, but it seems taht is.na does not work with S4
object, I get a warnings:

--- 8< ------------
setClass("myClass",slots=c(x="numeric"))
if(runif(1)>0.5){a <- new("myClass")}else{a <- NA}
is.na(a)
--- 8< ------------

Any solution?
Thanks 

Christophe




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