[Rd] setMethod() and log()

[Robin Hankin]

Hi

I am having difficulty with setMethod().  I have a "brob" class of  
objects whose
representation has two slots: "x" and "positive".  Slot "x"  (double)  
holds the log
of a number and slot "positive" (logical) its sign.   The idea is  
that large numbers
can be handled.

I'm trying to implement a log() method using an analogue of the  
setMethod() example
for polynomials on page 117 of V&R.  abs() works [I think], but log()  
doesn't:

[transcript follows, source for  the classes below]



 > a <- as.brob((-4:4 )+0.4)
 > a
An object of class "brob"
Slot "x":
[1]  1.2809338  0.9555114  0.4700036 -0.5108256 -0.9162907   
0.3364722  0.8754687
[8]  1.2237754  1.4816045

Slot "positive":
[1] FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE

 > abs(a)
An object of class "brob"
Slot "x":
[1]  1.2809338  0.9555114  0.4700036 -0.5108256 -0.9162907   
0.3364722  0.8754687
[8]  1.2237754  1.4816045

Slot "positive":
[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE

# This works: each element is now positive.

 > log(a)
Error in log(x) : Non-numeric argument to mathematical function
 >



What's wrong with my setMethod() call below?





setClass("brob",
          representation = representation 
(x="numeric",positive="logical"),
          prototype      = list(x=numeric(),positive=logical())
          )

setAs("brob", "numeric", function(from){
   out <- exp(from at x)
   out[!from at positive] <- -out[!from at positive]
   return(out)
}
       )

setMethod("as.numeric",signature(x="brob"),function(x){as(x,"numeric")})

is.brob <- function(x){is(x,"brob")}

"brob" <- function(x,positive){
   if(missing(positive)){
     positive <- rep(TRUE,length(x))
   }
   if(length(positive)==1){
     positive <- rep(positive,length(x))
   }
   new("brob",x=x,positive=positive)
}

as.brob <- function(x){
   brob(log(abs(x)),x>0)
}

setMethod("Math", "brob",
           function(x){
             switch(.Generic,
                    abs    = brob(x at x),
                    log    = { out <- x at x
                               out[!x at positive] <- NaN
                               out
                             },
                    acos   =,
                    acosh  =,
                    asin   =,
                    asinh  =,
                    atan   =,
                    atanh  =,
                    ceiling=,
                    cos    =,
                    cosh   =,
                    cumsum =,
                    exp    =,
                    floor  =,
                    gamma  =,
                    lgamma =,
                    sin    =,
                    sinh   =,
                    tan    =,
                    tanh   =,
                    trunc  = as.brob(callGeneric(as.numeric(x))),
                    stop(paste(.Generic, "not allowed on  
Brobdingnagian numbers"))
                      )
           }
           )






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