[Rd] setReplaceMethod
John Chambers
jmc at r-project.org
Wed Nov 1 00:18:08 CET 2006
There's a two-level issue here, principle and practice.
The principle is that the behavior of basic R functions on basic R data
types is considered "sealed". Math functions, arithmetic, etc. on basic
vectors are not supposed to be alterable.
The function "[<-" is one of those functions. If x is a basic vector
type, then x[i] <- value is considered sealed. That's what the error
message is telling you. (By the way setReplaceMethod() has nothing to
do with the issue; it's just a way of relieving the programmer from
knowing that x[i]<-value is really a call to the function "[<-"; again,
the error message shows that.)
Since you are writing the method for x having class "ANY", that is even
stronger than changing the behavior for a specific basic type.
If you were to convince the R management that in principle having a
special class on the right side justified breaking the seal (and that
would take some arguing), then there is a practical problem.
The current implementation of primitives in C code examines the first
argument only (except for binary operators, where both arguments are
examined). The object is examined for having a non-basic class. If
it's a basic data type, no further checking for methods occurs. So even
if you were allowed to define the method, it wouldn't get called if x
was a numeric vector, for example. Changing that would require changes
at a low level, and almost certainly add some overhead to all calls.
Expect opposition.
Otherwise, you must define some class and only expect your assignment to
promote x if x inherits from that class. Or have another function, not
"[<-", as the generic.
By the way, your method definition is not what you wanted in any case.
The signature you meant to have was signature(x="ANY", value = "brob").
At least I assume it was the right-hand side and not argument "i" that
you meant to have class "brob". You don't actually need the x="ANY"
part, that is the default for omitted arguments--but it's clearer in
this case to be explicit.
Robin Hankin wrote:
> Hi
>
> If x <- 1:10 then x[5] <- 1i will promote
> x to be a complex vector.
>
> Suppose I have an S4 class "brob", and have functions
> is.brob(), as.brob(), as.numeric() and so forth (minimal self-contained
> code below).
>
> If x is numeric (1:10, say) and y is a brob, what
> is the best way to make
>
> x[5] <- y
>
> promote x to a brob in the same way as the complex example?
>
> Or is this not desirable for some reason?
>
>
> My first idea was to use
>
> setReplaceMethod("[",signature("ANY","brob"), ...)
>
> but this gives a seal error:
>
> Error in setMethod(paste(f, "<-", sep = ""), ..., where = where) :
> the method for function "[<-" and signature x="ANY", i="brob" is
> sealed and cannot be re-defined
>
>
>
>
> so this can't be right.
>
>
>
>
>
>
>
>
>
>
>
> setClass("swift",
> representation = "VIRTUAL"
> )
>
> setClass("brob",
> representation = representation
> (x="numeric",positive="logical"),
> prototype = list(x=numeric(),positive=logical()),
> contains = "swift"
> )
>
> 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=double(),positive){
> if(missing(positive)){
> positive <- rep(TRUE,length(x))
> }
> if(length(positive)==1){
> positive <- rep(positive,length(x))
> }
> new("brob",x=as.numeric(x),positive=positive)
> }
>
> "as.brob" <- function(x){
> if(is.brob(x)){
> return(x)
> } else if(is.complex(x)) {
> warning("imaginary parts discarded")
> return(Recall(Re(x)))
> } else if(is.glub(x)){
> warning("imaginary parts discarded")
> return(Re(x))
> } else {
> return(brob(log(abs(x)), x>=0))
> }
> }
>
> setMethod("[", "brob",
> function(x, i, j, drop){
> brob(x at x[i], x at positive[i])
> } )
>
> setReplaceMethod("[",signature(x="brob"),
> function(x,i,j,value){
> jj.x <- x at x
> jj.pos <- x at positive
> if(is.brob(value)){
> jj.x[i] <- value at x
> jj.pos[i] <- value at positive
> return(brob(x=jj.x,positive=jj.pos))
> } else {
> x[i] <- as.brob(value)
> return(x)
> }
> } )
>
>
> setReplaceMethod("[",signature("ANY","brob"),
>
> function(x,i,j,value){
> x <- as.brob(x)
> x[i] <- as.brob(value)
> return(x)
> }
> )
>
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
> tel 023-8059-7743
>
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>
>
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