[R] Enumeration in R

[Paul Roebuck]

On Tue, 7 Jul 2004, Peter Dalgaard wrote:

> Paul Roebuck <roebuck at odin.mdacc.tmc.edu> writes:
>
> > I want the equivalent of this 'C' declaration.
> >     enum StoplightColor {
> >         green = 3,
> >         yellow = 5,
> >         red = 7
> >     };
>
> I think you *dis*abled it by specifying an initializer which doesn't
> check the validity:

Thanks. It all seems obvious once it's pointed out to you.
I have a couple related questions:

1) When I was reading Chambers book (pg 288), my initial
impression was that there was a way I could have done this
class without specifying the representation in 'setClass'
- I just couldn't figure out how to assign values that
way. Could this class have been defined 'slotless'? If so,
how?

2) How do you define a class and instantiate some, yet
prevent more from being created after that. Possibly better
stated, from the package's API view, I would like these to
be instance variables of opaque types. So I would like to
create my 'global' constants in an initialization routine
then prevent use of 'new' to create any more. I tried the
following with no success. Possible?

setMethod("new",
          "stoplightColor",
          function(Class, ...) stop("can't make any more"))

3) This seems kind of painful for trivial stuff. My idea
was to move some of the validation error checking out of my
project by converting certain function arguments into classes
that could be validated upon creation, improving the clarity
of project routines. What is the canonical style used in R
package authoring?




--------- stoplightColor.R ------------------------------------
setClass("stoplightColor",
         representation(value = "integer"),
         prototype = integer(1))
stoplightColor <- function(value) {
    new("stoplightColor", value)
}
valid.stoplightColor <- function(object) {
    valid <- switch(as(object at value, "character"),
                    "3" = TRUE,
                    "5" = TRUE,
                    "7" = TRUE,
                    FALSE)
    if (valid == FALSE)
        return('Invalid value - must be [3|5|7]');
    return(TRUE);
}
setValidity("stoplightColor", valid.stoplightColor)
initialize.stoplightColor <- function(.Object, value) {
    if (missing(value) || is.na(value))
        stop('Argument "value" is missing or NA')
    .Object at value <- as.integer(value)
    validObject(.Object)
    .Object
}
setMethod("initialize",
          signature(.Object = "stoplightColor"),
          initialize.stoplightColor)
stoplightColor.as.integer <- function(from) {
    return(from at value)
}
setAs("stoplightColor", "integer", stoplightColor.as.integer)

green <- stoplightColor(3)
yellow <- stoplightColor(5)
red <- stoplightColor(7)

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SIGSIG -- signature too long (core dumped)