##
#define a class
##use Splus6

 setClass("point", representation(x="numeric", y="numeric"))

 pt1 <- new("point", x=1,y=3)

 pt1@x

 pt1@y

 class(pt1)

 setClass("otherpt", representation(r="numeric", theta="numeric"))

 pt2 <- new("otherpt", r=4,theta=pi/4)

 pt2@r

 pt2@theta

 class(pt2)

## access via methods

 setGeneric("Xpos", function(obj) standardGeneric("Xpos"))

 setMethod("Xpos", signature(obj="point"), function(obj) obj@x)

 setGeneric("Ypos", function(obj) standardGeneric("Ypos"))

 setMethod("Ypos", signature(obj="point"), function(obj) obj@y)

##

 Xpos(pt1)
 Ypos(pt1)


##methods for the r-theta class

 setMethod("Xpos", signature(obj="otherpt"), function(obj)
           obj@r*cos(obj@theta))

 setMethod("Ypos", signature(obj="otherpt"), function(obj)
           obj@r*sin(obj@theta))

  Xpos(pt2)
  Ypos(pt2)

##what have we gained at this point?
##we just call Xpos -- we don't need to call anything else
##the generic function handles the dispatching

##printing:
##use cat etc to customize the display of your objects

 setMethod("show", signature("otherpt"), function(object)
           print("Hiya"))

 pt2

 removeMethod("show", signature("otherpt"))

 pt2

##plotting
##we could make this a
##for now we ignore user supplied col, xlim and ylim values
##'cause we like our own better ;-)

 plotxy <- function(x, col, xlim, ylim, ...) {
  xval <- Xpos(x)
  yval <- Ypos(x)
  plot(xval,yval, xlim=c(0,xval+1), ylim=c(0,yval+1), ...)
 }




 plotxy(pt2)