[R] attempt at making a polygon class failed

[Leeds, Mark (IED)]

I was reading a presentation of Professor Peng's and typed the
presentation code into R but I changed it to make plot.polygon a 
separate function instread of defining the function in SetMethod itself
as he did. Is that the problem with the code below because
plot(p) just gives me zero. Thanks.


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

plot <- function(object)
	{
		return(0)
	}

setGeneric("plot")

plot.polygon <- function(x, y, ...) {
	xlim <- range(x at x)
	ylim <- range(x at y)
	plot(0,0, type = "n", xlim = xlim, ylim = ylim, ...)
	xp <- c(x at x, x at x[1])
	yp <- c(x at y, x at y[1])
	lines(xp, yp)
}


setMethod("plot","polygon",plot.polygon)

p <- new("polygon", x = c(1,2,3,4), y = c(1,2,3,1))

plot(p)
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