# [R] how to draw a multivariate function

Spencer Graves spencer.graves at pdf.com
Sun Oct 17 06:06:39 CEST 2004

```      Have you studied the documentation for "wireframe" including the
(2002) Modern Applied Statistics with S, 4th ed. (Springer).

It might help if you write your function as follows:

f <- function(x,y ,n, pa, pb) (factorial(n)/
(factorial(x) * factorial(y) * factorial(n-x-y))*
pa^x * pb^y * ((1-pa-pb)^(n-x-y)))

I hope this fills in enough gaps that you will be able to complete
the remaining steps.

Good luck.  spencer graves

Sun wrote:

>Hi, Rusers:
>
>Thanks for answering my last questions. I am frustrated in plotting a trinomial pmf function
>
>f(x,y | n, pa, pb) = factorial(n)/ (factorial(x) * factorial(y) * factorial (n-x-y))* pa^x * pb^y * ((1-pa-pb)^(n-x-y))
>
>obviously it is a bivariate function of x and y. But I have put a lot of time on this.
>
>**********************************
>x <- seq(0, n, len = n/2+1) # for now I set it to n/2 to control x+y <= n
>y <- seq(0, n, len = n/2+1)
>f = factorial(n)/ (factorial(x) * factorial(y) * factorial (n-x-y))* pa^x * pb^y * ((1-pa-pb)^(n-x-y))
>wireframe(f ~ x * y, shade = TRUE)
>**********************************
>
>well, but it plots nothing out.
>
>I wonder if you could help me? Seems R is hard to learn without your help.
>
>Many thanks,
>
>Sun
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>
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