as.function()

2008/1/14, Robin Hankin <r.hankin at noc.soton.ac.uk>:

Hi

[this after some considerable thought as to R-help vs R-devel]



I want to write a (S3) method for as.function();
toy example follows.

Given a matrix "a", I need to evaluate trace(ax) as a function of
(matrix) "x".

Here's a trace function:

tr <-  function (a)  {
    i <- seq_len(nrow(a))
    return(sum(a[cbind(i, i)]))
}


How do I accomplish the following:


a <- crossprod(matrix(rnorm(12),ncol=3))
class(a) <- "foo"

f <- as.function(a)       # need help to write as.function.foo()
x <- diag(3)

f(x)             #should give tr(ax)

What about the following?

as.function.foo <- function(a, ...)
 function(x)
   sum(diag(a*x))

However, I don't see the need for an S3 method. Why don't simply use  
(?):
mulTraceFun <- function(a)
 function(x)
  sum(diag(a*x))

So you also have a more meaningful name than an anonymous  
'as.function'.

HTH,
Antonio.

a <- 4
f(x)           # should still give tr(ax) even though "a" has been
reassigned.

This would'nt work with my proposal, because of lexical scoping.

[my real example is very much more complicated than this but
I need this toy one too and I can't see how to modify
as.function.polynomial()
to do what I want]




--
Robin Hankin
Uncertainty Analyst and Neutral Theorist,
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
 tel  023-8059-7743

-- 
Antonio, Fabio Di Narzo
Ph.D. student at
Department of Statistical Sciences
University of Bologna, Italy