R devel - Jan 2008 - as.function()

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)

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





[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

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
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

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

Robin Hankin wrote:
> 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)
>
> a <- 4
> f(x)           # should still give tr(ax) even though "a" has
been
> reassigned.
>
>   
Brian's answer was what you want.  A less general version is this:

 > as.function.foo <- function(x, ...) {
+    function(b) tr(x %*% b)
+ }

(I switched the names of the args, because the first arg to 
as.function.foo should match the name of the first arg to as.function).

I was a little surprised that this worked even if a was changed without 
ever evaluating f, because I thought lazy evaluation would mess up that 
case.  But of course the value of x is forced when R evaluates it to 
find out the class for dispatch to the method.

Duncan Murdoch