as.function()

library(gsubfn)
fn$force(~ 2*x + 3*y^2)

fn$force(x + y ~ 2*x + a*y^2)

How about this as a version  that automatically constructs the argument
list (and make into a method for as.function as appropriate)?

makefun <- function(expr)
{
   f <- function() {}
   body(f) <- expr
   vars <- all.vars(expr)
   if (length(vars)) {
       args <- alist(x=)[rep(1,length(vars))]
       names(args) <- vars
       formals(f) <- args
   }
   environment(f) <- globalenv()
   return(f)
}

 > makefun(expression(2*x + 3*y^2))

function (x, y)
2 * x + 3 * y^2

 > makefun(expression(2*x + 3*y^2 - z))

function (x, y, z)
2 * x + 3 * y^2 - z

 > makefun(expression(p1 + p2))

function (p1, p2)
p1 + p2

 >

-- Tony Plate




Henrique Dallazuanna wrote:

Try this:

as.function.foo <- function(obj, ...)
{
newobj <- function(x, ...){}
body(newobj) <- obj
return(newobj)
}

x <- expression(2*x + 3*x^2)

foo <- as.function.foo(x)
foo(2)


Hope this help

On 14/01/2008, Robin Hankin <r.hankin at noc.soton.ac.uk> wrote:

Antonio


thanks for your help here, but it doesn't answer my question.

Perhaps if I outline my motivation it would help.


I want to recreate the ability of
the "polynom" package to do the following:

 > library(polynom)
 > p <- polynomial(1:4)
 > p

1 + 2*x + 3*x^2 + 4*x^3

 > MySpecialFunction <- as.function(p)
 > MySpecialFunction(1:10)

  [1]   10   49  142  313  586  985 1534 2257 3178 4321

 > p <- 4
 > MySpecialFunction(1:10)

  [1]   10   49  142  313  586  985 1534 2257 3178 4321

 >

See how the user can define object "MySpecialFunction",
  which outlives short-lived polynomial "p".

Unfortunately, I don't see a way to modify as.function.polynomial()
to do what I want.


best wishes


rksh









On 14 Jan 2008, at 08:45, Antonio, Fabio Di Narzo wrote:

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

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