[R] Folding ?

[Bill Venables]

> Hi,
> 
> I need to write a function that would look something like this:
> 
>  S <- function(b=betas){
>      expression(b[1] * f(b[2] * x * f(b[3] * x * f(...b[n-1] * x * f(b[n] *
> x)))...)
> }
> 
> Where n is the number of element in b.

I see Saikat Debroy has already responded to this, but here is
(possibly) a simpler version.  It is a function that returns a list of
two functions, one of which is S as above (assuming the leading part
is missing an "x") and the other of which is the derivative, dS/dx:

makeS <- function(b, f) {
  f <- deparse(substitute(f))
  b <- rev(b)
  ex <- call("*", b[1], as.name("x"))
  b <- b[-1]
  while(length(b) > 0) {
    ex <- call("*", b[1],
               call("*", as.name("x"),
                    call(f, ex)))
    b <- b[-1]
  }
  S <- function(x) NULL
  body(S) <- ex
  dS <- function(x) NULL
  body(dS) <- D(ex, "x")
  list(S = S, dS = dS)
}

Here is a small example:

> makeS(5:1, sqrt)
$S
function (x) 
5 * x * sqrt(4 * x * sqrt(3 * x * sqrt(2 * x * sqrt(1 * x))))
<environment: 678810>

$dS
function (x) 
5 * (sqrt(4 * (x * sqrt(3 * (x * sqrt(2 * (x * sqrt(1 * x))))))) + 
    x * (0.5 * (4 * (sqrt(3 * (x * sqrt(2 * (x * sqrt(1 * x))))) + 
        x * (0.5 * (3 * (sqrt(2 * (x * sqrt(1 * x))) + x * (0.5 * 
            (2 * (sqrt(1 * x) + x * (0.5 * 1 * x^-0.5)) * 2 * 
                (x * sqrt(1 * x))^-0.5))) * 3 * (x * sqrt(2 * 
            (x * sqrt(1 * x))))^-0.5))) * 4 * (x * sqrt(3 * (x * 
        sqrt(2 * (x * sqrt(1 * x))))))^-0.5)))
<environment: 678810>

You could probably do it a little more cleverly if you closures but 
you would not be able to read the functions you get very well.

> Further I need to be able to evaluate S at some x numerically of course and
> I need to use "deriv" and produce dS/dx such that I can evaluate it also at
> some x.

> ms <- makeS(5:1, sqrt)
> S <- ms[[1]]
> S(1:10)
 [1]   14.352   54.974  120.596  210.572  324.462  461.931
 [7]  622.711  806.577 1013.337 1242.822
> dS <- ms[[2]]
> dS(1:10)
 [1]  27.807  53.256  77.885 101.996 125.729 149.165 172.358
 [8] 195.343 218.149 240.797

> I tried building the S expression manually to test the deriv (D) function,
> evaluate them both and everything work's fine.
> 
> My trouble is automating the building of the expression S that is dependent
> on the length of b.
> 
> Any suggestion are welcome.

Use a loop.

Bill Venables.
-- 
Bill Venables,      Statistician,     CMIS Environmetrics Project
CSIRO Marine Labs, PO Box 120, Cleveland, Qld,  AUSTRALIA.   4163
Tel: +61 7 3826 7251           Email: Bill.Venables at cmis.csiro.au    
Fax: +61 7 3826 7304      http://www.cmis.csiro.au/bill.venables/



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