[R] Functional programming?

[Bert Gunter]

Does this do what you want:

f <- function(u) function(x) u * x^2
g <- function(u) function(x) u * log(x)
set.seed(3)
a <- runif(5)
h <- list()
hit <- list()
h[[1]] <- f(a[1])
hit[[1]] <- f(a[1])
for(i in 2:5)h[[i]] <- eval(bquote(function(x).(h[[i-1]])(x) * g(a[i])(x)))
x <- 1:99/10
plot(x, h[[1]](x), type = "l")
for(i in 2:5){
  i
  lines(x, h[[i]](x), col = i)
}

Cheers,
Bert


Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, Mar 2, 2016 at 8:47 AM, Roger Koenker <rkoenker at illinois.edu> wrote:
> I have a (remarkably ugly!!) code snippet  (below) that, given
> two simple functions, f and g,  generates
> a list of new functions h_{k+1} =  h_k * g, k= 1, …, K.  Surely, there are vastly
> better ways to do this.  I don’t particularly care about the returned list,
> I’d be happy to have the final  h_K version of the function,
> but I keep losing my way and running into the dreaded:
>
> Error in h[[1]] : object of type 'closure' is not subsettable
> or
> Error: evaluation nested too deeply: infinite recursion / options(expressions=)?
>
> Mainly I’d like to get rid of the horrible, horrible paste/parse/eval evils.  Admittedly
> the f,g look a bit strange, so you may have to suspend disbelief to imagine that there is
> something more sensible lurking beneath this minimal (toy)  example.
>
>     f <- function(u) function(x) u * x^2
>     g <- function(u) function(x) u * log(x)
>     set.seed(3)
>     a <- runif(5)
>     h <- list()
>     hit <- list()
>     h[[1]] <- f(a[1])
>     hit[[1]] <- f(a[1])
>     for(i in 2:5){
>         ht <- paste("function(x) h[[", i-1, "]](x) * g(", a[i], ")(x)")
>         h[[i]] <- eval(parse(text = ht))
>         hit[[i]] <- function(x) {force(i); return(h[[i]] (x))}
>         }
>     x <- 1:99/10
>     plot(x, h[[1]](x), type = "l")
>     for(i in 2:5)
>         lines(x, h[[i]](x), col = i)
>
> Thanks,
> Roger
>
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