On 02.03.2016 18:47, Roger Koenker 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)
Here is my (ugly?) suggestion:
f <- function(u) function(x) u * x^2
g <- function(u) function(x) u * log(x)
set.seed(3)
a <- runif(5)
h <- f(a[1])
for (i in 2:5) {
body(h) <- call("*", body(h),
as.call(list(do.call("g", list(a[i])), quote(x))))
}
--
Mikko Korpela
Aalto University School of Science
Department of Computer Science
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