[R] Question about creating lists with functions as elements

[Barry Rowlingson]

On Tue, Jun 30, 2009 at 1:00 PM, Craig P. Pyrame<crappyr at gmail.com> wrote:

> But this fails, as above.  Why?  Why can c(character, character) create a
> list of two functions, but rep(character, 2) can't?
>
> Another solution to my problem I could find (and you'll hopefully suggest an
> even better one) is to use class names instead, like so:
>
>> types = lapply(c(rep('character', 2), 'integer', 'numeric', ...),
>> function(type) vector(type, 0))
>
> but I am still curious why the above doesn't work as I would expect it to.
>

 I hope this doesn't turn into a debate about expectations vs what
actually happens :)

 I've discovered that rep(list(f),n) might do what you want:

 > f=function(x){x*2}
 > l=rep(list(f),5)
 > l[[1]](4)
 [1] 8

 help(rep) says "It is a generic function" which translates to "you
may have some problem finding out exactly what function is called when
you do rep(thing)". I don't think there's a method for functions, so
it's probably being passed to the generic method which is expecting
vectors. It possibly then tries to get an element of f, and fails, a
bit like this:

 > f[1]
 Error in f[1] : object of type 'closure' is not subsettable

 - without first checking if the object is a vector (you can do
rep(c(1,2),5) you see). Maybe a bug, or a misleading error message, or
a documentation problem: check with the latest version of R etc etc
etc read the FAQ before submitting a bug etc etc etc.

Barry