[R] Error: object of type 'closure' is not subsettable

[Romain Francois]

On 01/17/2010 09:00 PM, Rolf Turner wrote:
> I thought it would be possible to make rep() work for functions
> by writing a method for the function class. I tried:
>
> rep.function <- function(x,...) {
> times <- as.list(...)[[1]]
> rslt <- vector("list",times)
> rslt[1:times] <- list(x)
> rslt
> }
>
> But then doing
>
> rep(sin,2)
>
> still gave an error --- Error: object of type 'builtin' is not subsettable
>
> Note the difference: ``builtin'' rather than ``closure''.
>
> I then noticed that there is no generic function for rep, although
> there are ***methods*** for rep. I don't understand this at all!
>
> So I did:
>
> rep.default <- rep
> rep <- function(x,...){UseMethod("rep")}
>
> Having taken these steps, rep(sin,2) worked as expected.
>
> But why doesn't rep() have a generic form? And how can there
> be methods ***without*** a generic? Can anyone explain the
> issues to me, preferably in terms that are comprehensible to
> the human mind (which is what I'm equipped with)?

rep is an internal generic, so the  dispatch happens internally (in the 
c code). Here is the relevant C code fragment :

SEXP attribute_hidden do_rep(SEXP call, SEXP op, SEXP args, SEXP rho)
{
     SEXP ans, x, ap, times = R_NilValue /* -Wall */, ind;
     int i, lx, len = NA_INTEGER, each = 1, nt, nprotect = 4;

     if (DispatchOrEval(call, op, "rep", args, rho, &ans, 0, 0))
	return(ans);

     ...

}

Romain

> cheers,
>
> Rolf Turner
>
> On 14/01/2010, at 2:24 PM, Gabor Grothendieck wrote:
>
>> See ?rep where it says that the argument must be a vector. Try
>> rep(list(sin), 3)
>>
>> On Wed, Jan 13, 2010 at 8:11 PM, Matthew Walker
>> <matthew.walker.1 at ulaval.ca> wrote:
>>> Hi everyone,
>>>
>>> Would somebody please explain (or point me to a reference that
>>> explains) the
>>> following error:
>>>
>>> "Error: object of type 'closure' is not subsettable"
>>>
>>> I was trying to use rep() to replicate a function:
>>>
>>>> example_function <- function() { return(TRUE) }
>>>> rep(example_function, 3)
>>> Error: object of type 'closure' is not subsettable
>>>
>>> But I just cannot understand this error. I can combine functions
>>> using "c"
>>> without any problems:
>>>
>>>> c(example_function, example_function)
>>> [[1]]
>>> function ()
>>> {
>>> return(TRUE)
>>> }
>>>
>>> [[2]]
>>> function ()
>>> {
>>> return(TRUE)
>>> }
>>>
>>> What am I doing wrong when I use rep()?
>>>
>>> Thanks in advance,
>>>
>>> Matthew Walker

-- 
Romain Francois
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http://romainfrancois.blog.free.fr
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