R help - Jun 2009 - Question about creating lists with functions as elements

Dear list,

I am trying to construct a list of functions using rep.  I can't 
understand the following:

 > c(character, character) => list with two functions
 > rep(character, 2) => error

The error says that "object of type 'special' is not
subsettable", and I
have no idea what this means.  Would you please help me.

The purpose of doing the above is that I need to use scan for reading 
files that happen to be too large and a bit irregular for read.table.  
To make scan work, I need to specify the types of values in each column 
(record field).  I can specify the 'what' argument as follows:

 > records <- scan(..., what = list(character(0), character(0), 
integer(0), numeric(0), character(0), ...), ...)

but this quickly becomes boring for large enough records. (Why does not 
scan take a character string with class names, as read.table does, 
instead of a list with dummy objects?) So I am trying to do tricks, and 
one idea that seems pretty simple is to create a list of functions that 
create vectors of a particular type, and apply them (using lapply) to 
get a list of prototype objects:

 > types = lapply(c(rep(character, 2), integer, numeric, ...), 
function(type) type(0)) => error

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.

Best regards,
Craig

>From the help page of rep:

"Value:

     An object of the same type as 'x' (except that 'rep' will
coerce
     pairlists to vector lists)."

So, you can do:

rep(list(character), 2)

On Tue, Jun 30, 2009 at 9:00 AM, Craig P. Pyrame <crappyr@gmail.com>
wrote:
> Dear list,
>
> I am trying to construct a list of functions using rep.  I can't
understand
> the following:
>
> > c(character, character) => list with two functions
> > rep(character, 2) => error
>
> The error says that "object of type 'special' is not
subsettable", and I
> have no idea what this means.  Would you please help me.
>
> The purpose of doing the above is that I need to use scan for reading files
> that happen to be too large and a bit irregular for read.table.  To make
> scan work, I need to specify the types of values in each column (record
> field).  I can specify the 'what' argument as follows:
>
> > records <- scan(..., what = list(character(0), character(0),
integer(0),
> numeric(0), character(0), ...), ...)
>
> but this quickly becomes boring for large enough records. (Why does not
> scan take a character string with class names, as read.table does, instead
> of a list with dummy objects?) So I am trying to do tricks, and one idea
> that seems pretty simple is to create a list of functions that create
> vectors of a particular type, and apply them (using lapply) to get a list
of
> prototype objects:
>
> > types = lapply(c(rep(character, 2), integer, numeric, ...),
> function(type) type(0)) => error
>
> 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.
>
> Best regards,
> Craig
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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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