Re: [Rd] Copying objects prior to .Call

From: Simon Urbanek <simon.urbanek_at_r-project.org>
Date: Wed, 11 Jan 2012 13:21:05 -0500

On Jan 11, 2012, at 1:04 PM, Uwe Ligges wrote:

> 
> 
> On 11.01.2012 18:49, Simon Urbanek wrote:
>> 
>> On Jan 11, 2012, at 12:08 PM, Taylor Arnold wrote:
>> 

>>> R-devel,
>>>
>>> I have noticed that making a copy of an object in R prior to using
>>> .Call on the original object can
>>> cause the C code to alter not only the object passed to it but also
>>> the copy in R.
>> 
>> Please see the docs - .Call does *NOT* have a DUP argument - you are responsible for duplication at all times if you make modifications (e.g. using duplicate()).
>> 
>> Cheers,
>> Simon
>> 
>> 

>>> A simple example
>>> is:
>>>
>>>> x<- 2
>>>> y<- x
>>>> .Call("addOne", x, DUP=TRUE) # Changing DUP does not alter output

>>> NULL
>>>> x

>>> [1] 3
>>>> y

>>> [1] 3
>>>
>>> And corresponding simple C code:
>>>
>>> "test.c":
>>> #include<R.h>
>>> #include<Rinternals.h>
>>> #include<Rmath.h>
>>>
>>> SEXP addOne(SEXP input) {
>>> REAL(input)[0] = REAL(input)[0] + 1;
>>> return R_NilValue;
>>> }
>>>
>>> I assume that this is simply a result of lazy loading
> 
> In addition to Simon: it is "lazy evalution" rather than lazy loading in this case.
> 

It is actually neither. `x` gets evaluated, but the value is shared with `y` because R has no reason to create a copy of identical information until modified. That's why the .Call() code must create a copy if it wants to touch the value that it received. Note that .Call does *not* get `x` itself - it gets a value obtained from the binding of `x` so the only legal way to modify `x` is to assign a value to it. You can try to be more efficieint and check if a value has references to it and prevent copying if it doesn't (see NAMED), but if it does, you have to copy it.

Cheers,
Simon  

> Uwe
> 
> 

>>> in R, and well
>>> documented. My question is, do
>>> there exist functions to (1) force R to make a copy of an object
>>> (force() does not work), and (2) to check
>>> whether two objects are actually pointing to the same memory address.
>>> For question 1, I have
>>> found specific operations which force a copy of a given datatype, but
>>> would prefer a more general
>>> solution.
>>>
>>> Thank you,
>>>
>>> Taylor
>>>
>>>> sessionInfo()

>>> R version 2.14.1 (2011-12-22)
>>> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
>>>
>>> locale:
>>> [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
>>>
>>> attached base packages:
>>> [1] stats graphics grDevices utils datasets methods base
>>>
>>> loaded via a namespace (and not attached):
>>> [1] tools_2.14.1
>>>
>>>
>>> --
>>> Taylor B. Arnold
>>> Department of Statistics
>>> Yale University
>>> 24 Hillhouse Avenue
>>> New Haven, CT 06520
>>>
>>> e-mail: taylor.arnold_at_yale.edu
>>>
>>> ______________________________________________
>>> R-devel_at_r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>>
>> 
>> ______________________________________________
>> R-devel_at_r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
> 

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