[Rd] what is the correct way to force a copy of an object?

[Duncan Murdoch]

On 13-11-22 4:43 PM, Skye Bender-deMoll wrote:
> Dear R-devs,
>
> I'm working on a package where we have a function that modifies an
> Object via .Call  to a C function. Unfortunately in some situations this
> counterintuitive modifies a previously made copy of the object passed to
> the function.  For example, if I first make an assignment to "copy" the
> object,
>
> b<-a
>
> and then modify 'a' , the value of 'b' will be modified as well.  I
> understand that this is probably due to lazy evaluation--the copy of 'a'
> is not made until the 'a' or 'b' instance is further modified. Peeking
> at 'a' and 'b' using .Internal(inspect(a)) seems to confirm this.  But
> for some reason our function is not triggering the copy.

No, lazy evaluation is a higher level concept.  Your problem is that you 
are modifying something without guaranteeing nobody else is using it.
>
> We have been able to kludge it to force a copy and behave as expected by
> making a slight modification to the object before passing it to .Call:
>
> modificationFun <- function(x){
>     x$ForceCopy<-NULL
>     x<-.Call("do_the_mod_in_c_R", package='mypackage')
>     invisible(x)
> }

That's not the right way to fix this.
>
>
> What is the correct way to force the copy to occur to avoid
> inadvertently modifying both instances of the object?  Neither force()
> or eval() seem to do this.
>
> Would it be faster to call duplicate(x) inside the C function instead of
> forcing the copy at the R level?

duplicate(x) at the C level is the only way to do this.  You can look at 
NAMED(x) to decide if this is necessary, or just do it unconditionally.

Duncan Murdoch

>
> thanks for your help,
>    -skye
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>