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

3 messages · Skye Bender-deMoll, Kevin Ushey, Duncan Murdoch

Original
Skye Bender-deMoll
# 
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.

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)
}


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?

thanks for your help,
  -skye
Duncan Murdoch
#
On 13-11-22 4:43 PM, Skye Bender-deMoll wrote:
No, lazy evaluation is a higher level concept.  Your problem is that you 
are modifying something without guaranteeing nobody else is using it.
That's not the right way to fix this.
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