[Rd] modifying large R objects in place

[Peter Dalgaard]

Petr Savicky wrote:
> On Wed, Sep 26, 2007 at 10:52:28AM -0700, Byron Ellis wrote:
>   
>> For the most part, doing anything to an R object result in it's
>> duplication. You generally have to do a lot of work to NOT copy an R
>> object.
>>     
>
> Thank you for your response. Unfortunately, you are right. For example,
> the allocated memory determined by top command on Linux may change during
> a session as follows:
>   a <- matrix(as.integer(1),nrow=14100,ncol=14100) # 774m
>   a[1,1] <- 0 # 3.0g
>   gc() # 1.5g
>
> In the current applicatin, I modify the matrix only using my own C code
> and only read it on R level. So, the above is not a big problem for me
> (at least not now).
>
> However, there is a related thing, which could be a bug. The following
> code determines the value of NAMED field in SEXP header of an object:
>
>   SEXP getnamed(SEXP a)
>   {
>       SEXP out;
>       PROTECT(out = allocVector(INTSXP, 1));
>       INTEGER(out)[0] = NAMED(a);
>       UNPROTECT(1);
>       return(out);
>   }
>
> Now, consider the following session
>
>   u <- matrix(as.integer(1),nrow=5,ncol=3) + as.integer(0)
>   .Call("getnamed",u) # 1 (OK)
>
>   length(u)
>   .Call("getnamed",u) # 1 (OK)
>
>   dim(u)
>   .Call("getnamed",u) # 1 (OK)
>
>   nrow(u)
>   .Call("getnamed",u) # 2 (why?)
>
>   u <- matrix(as.integer(1),nrow=5,ncol=3) + as.integer(0)
>   .Call("getnamed",u) # 1 (OK)
>   ncol(u)
>   .Call("getnamed",u) # 2 (so, ncol does the same)
>
> Is this a bug?
>   
No. It is an infelicity.

The issues are that
1. length() and dim() call .Primitive directly, whereas nrow() and 
ncol() are "real" R functions
2. NAMED records whether an object has _ever_  had  0, 1, or 2+ names

During the evaluation of ncol(u). the argument x is evaluated, and at
that point the object "u" is also named "x" in the evaluation frame of
ncol(). A full(er) reference counting system might drop NAMED back to 1
when exiting ncol(), but currently, R  can only count up (and trying to
find the conditions under which it is safe to reduce NAMED will make
your head spin, believe me! )
> Petr Savicky.
>
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> https://stat.ethz.ch/mailman/listinfo/r-devel
>   


-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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