From savicky at cs.cas.cz  Thu Sep 27 15:13:17 2007
From: savicky at cs.cas.cz (Petr Savicky)
Date: Thu, 27 Sep 2007 15:13:17 +0200
Subject: [Rd] modifying large R objects in place
In-Reply-To: <7098abec0709261052q3a6b01e2v562f9a88ed81c513@mail.gmail.com>
References: <20070926092924.GA9879@cs.cas.cz>
	<7098abec0709261052q3a6b01e2v562f9a88ed81c513@mail.gmail.com>
Message-ID: <20070927131317.GB7605@cs.cas.cz>

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?

Petr Savicky.



[Rd] modifying large R objects in place
Petr Savicky