[Rd] unset() function?

[William Dunlap]

Does R have a function like the S/S++ unset() function?
unset(name) would remove 'name' from the current evaluation
frame and return its value.  It allowed you to safely avoid
some memory copying when calling .C or .Call.

E.g., suppose you had C code like
  #include <R.h>
  #include <Rinternals.h>
  SEXP add1(SEXP pX)
  {
      int nProtected = 0;
      int n = Rf_length(pX);
      int i;
      double* x;
      Rprintf("NAMED(pX)=%d: ", NAMED(pX));
      if (NAMED(pX)) {
          Rprintf("Copying pX before adding 1\n");
          PROTECT(pX = duplicate(pX)); nProtected++;
      } else {
          Rprintf("Changing pX in place\n");
      }
      x = REAL(pX);
      for(i=0 ; i<n ; i++) {
        x[i] = x[i] + 1.0;
      }
      UNPROTECT(nProtected);
      return pX;
  }

If I call this from an R function
  add1 <- function(x) {
      stopifnot(inherits(x, "numeric"))
     .Call("add1", x)
  }
it will will always copy 'x', even though not copying would
be safe (since add1 doesn't use 'x' after calling .Call()).
  > add1(c(1.2, 3.4))
  NAMED(pX)=2: Copying pX before adding 1
  [1] 2.2 4.4
If I make the .Call directly, without a nice R function around it
then I can avoid the copy
  > .Call("add1", c(1.2, 3.4))
  NAMED(pX)=0: Changing pX in place
  [1] 2.2 4.4

If something like S's unset() were available I could avoid the copy,
when safe to do so, by making the .Call in add1
   .Call("add1", unset(x))

If you called this new add1 with a named variable from another
function the copying would be done, since NAMED(x) would be
2 even after the local binding was removed.  It actually requires some
care to to eliminate the copying, as all the functions in the call
chain would have to use unset() when possible.

I ask this because I ran across a function in the 'bit' package that
does not have its C code call duplicate but instead assumes the
x[1] <- x[1] will force x to be copied:
  "!.bit" <- function(x){
    if (length(x)){
      ret <- x
      ret[1] <- ret[1]  # force duplication
      .Call("R_bit_not", ret, PACKAGE="bit")
    }else{
      x
    }
  }
If you optimize things so that 'ret[1] <- ret[1]' does not copy 'ret',
then this function alters its input.  It a function like unset()
were there then the .Call could be
     .Call("R_bit_not", unset(x))

I suppose the compiler could analyze the code and see that
x was not used after the .Call and thus feel free to avoid the
copy.

In any case bit's maintainer should add something like
    if(NAMED(x) {
        PROTECT(x=duplicate(x));
        nProtect++;
    }
    ...
    UNPROTECT(nProtect);
in the C code, but unset() would help avoid unneeded duplications.


Bill Dunlap
TIBCO Software
wdunlap tibco.com

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