R devel - Mar 2001 - DUP=FALSE

Last week Paul Gilbert asked about DUP=FALSE and whether parent
environments can be affected.  I have experimented and they can.
Consider the R functions
> f
function(x){ .C("badidea",x,DUP=FALSE)}
> g
function(y) f(y)
> h
function(z){
        z<-as.integer(z)
        g(z)
        z
}

and the C function
	void badidea(int *x){
	    *x=7;
	}

This gives
> h(4)
[1] 7

so the modification of x in f() reaches up past g() to modify z in h().
The conclusion is that one should not pass a formal parameter of a
function using DUP=FALSE if it might be modified.  Assigning the parameter
seems to be sufficient to prevent the problem: replacing g by

 g<-function(y) {
	w<-y
	f(w)
 }
gives
> h(4)
[1] 4
as expected.


	-thomas

Thomas Lumley			Asst. Professor, Biostatistics
tlumley@u.washington.edu	University of Washington, Seattle

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Thomas

Thanks for verifying that this actually does work the way you thought it
did. In the example I asked about  (qr.qty ) I am fairly sure that it
does not modify the y argument. Unfortunately this seems relatively
difficult to trace for certain, and experimenting works but may not hit
all cases.

Another thing I find I do not understand is how many copies of an
argument are made. For example, in a call to a function like

f <- function(x){ .C("goodidea", y=double(1),  x, DUP=TRUE)[[y]] }

there is the original x and the duplicated x in the argument to .C, but
is there a third in the list returned by .C, or is that somehow the same
one as in the argument to .C.  My problem is that my x is very big.

Thanks,
Paul Gilbert



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