From pgilbert@bank-banque-canada.ca  Tue Mar 27 03:00:47 2001
From: pgilbert@bank-banque-canada.ca (Paul Gilbert)
Date: Mon, 26 Mar 2001 21:00:47 -0500
Subject: [Rd] DUP=FALSE
References: <Pine.A41.4.33.0103261449180.14166-100000@homer16.u.washington.edu>
Message-ID: <3ABFF44E.98D7BFAC@bank-banque-canada.ca>

Thomas

>> 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.

>There's a third one. In dotcode.c the following happens

>a) x is copied to a C array at the top of the R heap
>b) the C function is called
>c) the C array is copied into a newly created R object
>d) this new R object is returned.

It seems like there is an unnecessary extra copy here, unless of course
the array in the R object cannot be the C array (but your example of
messing up things in the parent environment suggests it can be). If
DUP=TRUE then there should be no harm in the result being just a pointer
to the copy which is the argument, and if DUP=FALSE then the result is
already in the copy as illustrated in the example circulated by Martin
Schlather on Martin Maechler suggestion a few days ago, so again there
is no harm. I won't pretend to understand the internals of R but I know
I am getting clobbered by a big array in an argument.

>I don't know why you are getting slightly different results with
>DUP=FALSE. I can't think of any good explanation.

I'll try to track this down sometime. If you think of anything please
let me know.

Paul Gilbert

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[Rd] DUP=FALSE
Paul Gilbert