Andrew, On Mar 15, 2007, at 8:01 AM, Andrew Piskorski wrote:
> On Wed, Mar 14, 2007 at 05:44:04PM -0000, Matthew Dowle wrote: >> >> Hi, >> >> I'm trying to write a function to return an R vector which points >> directly to a contiguous subset of another vector, without taking >> a copy. > > Matthew, I don't know the answer to your question, but this all > seems related to support for references in R. I've included my > notes on R references below. > [...] >> Since SEXPREC is a header followed by the data, rather than the >> header plus a pointer to the data, I'm not sure what I'm trying to >> do is possible. Is there a way of doing this? Similar in spirit >> to how the R assignment "x=y" does not copy y until x is modified, > > Is you last statement above in fact true? Yes. Just try a=1:100000000 gc() b=a gc() b[0]=0:0 gc() > I was under the impression that R does NOT do copy on write, that > when you make a copy of a variable, R immediately allocates memory > and makes a deep copy of the value. > Nope. > But you're using old deprecated "=" for assignment, "old deprecated"? It was introduced in 1.4.0 and it is the most recently introduced assignment operator in R. I think that you are mistaking it for "_" which is no assignment operator anymore but used to be. > which is weird, so maybe you mean when you pass named arguments to > a function? Function args are evaluated lazily, and I think that > is used (I don't know how exactly) to give copy on write behavior - > but only for function arguments. Cheers, Simon ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
