Right. A semi-short explanation is that R doesn't do full reference counting (*), hence copying happens when you modify an object that _at any previous point_ has been known by two or more names (incl. same name in different environments).
In the present case, there has been local variable x inside foo(), and global variable x. -pd (*) ...yet. Luke Tierney has been working on this; I have temporarily forgotten how much of his work is included in released versions of R. On 03 Mar 2017, at 00:46 , Bert Gunter <bgunter.4...@gmail.com> wrote: > Your assumptions are wrong. > > a full discussion and answer to your question can be found here: > http://adv-r.had.co.nz/memory.html > > This *is* complex and probably off topic for this list. > > Cheers, > Bert > > > Bert Gunter > > "The trouble with having an open mind is that people keep coming along > and sticking things into it." > -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip ) > > > On Thu, Mar 2, 2017 at 7:35 AM, Jiucang Hao <jiuc...@goraltrading.com> wrote: >> Hi Everyone, >> >> >> >> Some R function will make R copy the object AFTER the function call, like >> nrow, while some others don't, like sum. For example the following code: >> >> x = as.double(1:1e8) >> >> system.time(x[1] <- 100) >> >> y = sum(x) >> >> system.time(x[1] <- 200) ## Fast (takes 0s), after calling sum >> >> foo = function(x) { >> >> return(sum(x)) >> >> } >> >> y = foo(x) >> >> system.time(x[1] <- 300) ## Slow (takes 0.35s), after calling foo >> >> Calling foo is NOT slow, because x isn't copied. However, changing x again >> is very slow, as x is copied. My guess is that calling foo will leave a >> reference to x, so when changing it after, R makes another copy. >> >> Any one knows why R does this? Even when the function doesn't change x at >> all? Thanks. >> >> >> >> >> >> Regards, >> >> JiuCang Hao >> >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Office: A 4.23 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
