This is because R keeps track of the names of an object, until there are 2
names. Thus, once it reaches 2, it can no longer decrement the named count.
In this example, 'a' reaches 2 names ('a' and 'b'), thus R does not know
that 'a' only has one name at the end.Luke has added reference counting to R 3.1 to get around these types of problems. If you want to try it out, make the necessary change in Rinternals.h and recompile. With reference counting, R knows that 'a' only has one reference and avoids the copy. > a <- seq.int(10) > a[1:4] <- 4:1 > b <- a > .Internal(inspect(a)) @2b71608 13 INTSXP g0c4 [REF(2)] (len=10, tl=0) 4,3,2,1,5,... > b[1:4] <- 1:4 > .Internal(inspect(b)) @2b715a0 13 INTSXP g0c4 [REF(1)] (len=10, tl=0) 1,2,3,4,5,... > .Internal(inspect(a)) @2b71608 13 INTSXP g0c4 [REF(1)] (len=10, tl=0) 4,3,2,1,5,... > a[1:4] <- 1:4 > .Internal(inspect(a)) @2b71608 13 INTSXP g0c4 [REF(1)] (len=10, tl=0) 1,2,3,4,5,... On Mon, Jun 2, 2014 at 9:31 AM, Dénes Tóth <toth.de...@ttk.mta.hu> wrote: > > Hi, > > Please consider the following code: > > a <- seq.int(10) # create a > tracemem(a) > a[1:4] <- 4:1 # no internal copy > b <- a # no internal copy > b[1:4] <- 1:4 # copy, b is not a any more > a[1:4] <- 1:4 # copy, but why? > > With results: > > a <- seq.int(10) > > tracemem(a) > [1] "<0x1792bc0>" > > a[1:4] <- 4:1 > > b <- a > > b[1:4] <- 1:4 > tracemem[0x1792bc0 -> 0x1792b58]: > > a[1:4] <- 1:4 > tracemem[0x1792bc0 -> 0x1792af0]: > > > ## > > Could you provide a brief explanation or point me to a source why R needs > a copy in the final step? > > > Best, > Denes > > > > > sessionInfo() > R version 3.1.0 (2014-04-10) > Platform: x86_64-pc-linux-gnu (64-bit) > > locale: > [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C > [3] LC_TIME=en_US.UTF-8 LC_COLLATE=en_US.UTF-8 > [5] LC_MONETARY=en_US.UTF-8 LC_MESSAGES=en_US.UTF-8 > [7] LC_PAPER=en_US.UTF-8 LC_NAME=C > [9] LC_ADDRESS=C LC_TELEPHONE=C > [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C > > attached base packages: > [1] stats graphics grDevices utils datasets methods base > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > [[alternative HTML version deleted]]
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