>>>>> John Chambers <j...@stat.stanford.edu> >>>>> on Tue, 5 May 2015 08:39:46 -0700 writes:
> When someone suggests that we "might have had a reason" for some peculiarity in the original S, my usual reaction is "Or else we never thought of the problem". > In this case, however, there is a relevant statement in the 1988 "blue book". In the discussion of subscripting (p 358) the definition for negative i says: "the indices consist of the elements of seq(along=x) that do not match any elements in -i". > Suggesting that no bounds checking on -i takes place. > John Indeed! Thanks a lot John, for the perspective and clarification! I'm committing a patch to the documentation now. Martin > On May 5, 2015, at 7:01 AM, Martin Maechler <maech...@lynne.stat.math.ethz.ch> wrote: >>>>>>> Henrik Bengtsson <henrik.bengts...@ucsf.edu> >>>>>>> on Mon, 4 May 2015 12:20:44 -0700 writes: >> >>> In Section 'Indexing by vectors' of 'R Language Definition' >>> (http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Indexing-by-vectors) >>> it says: >> >>> "Integer. All elements of i must have the same sign. If they are >>> positive, the elements of x with those index numbers are selected. If >>> i contains negative elements, all elements except those indicated are >>> selected. >> >>> If i is positive and exceeds length(x) then the corresponding >>> selection is NA. A negative out of bounds value for i causes an error. >> >>> A special case is the zero index, which has null effects: x[0] is an >>> empty vector and otherwise including zeros among positive or negative >>> indices has the same effect as if they were omitted." >> >>> However, that "A negative out of bounds value for i causes an error" >>> in the second paragraph does not seem to apply. Instead, R silently >>> ignore negative indices that are out of range. For example: >> >>>> x <- 1:4 >>>> x[-9L] >>> [1] 1 2 3 4 >>>> x[-c(1:9)] >>> integer(0) >>>> x[-c(3:9)] >>> [1] 1 2 >> >>>> y <- as.list(1:4) >>>> y[-c(1:9)] >>> list() >> >>> Is the observed non-error the correct behavior and therefore the >>> documentation is incorrect, or is it vice verse? (...or is it me >>> missing something) >> >>> I get the above on R devel, R 3.2.0, and as far back as R 2.11.0 >>> (haven't check earlier versions). >> >> Thank you, Henrik! >> >> I've checked further back: The change happened between R 2.5.1 and R 2.6.0. >> >> The previous behavior was >> >>> (1:3)[-(3:5)] >> Error: subscript out of bounds >> >> If you start reading NEWS.2, you see a *lot* of new features >> (and bug fixes) in the 2.6.0 news, but from my browsing, none of >> them mentioned the new behavior as feature. >> >> Let's -- for a moment -- declare it a bug in the code, i.e., not >> in the documentation: >> >> - As 2.6.0 happened quite a while ago (Oct. 2007), >> we could wonder how much R code will break if we fix the bug. >> >> - Is the R package authors' community willing to do the necessary >> cleanup in their packages ? >> >> ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- >> >> >> Now, after reading the source code for a while, and looking at >> the changes, I've found the log entry >> >> ------------------------------------------------------------------------ >> r42123 | ihaka | 2007-07-05 02:00:05 +0200 (Thu, 05 Jul 2007) | 4 lines >> >> Changed the behaviour of out-of-bounds negative >> subscripts to match that of S. Such values are >> now ignored rather than tripping an error. >> >> ------------------------------------------------------------------------ >> >> So, it was changed on purpose, by one of the true "R"s, very >> much on purpose. >> >> Making it a *warning* instead of the original error >> may have been both more cautious and more helpful for >> detecting programming errors. >> >> OTOH, John Chambers, the father of S and hence grandfather of R, >> may have had good reasons why it seemed more logical to silently >> ignore such out of bound negative indices: >> One could argue that >> >> x[-5] means "leave away the 5-th element of x" >> >> and if there is no 5-th element of x, leaving it away should be a no-op. >> >> After all this musing and history detection, my gut decision >> would be to only change the documentation which Ross forgot to change. >> >> But of course, it may be interesting to hear other programmeR's feedback on this. >> >> Martin ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel