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 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
