# [Rd] Shouldn't vector indexing with negative out-of-range index give an error?

Martin Maechler maechler at lynne.stat.math.ethz.ch
Wed May 6 10:33:50 CEST 2015

```>>>>> John Chambers <jmc at 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 <maechler at lynne.stat.math.ethz.ch> wrote:

>>>>>>> Henrik Bengtsson <henrik.bengtsson at 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

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