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

John Chambers jmc at r-project.org
Tue May 5 17:45:30 CEST 2015

```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 <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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