Re: [Rd] Out of bounds negative index

From: Vincent Goulet <vincent.goulet_at_act.ulaval.ca>
Date: Fri, 02 May 2008 16:11:04 -0400

Le ven. 02 mai à 15:46, Duncan Murdoch a écrit :

> On 5/2/2008 3:23 PM, Vincent Goulet wrote:
>> Hi,
>> From the R Language Definition, Section 3.4.1:
>> "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."
>> (This is also mentioned in S Programming, footnote of page 24.)
>> Can someone please provide an example triggering the error? Looking
>> in src/main/subscript.c I could not find exception handling for |
>> i| > length(x), unless the negative subscript is mixed with NAs.
>> In other cases, out of bounds negative indexes just seem silently
>> ignored.
>> Did I missed or misinterpreted something, here?
>
> Looks to me like a documentation error. I would expect from that
> description that executing
>
> > x <- 1:5
> > x[7]
> [1] NA
> > x[-7]
> [1] 1 2 3 4 5
>
> would have given an error on x[-7], but clearly it didn't. This
> behaviour appears to have started with 2.6.0; 2.5.1 gives the
> error. I don't see a NEWS entry about it...but revision r42123 says
>
> Changed the behaviour of out-of-bounds negative
> subscripts to match that of S. Such values are
> now ignored rather than tripping an error.

Ah ha, there it is. The svn log is one place I didn't look. I had screened NEWS, ONEWS and OONEWS, though...

Thanks, Duncan.

>
>
> so apparently it was intentional.
>
> Duncan Murdoch

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