[Rd] Out of bounds negative index

[Duncan Murdoch]

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.

so apparently it was intentional.

Duncan Murdoch