R devel - Nov 2009 - Negative integer subscripts in [[?

Should negative subscripts be outlawed in
   x[[subscript]]
?  

Currently, if subscript is a scalar then it can only
be negative if length(x)==1 (otherwise [[ throws an
error).  If length(subscript)>1 then it gets treated
as an attempt to recursively extract an element of
a nested list.
> list(10,20)[[-1]] # get the last element out of 2
[1] 20
> list(10,20,30)[[-(1:2)]] # get the last of 3?  No.
Error in list(10, 20, 30)[[-(1:2)]] :
  attempt to select more than one element
> list(10,list(20,30))[[-c(1:2)]] # see how recursive subscripting is
done
[1] 20

If negative subscripts were not allowed in [[ then 
there might be a little less confusion about [[.

(If recursive subscripting were done by a list instead
of by an integer or character vector there might be
less confusion and it would be more flexible.)


Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

Hmm.  It's something that is worth discouraging, because the user 
probably didn't mean it.

But it is more or less consistent with the language, and prohibiting it 
at this date is unlikely to be worth the possible breakage (I know, who 
could possibly rely on it?  But with a few thousand packages, somebody 
likely does.)

John

William Dunlap wrote:
> Should negative subscripts be outlawed in
>    x[[subscript]]
> ?  
>
> Currently, if subscript is a scalar then it can only
> be negative if length(x)==1 
Actually, length(x)==2 as in your example.
> (otherwise [[ throws an
> error).  If length(subscript)>1 then it gets treated
> as an attempt to recursively extract an element of
> a nested list.
>   
Yes, but the interpretation is essentially consistent with multiple 
elements and positive values:

 > xx = list(c(1,2),c(3,4))
 > xx[[-1]]
[1] 3 4
 > xx[[-2]]
[1] 1 2
 > xx[[c(-1,-2)]]
[1] 3
 > xx[[c(2,1)]]
[1] 3
>   
>> list(10,20)[[-1]] # get the last element out of 2
>>     
> [1] 20
>   
>> list(10,20,30)[[-(1:2)]] # get the last of 3?  No.
>>     
> Error in list(10, 20, 30)[[-(1:2)]] :
>   attempt to select more than one element
>   
>> list(10,list(20,30))[[-c(1:2)]] # see how recursive subscripting is
>>     
> done
> [1] 20
>
> If negative subscripts were not allowed in [[ then 
> there might be a little less confusion about [[.
>
> (If recursive subscripting were done by a list instead
> of by an integer or character vector there might be
> less confusion and it would be more flexible.)
>
>
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com 
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>   
	[[alternative HTML version deleted]]

On 09/11/2009 4:38 PM, William Dunlap wrote:
> Should negative subscripts be outlawed in
>    x[[subscript]]
> ?  
> 
> Currently, if subscript is a scalar then it can only
> be negative if length(x)==1 (otherwise [[ throws an
> error).  If length(subscript)>1 then it gets treated
> as an attempt to recursively extract an element of
> a nested list.
> 
>> list(10,20)[[-1]] # get the last element out of 2
> [1] 20
>> list(10,20,30)[[-(1:2)]] # get the last of 3?  No.
> Error in list(10, 20, 30)[[-(1:2)]] :
>   attempt to select more than one element
>> list(10,list(20,30))[[-c(1:2)]] # see how recursive subscripting is
> done
> [1] 20
> 
> If negative subscripts were not allowed in [[ then 
> there might be a little less confusion about [[.
I agree, it would be better not to allow negatives here, but as John 
said it's probably too late to do away with them.
> (If recursive subscripting were done by a list instead
> of by an integer or character vector there might be
> less confusion and it would be more flexible.)
I don't follow this.  Recursive lists are trees, and you specify a 
single element of a tree by specifying a sequence of indices.  Why would 
it be less confusing to give a list?  What extra flexibility could there 
be?  I suppose you could mix integer and character indices, but what 
would be meant by x[[ list(1, list(2,3), 4) ]] ?

I don't know the original motivation for allowing vector indexing to 
lists, but I extended it to pairlists so that it would be possible to 
specify a location within a function exactly, by walking down the parse 
tree.  I think it's something that would be rarely used, but when you 
need it, it's very handy.

Duncan Murdoch