[Rd] (PR#8192) [ subscripting sometimes loses names

Duncan Murdoch murdoch at stats.uwo.ca
Sat Jan 31 15:04:58 CET 2009

On 31/01/2009 7:31 AM, Andrew Piskorski wrote:
> This (tangential) discussion really should be a separate thread so I
> changed the subject line above.
> On Fri, Jan 30, 2009 at 11:51:00AM -0500, Simon Urbanek wrote:
>> Subject: Re: [Rd] (PR#13487) Segfault when mistakenly calling [.data.frame
>>> My boss was debugging an issue in our R code.  We have our own "[...."
>>> functions, because stock R drops names when subscripting.
>> ... if you tell it to do so, yes. If you tell it to not do that, it  
>> won't ... ever tried drop=FALSE ?
> Simon, no, the drop=FALSE argument has nothing to do with what
> Christian was talking about.  The kind of thing he meant is PR# 8192,
> "Subject: [ subscripting sometimes loses names":
>   http://bugs.r-project.org/cgi-bin/R/wishlist?id=8192

In that bug report you were asked to provide simple examples, and you 
didn't.  I imagine that's why there was no action on it.  It is not that 
easy for someone else to actually find the simple example that led you 
to print

BAD  $vec.1[[1]]           $vec.1[[2]]
         a    c <NA>         a  c no
         1    3   NA         1  3 NA

I just tracked this one down, and can put together this simple example:

 > (1:3)["no"]
[1] NA

where I think you would want the name "no" attached to the output.  (Or 
maybe your more complicated example is wanted?  You don't explain.)  But 
that looks like documented behaviour to me:  according to my reading of 
"Indexing by vectors" in the R Language Definition manual, it should 
give the same answer as (1:3)[4], and it does.  So it's not a bug, but a 
wishlist item.

And the other two cases where you list "BAD" behaviour?  I didn't track 
them down.

I know you spent a lot of time putting together that bug report; it 
seems a shame that it is being ignored because you put in too much:  you 
really should simplify it as you were asked to do.

Duncan Murdoch

> In R, subscripting with "[" USUALLY retains names, but R has various
> edge cases where it (IMNSHO) inappropriately discards them.  This
> occurs with both .Primitive("[") and "[.data.frame".  This has been
> known for years, but I have not yet tried digging into R's
> implementation to see where and how the names are actually getting
> lost.
> Incidentally, versions of S-Plus since approximately S-Plus 6.0 back
> in 2001 show similar buggy edge case behavior.  Older versions of
> S-Plus, c. S-Plus 3.3 and earlier, had the correct, name preserving
> behavior.  I presume that the original Bell Labs S had correct
> name-preserving behavior, and then the S-Plus developers broke it
> sometime along the way.

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