[Rd] Subscripting fails if name of element is "" (PR#8161)

[Prof Brian Ripley]

On Fri, 7 Oct 2005, Duncan Murdoch wrote:

> I haven't been following this conversation in order, but I think there's 
> another bug here besides the one(s?) you identified:
>
> Jens had this example:
>
>> x <- 1:4
>> names(x) <- c(NA, "NA", "a", "")
>> x[names(x)]
> <NA> <NA>    a <NA>
>   1    1    3   NA
>
> Shouldn't the second entry in the result be 2, with name "NA"?  It seems the 
> string "NA" has been converted to <NA> here.

Yes, but I don't see it in PR#8161 where there is no name "NA" that I can
see.  (In other words it is not an instance of the subject line.)

The issue is that <NA> is matching "NA", and it should not.  As in the
code

Rboolean NonNullStringMatch(SEXP s, SEXP t)
{
     if (CHAR(s)[0] && CHAR(t)[0] && strcmp(CHAR(s), CHAR(t)) == 0)
 	return TRUE;
     else
 	return FALSE;
}

and there are more instances around.



>
> Duncan Murdoch
>
>
> Prof Brian Ripley wrote:
>> On Thu, 6 Oct 2005, "Jens Oehlschlägel" wrote:
>> 
>> 
>>> Dear Thomas,
>>> 
>>> 
>>>> This looks deliberate (there is a function NonNullStringMatch that does
>>>> the matching).  I assume this is because there is no other way to
>>>> indicate that an element has no name.
>>> 
>>>> If so, it is a documentation bug -- help(names) and FAQ 7.14 should
>>>> specify this behaviour.  Too late for 2.2.0, unfortunately.
>>> 
>>> I respectfully disagree: the element has a name, its an empty string. Of
>>> course "" is a doubtful name for an element, but as long as we allow this
>>> name when assigning names()<- we also should handle it like a name in
>>> subscripting. The alternative would be to disallow "" in names at all.
>>> However, both alternatives rather look like code changes, not only
>>> documentation.
>> 
>> 
>> I think Thomas is right as to how S interprets this: "" is no name on 
>> assignment, wheread NA as a name is a different thing (there probably is a 
>> name, we just do not know what it is).
>> 
>> Here is the crux of the example.
>> 
>> p <- c(a=1, 2)
>> 
>>> p <- c(a=1, 2)
>>> names(p)
>> 
>> [1] "a" ""
>> 
>>> p
>> 
>> a
>> 1 2
>> 
>>> p2 <- c(1,2)
>>> names(p2) <- c("a", "")
>>> identical(p, p2)
>> 
>> [1] TRUE
>> 
>> so giving the name is "" really is the same as giving no name.
>> 
>> `Error 1' is said to be
>> 
>> 
>>> p[""]
>> 
>> <NA>
>>    NA
>> 
>> You haven't given a name, so I think that is right.  S (which has no 
>> character NAs) uses "" as the name, but here there may be a name or not.
>> 
>> 
>>> P <- list(a=1, 2)
>> 
>> 
>> I think Jens then meant as `error 2' that
>> 
>> 
>>> P
>> 
>> $a
>> [1] 1
>> 
>> [[2]]
>> [1] 2
>> 
>> shows no name for the second element, and that seems right to me (although 
>> S shows "" here).
>> 
>> Finally (`error 3')
>> 
>> 
>>> P[""]
>> 
>> $"NA"
>> NULL
>> 
>> is a length-one list with name character-NA.  (S has no name here.)  That 
>> seems the right answer but if so is printed inconsistently.
>> 
>> I would say that
>> 
>> 
>>> Q <- list(1, 2)
>>> names(Q) <- c("a", NA)
>>> Q
>> 
>> $a
>> [1] 1
>> 
>> $"NA"
>> [1] 2
>> 
>> was the only bug here (the name should be printed as <NA>).  Now that
>> comes from this bit of code
>> 
>>  		    if( isValidName(CHAR(PRINTNAME(TAG(s)))) )
>>  			sprintf(ptag, "$%s", CHAR(PRINTNAME(TAG(s))));
>>  		    else
>>  			sprintf(ptag, "$\"%s\"", CHAR(PRINTNAME(TAG(s))));
>> 
>> so non-syntactic names are printed surrounded by "".  Nowadays I think we 
>> would prefer ``, as in
>> 
>> 
>>> A <- list("a+b"=1)
>>> A
>> 
>> $"a+b"
>> [1] 1
>> 
>> 
>>> A$"a+b"
>> 
>> [1] 1
>> 
>>> A$`a+b`
>> 
>> [1] 1
>> 
>> but NA needs to be a special case as in
>> 
>> 
>>> A <- list(1, 2)
>>> names(A) <- c("NA", NA)
>>> A
>> 
>> $"NA"
>> [1] 1
>> 
>> $"NA"
>> [1] 2
>> 
>> 
>>> is.na(names(A))
>> 
>> [1] FALSE  TRUE
>> 
>> 
>> 
>> 
>> ------------------------------------------------------------------------
>> 
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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